Re: [PATCH] dma-buf: mmap support
Hi Daniel, Rob, On 11 May 2012 21:00, Rob Clark rob.cl...@linaro.org wrote: On Tue, Apr 24, 2012 at 4:08 AM, Daniel Vetter daniel.vet...@ffwll.ch wrote: Compared to Rob Clark's RFC I've ditched the prepare/finish hooks snip Cc: Rob Clark rob.cl...@linaro.org Cc: Rebecca Schultz Zavin rebe...@android.com Signed-Off-by: Daniel Vetter daniel.vet...@ffwll.ch Acked-by: Rob Clark rob.cl...@linaro.org Thanks, applied to my for-next. Sorry, I was away due to some medical reasons for some time, hence the delay. snip Best regards, ~Sumit. -- To unsubscribe from this list: send the line unsubscribe linux-media in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [PATCH] dma-buf: mmap support
On Tue, Apr 24, 2012 at 4:08 AM, Daniel Vetter daniel.vet...@ffwll.ch wrote: Compared to Rob Clark's RFC I've ditched the prepare/finish hooks and corresponding ioctls on the dma_buf file. The major reason for that is that many people seem to be under the impression that this is also for synchronization with outstanding asynchronous processsing. I'm pretty massively opposed to this because: - It boils down reinventing a new rather general-purpose userspace synchronization interface. If we look at things like futexes, this is hard to get right. - Furthermore a lot of kernel code has to interact with this synchronization primitive. This smells a look like the dri1 hw_lock, a horror show I prefer not to reinvent. - Even more fun is that multiple different subsystems would interact here, so we have plenty of opportunities to create funny deadlock scenarios. I think synchronization is a wholesale different problem from data sharing and should be tackled as an orthogonal problem. Now we could demand that prepare/finish may only ensure cache coherency (as Rob intended), but that runs up into the next problem: We not only need mmap support to facilitate sw-only processing nodes in a pipeline (without jumping through hoops by importing the dma_buf into some sw-access only importer), which allows for a nicer ION-dma-buf upgrade path for existing Android userspace. We also need mmap support for existing importing subsystems to support existing userspace libraries. And a loot of these subsystems are expected to export coherent userspace mappings. So prepare/finish can only ever be optional and the exporter /needs/ to support coherent mappings. Given that mmap access is always somewhat fallback-y in nature I've decided to drop this optimization, instead of just making it optional. If we demonstrate a clear need for this, supported by benchmark results, we can always add it in again later as an optional extension. Other differences compared to Rob's RFC is the above mentioned support for mapping a dma-buf through facilities provided by the importer. Which results in mmap support no longer being optional. Note that this dma-buf mmap patch does _not_ support every possible insanity an existing subsystem could pull of with mmap: Because it does not allow to intercept pagefaults and shoot down ptes importing subsystems can't add some magic of their own at these points (e.g. to automatically synchronize with outstanding rendering or set up some special resources). I've done a cursory read through a few mmap implementions of various subsytems and I'm hopeful that we can avoid this (and the complexity it'd bring with it). Additonally I've extended the documentation a bit to explain the hows and whys of this mmap extension. In case we ever want to add support for explicitly cache maneged userspace mmap with a prepare/finish ioctl pair, we could specify that userspace needs to mmap a different part of the dma_buf, e.g. the range starting at dma_buf-size up to dma_buf-size*2. This works because the size of a dma_buf is invariant over it's lifetime. The exporter would obviously need to fall back to coherent mappings for both ranges if a legacy clients maps the coherent range and the architecture cannot suppor conflicting caching policies. Also, this would obviously be optional and userspace needs to be able to fall back to coherent mappings. v2: - Spelling fixes from Rob Clark. - Compile fix for !DMA_BUF from Rob Clark. - Extend commit message to explain how explicitly cache managed mmap support could be added later. - Extend the documentation with implementations notes for exporters that need to manually fake coherency. v3: - dma_buf pointer initialization goof-up noticed by Rebecca Schultz Zavin. Cc: Rob Clark rob.cl...@linaro.org Cc: Rebecca Schultz Zavin rebe...@android.com Signed-Off-by: Daniel Vetter daniel.vet...@ffwll.ch Acked-by: Rob Clark rob.cl...@linaro.org --- Documentation/dma-buf-sharing.txt | 98 ++--- drivers/base/dma-buf.c | 64 +++- include/linux/dma-buf.h | 16 ++ 3 files changed, 170 insertions(+), 8 deletions(-) diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt index 3bbd5c5..5ff4d2b 100644 --- a/Documentation/dma-buf-sharing.txt +++ b/Documentation/dma-buf-sharing.txt @@ -29,13 +29,6 @@ The buffer-user in memory, mapped into its own address space, so it can access the same area of memory. -*IMPORTANT*: [see https://lkml.org/lkml/2011/12/20/211 for more details] -For this first version, A buffer shared using the dma_buf sharing API: -- *may* be exported to user space using mmap *ONLY* by exporter, outside of - this framework. -- with this new iteration of the dma-buf api cpu access from the kernel has been - enable, see below for the details. - dma-buf operations for
[PATCH] dma-buf: mmap support
Compared to Rob Clark's RFC I've ditched the prepare/finish hooks and corresponding ioctls on the dma_buf file. The major reason for that is that many people seem to be under the impression that this is also for synchronization with outstanding asynchronous processsing. I'm pretty massively opposed to this because: - It boils down reinventing a new rather general-purpose userspace synchronization interface. If we look at things like futexes, this is hard to get right. - Furthermore a lot of kernel code has to interact with this synchronization primitive. This smells a look like the dri1 hw_lock, a horror show I prefer not to reinvent. - Even more fun is that multiple different subsystems would interact here, so we have plenty of opportunities to create funny deadlock scenarios. I think synchronization is a wholesale different problem from data sharing and should be tackled as an orthogonal problem. Now we could demand that prepare/finish may only ensure cache coherency (as Rob intended), but that runs up into the next problem: We not only need mmap support to facilitate sw-only processing nodes in a pipeline (without jumping through hoops by importing the dma_buf into some sw-access only importer), which allows for a nicer ION-dma-buf upgrade path for existing Android userspace. We also need mmap support for existing importing subsystems to support existing userspace libraries. And a loot of these subsystems are expected to export coherent userspace mappings. So prepare/finish can only ever be optional and the exporter /needs/ to support coherent mappings. Given that mmap access is always somewhat fallback-y in nature I've decided to drop this optimization, instead of just making it optional. If we demonstrate a clear need for this, supported by benchmark results, we can always add it in again later as an optional extension. Other differences compared to Rob's RFC is the above mentioned support for mapping a dma-buf through facilities provided by the importer. Which results in mmap support no longer being optional. Note that this dma-buf mmap patch does _not_ support every possible insanity an existing subsystem could pull of with mmap: Because it does not allow to intercept pagefaults and shoot down ptes importing subsystems can't add some magic of their own at these points (e.g. to automatically synchronize with outstanding rendering or set up some special resources). I've done a cursory read through a few mmap implementions of various subsytems and I'm hopeful that we can avoid this (and the complexity it'd bring with it). Additonally I've extended the documentation a bit to explain the hows and whys of this mmap extension. In case we ever want to add support for explicitly cache maneged userspace mmap with a prepare/finish ioctl pair, we could specify that userspace needs to mmap a different part of the dma_buf, e.g. the range starting at dma_buf-size up to dma_buf-size*2. This works because the size of a dma_buf is invariant over it's lifetime. The exporter would obviously need to fall back to coherent mappings for both ranges if a legacy clients maps the coherent range and the architecture cannot suppor conflicting caching policies. Also, this would obviously be optional and userspace needs to be able to fall back to coherent mappings. v2: - Spelling fixes from Rob Clark. - Compile fix for !DMA_BUF from Rob Clark. - Extend commit message to explain how explicitly cache managed mmap support could be added later. - Extend the documentation with implementations notes for exporters that need to manually fake coherency. v3: - dma_buf pointer initialization goof-up noticed by Rebecca Schultz Zavin. Cc: Rob Clark rob.cl...@linaro.org Cc: Rebecca Schultz Zavin rebe...@android.com Signed-Off-by: Daniel Vetter daniel.vet...@ffwll.ch --- Documentation/dma-buf-sharing.txt | 98 ++--- drivers/base/dma-buf.c| 64 +++- include/linux/dma-buf.h | 16 ++ 3 files changed, 170 insertions(+), 8 deletions(-) diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt index 3bbd5c5..5ff4d2b 100644 --- a/Documentation/dma-buf-sharing.txt +++ b/Documentation/dma-buf-sharing.txt @@ -29,13 +29,6 @@ The buffer-user in memory, mapped into its own address space, so it can access the same area of memory. -*IMPORTANT*: [see https://lkml.org/lkml/2011/12/20/211 for more details] -For this first version, A buffer shared using the dma_buf sharing API: -- *may* be exported to user space using mmap *ONLY* by exporter, outside of - this framework. -- with this new iteration of the dma-buf api cpu access from the kernel has been - enable, see below for the details. - dma-buf operations for device dma only -- @@ -313,6 +306,83 @@ Access to a dma_buf from the kernel context involves three steps: enum
Re: [PATCH] dma-buf: mmap support
Hi,
+static int dma_buf_mmap_internal(struct file *file, struct vm_area_struct
*vma)
+{
+ struct dma_buf *dmabuf;
+
+ if (!is_dma_buf_file(file))
+ return -EINVAL;
+
+ dmabuf = file-private_data;
+
+ /* check for overflowing the buffer's size */
+ if (vma-vm_pgoff + ((vma-vm_end - vma-vm_start) PAGE_SHIFT)
+ dmabuf-size PAGE_SHIFT)
is this condition right? your intention is for checking buffer's size
is valid or not. by the way why is vma-vm_pgoff added to vm region
size?
+ return -EINVAL;
+
+ return dmabuf-ops-mmap(dmabuf, vma);
+}
+
static const struct file_operations dma_buf_fops = {
.release = dma_buf_release,
+ .mmap = dma_buf_mmap_internal,
};
/*
@@ -82,7 +100,8 @@ struct dma_buf *dma_buf_export(void *priv, const struct
dma_buf_ops *ops,
|| !ops-unmap_dma_buf
|| !ops-release
|| !ops-kmap_atomic
- || !ops-kmap)) {
+ || !ops-kmap
+ || !ops-mmap)) {
return ERR_PTR(-EINVAL);
}
@@ -406,3 +425,46 @@ void dma_buf_kunmap(struct dma_buf *dmabuf, unsigned
long page_num,
dmabuf-ops-kunmap(dmabuf, page_num, vaddr);
}
EXPORT_SYMBOL_GPL(dma_buf_kunmap);
+
+
+/**
+ * dma_buf_mmap - Setup up a userspace mmap with the given vma
+ * @dma_buf: [in] buffer that should back the vma
+ * @vma: [in] vma for the mmap
+ * @pgoff: [in] offset in pages where this mmap should start within
the
+ * dma-buf buffer.
+ *
+ * This function adjusts the passed in vma so that it points at the file of
the
+ * dma_buf operation. It alsog adjusts the starting pgoff and does bounds
+ * checking on the size of the vma. Then it calls the exporters mmap
function to
+ * set up the mapping.
+ *
+ * Can return negative error values, returns 0 on success.
+ */
+int dma_buf_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma,
+ unsigned long pgoff)
+{
+ if (WARN_ON(!dmabuf || !vma))
+ return -EINVAL;
+
+ /* check for offset overflow */
+ if (pgoff + ((vma-vm_end - vma-vm_start) PAGE_SHIFT) pgoff)
ditto. isn't it checked whether page offset to be mmaped is placed
within vm region or not with the condition, if ((vma-vm_end -
vma-vm_start) PAGE_SHIFT) pgoff)?
+ return -EOVERFLOW;
+
+ /* check for overflowing the buffer's size */
+ if (pgoff + ((vma-vm_end - vma-vm_start) PAGE_SHIFT)
+ dmabuf-size PAGE_SHIFT)
+ return -EINVAL;
+
+ /* readjust the vma */
+ if (vma-vm_file)
+ fput(vma-vm_file);
+
+ vma-vm_file = dmabuf-file;
+ get_file(vma-vm_file);
+
+ vma-vm_pgoff = pgoff;
+
+ return dmabuf-ops-mmap(dmabuf, vma);
+}
+EXPORT_SYMBOL_GPL(dma_buf_mmap);
diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
index 3efbfc2..1f78d15 100644
--- a/include/linux/dma-buf.h
+++ b/include/linux/dma-buf.h
@@ -61,6 +61,10 @@ struct dma_buf_attachment;
* This Callback must not sleep.
* @kmap: maps a page from the buffer into kernel address space.
* @kunmap: [optional] unmaps a page from the buffer.
+ * @mmap: used to expose the backing storage to userspace. Note that the
+ * mapping needs to be coherent - if the exporter doesn't directly
+ * support this, it needs to fake coherency by shooting down any ptes
+ * when transitioning away from the cpu domain.
*/
struct dma_buf_ops {
int (*attach)(struct dma_buf *, struct device *,
@@ -92,6 +96,8 @@ struct dma_buf_ops {
void (*kunmap_atomic)(struct dma_buf *, unsigned long, void *);
void *(*kmap)(struct dma_buf *, unsigned long);
void (*kunmap)(struct dma_buf *, unsigned long, void *);
+
+ int (*mmap)(struct dma_buf *, struct vm_area_struct *vma);
};
/**
@@ -167,6 +173,9 @@ void *dma_buf_kmap_atomic(struct dma_buf *, unsigned
long);
void dma_buf_kunmap_atomic(struct dma_buf *, unsigned long, void *);
void *dma_buf_kmap(struct dma_buf *, unsigned long);
void dma_buf_kunmap(struct dma_buf *, unsigned long, void *);
+
+int dma_buf_mmap(struct dma_buf *, struct vm_area_struct *,
+ unsigned long);
#else
static inline struct dma_buf_attachment *dma_buf_attach(struct dma_buf
*dmabuf,
@@ -248,6 +257,13 @@ static inline void dma_buf_kunmap(struct dma_buf *dmabuf,
unsigned long pnum, void *vaddr)
{
}
+
+static inline int dma_buf_mmap(struct dma_buf *dmabuf,
+ struct vm_area_struct *vma,
+ unsigned long pgoff)
+{
+ return -ENODEV;
+}
#endif /* CONFIG_DMA_SHARED_BUFFER */
#endif /* __DMA_BUF_H__ */
--
Re: [PATCH] dma-buf: mmap support
On Wed, Apr 25, 2012 at 01:37:51AM +0900, InKi Dae wrote:
Hi,
+static int dma_buf_mmap_internal(struct file *file, struct vm_area_struct
*vma)
+{
+ struct dma_buf *dmabuf;
+
+ if (!is_dma_buf_file(file))
+ return -EINVAL;
+
+ dmabuf = file-private_data;
+
+ /* check for overflowing the buffer's size */
+ if (vma-vm_pgoff + ((vma-vm_end - vma-vm_start) PAGE_SHIFT)
+ dmabuf-size PAGE_SHIFT)
is this condition right? your intention is for checking buffer's size
is valid or not. by the way why is vma-vm_pgoff added to vm region
size?
This check here is to ensure that userspace cannot mmap beyong the end of
the dma_buf object. vm_pgoff is the offset userspace passed in at mmap
time and hence needs to be added. Note that vm_end and vm_start are in
bytes, wheres vm_pgoff is in pages.
+ return -EINVAL;
+
+ return dmabuf-ops-mmap(dmabuf, vma);
+}
+
static const struct file_operations dma_buf_fops = {
.release = dma_buf_release,
+ .mmap = dma_buf_mmap_internal,
};
/*
@@ -82,7 +100,8 @@ struct dma_buf *dma_buf_export(void *priv, const struct
dma_buf_ops *ops,
|| !ops-unmap_dma_buf
|| !ops-release
|| !ops-kmap_atomic
- || !ops-kmap)) {
+ || !ops-kmap
+ || !ops-mmap)) {
return ERR_PTR(-EINVAL);
}
@@ -406,3 +425,46 @@ void dma_buf_kunmap(struct dma_buf *dmabuf, unsigned
long page_num,
dmabuf-ops-kunmap(dmabuf, page_num, vaddr);
}
EXPORT_SYMBOL_GPL(dma_buf_kunmap);
+
+
+/**
+ * dma_buf_mmap - Setup up a userspace mmap with the given vma
+ * @dma_buf: [in] buffer that should back the vma
+ * @vma: [in] vma for the mmap
+ * @pgoff: [in] offset in pages where this mmap should start within
the
+ * dma-buf buffer.
+ *
+ * This function adjusts the passed in vma so that it points at the file
of the
+ * dma_buf operation. It alsog adjusts the starting pgoff and does bounds
+ * checking on the size of the vma. Then it calls the exporters mmap
function to
+ * set up the mapping.
+ *
+ * Can return negative error values, returns 0 on success.
+ */
+int dma_buf_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma,
+ unsigned long pgoff)
+{
+ if (WARN_ON(!dmabuf || !vma))
+ return -EINVAL;
+
+ /* check for offset overflow */
+ if (pgoff + ((vma-vm_end - vma-vm_start) PAGE_SHIFT) pgoff)
ditto. isn't it checked whether page offset to be mmaped is placed
within vm region or not with the condition, if ((vma-vm_end -
vma-vm_start) PAGE_SHIFT) pgoff)?
Nope, this check only checks for overflow. The pgoff is the offset within
the dma_buf object. E.g. a drm driver splits up it mmap space into pieces,
which map to individual buffers. If userspace just mmaps parts of such a
buffer, the importer can pass the offset in pgoff. But I expect this to be
0 for almost all cases.
Note that we don't need this overflow check in the internal mmap function
because do_mmap will do it for us. But here the importer potentially sets
a completely different pgoff, so we need to do it. dma_buf documentation
also mentions this (and that importers do not have to do these checks).
Yours, Daniel
+ return -EOVERFLOW;
+
+ /* check for overflowing the buffer's size */
+ if (pgoff + ((vma-vm_end - vma-vm_start) PAGE_SHIFT)
+ dmabuf-size PAGE_SHIFT)
+ return -EINVAL;
+
+ /* readjust the vma */
+ if (vma-vm_file)
+ fput(vma-vm_file);
+
+ vma-vm_file = dmabuf-file;
+ get_file(vma-vm_file);
+
+ vma-vm_pgoff = pgoff;
+
+ return dmabuf-ops-mmap(dmabuf, vma);
+}
+EXPORT_SYMBOL_GPL(dma_buf_mmap);
diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
index 3efbfc2..1f78d15 100644
--- a/include/linux/dma-buf.h
+++ b/include/linux/dma-buf.h
@@ -61,6 +61,10 @@ struct dma_buf_attachment;
* This Callback must not sleep.
* @kmap: maps a page from the buffer into kernel address space.
* @kunmap: [optional] unmaps a page from the buffer.
+ * @mmap: used to expose the backing storage to userspace. Note that the
+ * mapping needs to be coherent - if the exporter doesn't directly
+ * support this, it needs to fake coherency by shooting down any ptes
+ * when transitioning away from the cpu domain.
*/
struct dma_buf_ops {
int (*attach)(struct dma_buf *, struct device *,
@@ -92,6 +96,8 @@ struct dma_buf_ops {
void (*kunmap_atomic)(struct dma_buf *, unsigned long, void *);
void *(*kmap)(struct dma_buf *,
Re: [PATCH] dma-buf: mmap support
2012/4/25, Daniel Vetter dan...@ffwll.ch:
On Wed, Apr 25, 2012 at 01:37:51AM +0900, InKi Dae wrote:
Hi,
+static int dma_buf_mmap_internal(struct file *file, struct
vm_area_struct *vma)
+{
+ struct dma_buf *dmabuf;
+
+ if (!is_dma_buf_file(file))
+ return -EINVAL;
+
+ dmabuf = file-private_data;
+
+ /* check for overflowing the buffer's size */
+ if (vma-vm_pgoff + ((vma-vm_end - vma-vm_start)
PAGE_SHIFT)
+ dmabuf-size PAGE_SHIFT)
is this condition right? your intention is for checking buffer's size
is valid or not. by the way why is vma-vm_pgoff added to vm region
size?
This check here is to ensure that userspace cannot mmap beyong the end of
the dma_buf object. vm_pgoff is the offset userspace passed in at mmap
time and hence needs to be added. Note that vm_end and vm_start are in
bytes, wheres vm_pgoff is in pages.
You're right, vma area region would be decided by user-desired size
that this is passed by mmap syscall so user should set size and
vm_pgoff appropriately. it's my missing point. well if any part of
dmabuf buffer region had already been mmaped and after that user
requested mmap for another region of the dmabuf buffer region again
then isn't there any problem? I mean that dmabuf-size would always
have same value since any memory region allocated by any allocators
such as gem, ump and so on have been exported to dmabuf. so at second
mmap request, dmabuf-size wouldn't have reasonable value because with
first mmap request, any part of the dmabuf buffer region had already
beem mmaped. for example, dmabuf size is 1MB and 512Kb region of the
dmabuf was mmaped by first mmap request and then with second mmap
request, your code would check whether user-desired size is valid or
not with dmabuf-size but dmabuf-size would still have 1MB it means
at second mmap request, any size between 512KB ~ 1MB would be ok. it's
just my concern and there could be my missing point.
Thanks,
Inki Dae.
+ return -EINVAL;
+
+ return dmabuf-ops-mmap(dmabuf, vma);
+}
+
static const struct file_operations dma_buf_fops = {
.release= dma_buf_release,
+ .mmap = dma_buf_mmap_internal,
};
/*
@@ -82,7 +100,8 @@ struct dma_buf *dma_buf_export(void *priv, const
struct dma_buf_ops *ops,
|| !ops-unmap_dma_buf
|| !ops-release
|| !ops-kmap_atomic
- || !ops-kmap)) {
+ || !ops-kmap
+ || !ops-mmap)) {
return ERR_PTR(-EINVAL);
}
@@ -406,3 +425,46 @@ void dma_buf_kunmap(struct dma_buf *dmabuf,
unsigned long page_num,
dmabuf-ops-kunmap(dmabuf, page_num, vaddr);
}
EXPORT_SYMBOL_GPL(dma_buf_kunmap);
+
+
+/**
+ * dma_buf_mmap - Setup up a userspace mmap with the given vma
+ * @dma_buf: [in]buffer that should back the vma
+ * @vma: [in]vma for the mmap
+ * @pgoff: [in]offset in pages where this mmap should start
within the
+ * dma-buf buffer.
+ *
+ * This function adjusts the passed in vma so that it points at the
file of the
+ * dma_buf operation. It alsog adjusts the starting pgoff and does
bounds
+ * checking on the size of the vma. Then it calls the exporters mmap
function to
+ * set up the mapping.
+ *
+ * Can return negative error values, returns 0 on success.
+ */
+int dma_buf_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma,
+unsigned long pgoff)
+{
+ if (WARN_ON(!dmabuf || !vma))
+ return -EINVAL;
+
+ /* check for offset overflow */
+ if (pgoff + ((vma-vm_end - vma-vm_start) PAGE_SHIFT)
pgoff)
ditto. isn't it checked whether page offset to be mmaped is placed
within vm region or not with the condition, if ((vma-vm_end -
vma-vm_start) PAGE_SHIFT) pgoff)?
Nope, this check only checks for overflow. The pgoff is the offset within
the dma_buf object. E.g. a drm driver splits up it mmap space into pieces,
which map to individual buffers. If userspace just mmaps parts of such a
buffer, the importer can pass the offset in pgoff. But I expect this to be
0 for almost all cases.
Note that we don't need this overflow check in the internal mmap function
because do_mmap will do it for us. But here the importer potentially sets
a completely different pgoff, so we need to do it. dma_buf documentation
also mentions this (and that importers do not have to do these checks).
Yours, Daniel
+ return -EOVERFLOW;
+
+ /* check for overflowing the buffer's size */
+ if (pgoff + ((vma-vm_end - vma-vm_start) PAGE_SHIFT)
+ dmabuf-size PAGE_SHIFT)
+ return -EINVAL;
+
+ /* readjust the vma */
+ if (vma-vm_file)
+
Re: [PATCH] dma-buf: mmap support
I'd still rather see some form of explicit cache flusing api, but I can do that through my exporter (and probably will to get started). Otherwise this looks good to me except for comment inline. On Wed, Apr 18, 2012 at 6:52 AM, Daniel Vetter daniel.vet...@ffwll.ch wrote: Compared to Rob Clark's RFC I've ditched the prepare/finish hooks and corresponding ioctls on the dma_buf file. The major reason for that is that many people seem to be under the impression that this is also for synchronization with outstanding asynchronous processsing. I'm pretty massively opposed to this because: - It boils down reinventing a new rather general-purpose userspace synchronization interface. If we look at things like futexes, this is hard to get right. - Furthermore a lot of kernel code has to interact with this synchronization primitive. This smells a look like the dri1 hw_lock, a horror show I prefer not to reinvent. - Even more fun is that multiple different subsystems would interact here, so we have plenty of opportunities to create funny deadlock scenarios. I think synchronization is a wholesale different problem from data sharing and should be tackled as an orthogonal problem. Now we could demand that prepare/finish may only ensure cache coherency (as Rob intended), but that runs up into the next problem: We not only need mmap support to facilitate sw-only processing nodes in a pipeline (without jumping through hoops by importing the dma_buf into some sw-access only importer), which allows for a nicer ION-dma-buf upgrade path for existing Android userspace. We also need mmap support for existing importing subsystems to support existing userspace libraries. And a loot of these subsystems are expected to export coherent userspace mappings. So prepare/finish can only ever be optional and the exporter /needs/ to support coherent mappings. Given that mmap access is always somewhat fallback-y in nature I've decided to drop this optimization, instead of just making it optional. If we demonstrate a clear need for this, supported by benchmark results, we can always add it in again later as an optional extension. Other differences compared to Rob's RFC is the above mentioned support for mapping a dma-buf through facilities provided by the importer. Which results in mmap support no longer being optional. Note that this dma-buf mmap patch does _not_ support every possible insanity an existing subsystem could pull of with mmap: Because it does not allow to intercept pagefaults and shoot down ptes importing subsystems can't add some magic of their own at these points (e.g. to automatically synchronize with outstanding rendering or set up some special resources). I've done a cursory read through a few mmap implementions of various subsytems and I'm hopeful that we can avoid this (and the complexity it'd bring with it). Additonally I've extended the documentation a bit to explain the hows and whys of this mmap extension. In case we ever want to add support for explicitly cache maneged userspace mmap with a prepare/finish ioctl pair, we could specify that userspace needs to mmap a different part of the dma_buf, e.g. the range starting at dma_buf-size up to dma_buf-size*2. This works because the size of a dma_buf is invariant over it's lifetime. The exporter would obviously need to fall back to coherent mappings for both ranges if a legacy clients maps the coherent range and the architecture cannot suppor conflicting caching policies. Also, this would obviously be optional and userspace needs to be able to fall back to coherent mappings. v2: - Spelling fixes from Rob Clark. - Compile fix for !DMA_BUF from Rob Clark. - Extend commit message to explain how explicitly cache managed mmap support could be added later. - Extend the documentation with implementations notes for exporters that need to manually fake coherency. Cc: Rob Clark rob.cl...@linaro.org Cc: Rebecca Schultz Zavin rebe...@android.com Signed-Off-by: Daniel Vetter daniel.vet...@ffwll.ch --- Documentation/dma-buf-sharing.txt | 98 ++--- drivers/base/dma-buf.c | 64 +++- include/linux/dma-buf.h | 16 ++ 3 files changed, 170 insertions(+), 8 deletions(-) diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt index 3bbd5c5..5ff4d2b 100644 --- a/Documentation/dma-buf-sharing.txt +++ b/Documentation/dma-buf-sharing.txt @@ -29,13 +29,6 @@ The buffer-user in memory, mapped into its own address space, so it can access the same area of memory. -*IMPORTANT*: [see https://lkml.org/lkml/2011/12/20/211 for more details] -For this first version, A buffer shared using the dma_buf sharing API: -- *may* be exported to user space using mmap *ONLY* by exporter, outside of - this framework. -- with this new iteration of the dma-buf api cpu access from the kernel has
[PATCH] dma-buf: mmap support
Compared to Rob Clark's RFC I've ditched the prepare/finish hooks and corresponding ioctls on the dma_buf file. The major reason for that is that many people seem to be under the impression that this is also for synchronization with outstanding asynchronous processsing. I'm pretty massively opposed to this because: - It boils down reinventing a new rather general-purpose userspace synchronization interface. If we look at things like futexes, this is hard to get right. - Furthermore a lot of kernel code has to interact with this synchronization primitive. This smells a look like the dri1 hw_lock, a horror show I prefer not to reinvent. - Even more fun is that multiple different subsystems would interact here, so we have plenty of opportunities to create funny deadlock scenarios. I think synchronization is a wholesale different problem from data sharing and should be tackled as an orthogonal problem. Now we could demand that prepare/finish may only ensure cache coherency (as Rob intended), but that runs up into the next problem: We not only need mmap support to facilitate sw-only processing nodes in a pipeline (without jumping through hoops by importing the dma_buf into some sw-access only importer), which allows for a nicer ION-dma-buf upgrade path for existing Android userspace. We also need mmap support for existing importing subsystems to support existing userspace libraries. And a loot of these subsystems are expected to export coherent userspace mappings. So prepare/finish can only ever be optional and the exporter /needs/ to support coherent mappings. Given that mmap access is always somewhat fallback-y in nature I've decided to drop this optimization, instead of just making it optional. If we demonstrate a clear need for this, supported by benchmark results, we can always add it in again later as an optional extension. Other differences compared to Rob's RFC is the above mentioned support for mapping a dma-buf through facilities provided by the importer. Which results in mmap support no longer being optional. Note that this dma-buf mmap patch does _not_ support every possible insanity an existing subsystem could pull of with mmap: Because it does not allow to intercept pagefaults and shoot down ptes importing subsystems can't add some magic of their own at these points (e.g. to automatically synchronize with outstanding rendering or set up some special resources). I've done a cursory read through a few mmap implementions of various subsytems and I'm hopeful that we can avoid this (and the complexity it'd bring with it). Additonally I've extended the documentation a bit to explain the hows and whys of this mmap extension. In case we ever want to add support for explicitly cache maneged userspace mmap with a prepare/finish ioctl pair, we could specify that userspace needs to mmap a different part of the dma_buf, e.g. the range starting at dma_buf-size up to dma_buf-size*2. This works because the size of a dma_buf is invariant over it's lifetime. The exporter would obviously need to fall back to coherent mappings for both ranges if a legacy clients maps the coherent range and the architecture cannot suppor conflicting caching policies. Also, this would obviously be optional and userspace needs to be able to fall back to coherent mappings. v2: - Spelling fixes from Rob Clark. - Compile fix for !DMA_BUF from Rob Clark. - Extend commit message to explain how explicitly cache managed mmap support could be added later. - Extend the documentation with implementations notes for exporters that need to manually fake coherency. Cc: Rob Clark rob.cl...@linaro.org Cc: Rebecca Schultz Zavin rebe...@android.com Signed-Off-by: Daniel Vetter daniel.vet...@ffwll.ch --- Documentation/dma-buf-sharing.txt | 98 ++--- drivers/base/dma-buf.c| 64 +++- include/linux/dma-buf.h | 16 ++ 3 files changed, 170 insertions(+), 8 deletions(-) diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt index 3bbd5c5..5ff4d2b 100644 --- a/Documentation/dma-buf-sharing.txt +++ b/Documentation/dma-buf-sharing.txt @@ -29,13 +29,6 @@ The buffer-user in memory, mapped into its own address space, so it can access the same area of memory. -*IMPORTANT*: [see https://lkml.org/lkml/2011/12/20/211 for more details] -For this first version, A buffer shared using the dma_buf sharing API: -- *may* be exported to user space using mmap *ONLY* by exporter, outside of - this framework. -- with this new iteration of the dma-buf api cpu access from the kernel has been - enable, see below for the details. - dma-buf operations for device dma only -- @@ -313,6 +306,83 @@ Access to a dma_buf from the kernel context involves three steps: enum dma_data_direction dir); +Direct Userspace Access/mmap Support
Re: [PATCH] dma-buf: mmap support
On Wednesday 18 April 2012, Daniel Vetter wrote: + Because existing importing subsystems might presume coherent mappings for + userspace, the exporter needs to set up a coherent mapping. If that's not + possible, it needs to fake coherency by manually shooting down ptes when + leaving the cpu domain and flushing caches at fault time. Note that all the + dma_buf files share the same anon inode, hence the exporter needs to replace + the dma_buf file stored in vma-vm_file with it's own if pte shootdown is + requred. This is because the kernel uses the underlying inode's address_space + for vma tracking (and hence pte tracking at shootdown time with + unmap_mapping_range). + + If the above shootdown dance turns out to be too expensive in certain + scenarios, we can extend dma-buf with a more explicit cache tracking scheme + for userspace mappings. But the current assumption is that using mmap is + always a slower path, so some inefficiencies should be acceptable. + + Exporters that shoot down mappings (for any reasons) shall not do any + synchronization at fault time with outstanding device operations. + Synchronization is an orthogonal issue to sharing the backing storage of a + buffer and hence should not be handled by dma-buf itself. This is explictly + mentioned here because many people seem to want something like this, but if + different exporters handle this differently, buffer sharing can fail in + interesting ways depending upong the exporter (if userspace starts depending + upon this implicit synchronization). How do you ensure that no device can do DMA on the buffer while it's mapped into user space in a noncoherent manner? Arnd -- To unsubscribe from this list: send the line unsubscribe linux-media in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [PATCH] dma-buf: mmap support
On Wed, Apr 18, 2012 at 9:06 AM, Arnd Bergmann a...@arndb.de wrote: On Wednesday 18 April 2012, Daniel Vetter wrote: + Because existing importing subsystems might presume coherent mappings for + userspace, the exporter needs to set up a coherent mapping. If that's not + possible, it needs to fake coherency by manually shooting down ptes when + leaving the cpu domain and flushing caches at fault time. Note that all the + dma_buf files share the same anon inode, hence the exporter needs to replace + the dma_buf file stored in vma-vm_file with it's own if pte shootdown is + requred. This is because the kernel uses the underlying inode's address_space + for vma tracking (and hence pte tracking at shootdown time with + unmap_mapping_range). + + If the above shootdown dance turns out to be too expensive in certain + scenarios, we can extend dma-buf with a more explicit cache tracking scheme + for userspace mappings. But the current assumption is that using mmap is + always a slower path, so some inefficiencies should be acceptable. + + Exporters that shoot down mappings (for any reasons) shall not do any + synchronization at fault time with outstanding device operations. + Synchronization is an orthogonal issue to sharing the backing storage of a + buffer and hence should not be handled by dma-buf itself. This is explictly + mentioned here because many people seem to want something like this, but if + different exporters handle this differently, buffer sharing can fail in + interesting ways depending upong the exporter (if userspace starts depending + upon this implicit synchronization). How do you ensure that no device can do DMA on the buffer while it's mapped into user space in a noncoherent manner? you do unmap_mapping_range() before DMA.. if you have userspace accessing buffer simultaneously with DMA then the results are undefined, as they always have been (even w/ uncached mappings) BR, -R Arnd -- To unsubscribe from this list: send the line unsubscribe linux-media in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line unsubscribe linux-media in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [PATCH] dma-buf: mmap support
On Wed, Apr 18, 2012 at 02:06:13PM +, Arnd Bergmann wrote: On Wednesday 18 April 2012, Daniel Vetter wrote: + Because existing importing subsystems might presume coherent mappings for + userspace, the exporter needs to set up a coherent mapping. If that's not + possible, it needs to fake coherency by manually shooting down ptes when + leaving the cpu domain and flushing caches at fault time. Note that all the + dma_buf files share the same anon inode, hence the exporter needs to replace + the dma_buf file stored in vma-vm_file with it's own if pte shootdown is + requred. This is because the kernel uses the underlying inode's address_space + for vma tracking (and hence pte tracking at shootdown time with + unmap_mapping_range). + + If the above shootdown dance turns out to be too expensive in certain + scenarios, we can extend dma-buf with a more explicit cache tracking scheme + for userspace mappings. But the current assumption is that using mmap is + always a slower path, so some inefficiencies should be acceptable. + + Exporters that shoot down mappings (for any reasons) shall not do any + synchronization at fault time with outstanding device operations. + Synchronization is an orthogonal issue to sharing the backing storage of a + buffer and hence should not be handled by dma-buf itself. This is explictly + mentioned here because many people seem to want something like this, but if + different exporters handle this differently, buffer sharing can fail in + interesting ways depending upong the exporter (if userspace starts depending + upon this implicit synchronization). How do you ensure that no device can do DMA on the buffer while it's mapped into user space in a noncoherent manner? We don't. Letting userspace shoot into it's foot is part of the interface. drm drivers and afaik also v4l has some explicit interfaces where userspace and the kernel can communicate who is allowed to stomp on a buffer (and sync up access with various degrees of sophistication). Userspace still needs to use this interfaces to ensure that any dma activity it wants to have completed is completed. To ensure coherency for userspace that does not try to get unnecessary holes in its feet, the exporter needs to zap all ptes pointing to a dma buf object and flush relevant parts of the cpu cache before device dma starts (signalled with dma_buf_map_sg from the importer atm, but we'll eventually grow streaming support I guess). On page fault time it has then to invalidate any cpu caches, so that userspace does not read stale data. Like I've said in the above blurb in the documentation, I think synchronization is an orthogonal issue to sharing memory. -Daniel -- Daniel Vetter Mail: dan...@ffwll.ch Mobile: +41 (0)79 365 57 48 -- To unsubscribe from this list: send the line unsubscribe linux-media in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [PATCH] dma-buf: mmap support
How do you ensure that no device can do DMA on the buffer while it's mapped into user space in a noncoherent manner? Why do we want to enforce that ? We provide the appropriate base service but you need to know what you are doing. In reality a lot of use cases are going to need far more than a simple kernel API could try and guess coherency rules about. -- To unsubscribe from this list: send the line unsubscribe linux-media in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
