Re: [PATCH 1/9] dma-buf: start caching of sg_table objects

[Christian König]

Am 08.05.19 um 11:15 schrieb Daniel Vetter:
On Tue, May 07, 2019 at 10:13:30AM +0200, Christian König wrote:
To allow a smooth transition from pinning buffer objects to dynamic
invalidation we first start to cache the sg_table for an attachment.
Signed-off-by: Christian König <christian.koe...@amd.com>
---
  drivers/dma-buf/dma-buf.c | 24 ++++++++++++++++++++++++
  include/linux/dma-buf.h   | 14 ++++++++++++++
  2 files changed, 38 insertions(+)

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index 7c858020d14b..775e13f54083 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -573,6 +573,20 @@ struct dma_buf_attachment *dma_buf_attach(struct dma_buf 
*dmabuf,
        list_add(&attach->node, &dmabuf->attachments);
  
  	mutex_unlock(&dmabuf->lock);
+
+       if (!dma_buf_is_dynamic(dmabuf)) {
+               struct sg_table *sgt;
+
+               sgt = dmabuf->ops->map_dma_buf(attach, DMA_BIDIRECTIONAL);
+               if (!sgt)
+                       sgt = ERR_PTR(-ENOMEM);
+               if (IS_ERR(sgt)) {
+                       dma_buf_detach(dmabuf, attach);
+                       return ERR_CAST(sgt);
+               }
+               attach->sgt = sgt;
+       }
+
        return attach;
  
  err_attach:
@@ -595,6 +609,10 @@ void dma_buf_detach(struct dma_buf *dmabuf, struct 
dma_buf_attachment *attach)
        if (WARN_ON(!dmabuf || !attach))
                return;
  
+	if (attach->sgt)
+               dmabuf->ops->unmap_dma_buf(attach, attach->sgt,
+                                          DMA_BIDIRECTIONAL);
+
        mutex_lock(&dmabuf->lock);
        list_del(&attach->node);
        if (dmabuf->ops->detach)
@@ -630,6 +648,9 @@ struct sg_table *dma_buf_map_attachment(struct 
dma_buf_attachment *attach,
        if (WARN_ON(!attach || !attach->dmabuf))
                return ERR_PTR(-EINVAL);
  
+	if (attach->sgt)
+               return attach->sgt;
+
        sg_table = attach->dmabuf->ops->map_dma_buf(attach, direction);
        if (!sg_table)
                sg_table = ERR_PTR(-ENOMEM);
@@ -657,6 +678,9 @@ void dma_buf_unmap_attachment(struct dma_buf_attachment 
*attach,
        if (WARN_ON(!attach || !attach->dmabuf || !sg_table))
                return;
  
+	if (attach->sgt == sg_table)
+               return;
+
        attach->dmabuf->ops->unmap_dma_buf(attach, sg_table,
                                                direction);
  }
diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
index 58725f890b5b..52031fdc75bb 100644
--- a/include/linux/dma-buf.h
+++ b/include/linux/dma-buf.h
@@ -322,6 +322,7 @@ struct dma_buf_attachment {
        struct dma_buf *dmabuf;
        struct device *dev;
        struct list_head node;
+       struct sg_table *sgt;
        void *priv;
  };
  
@@ -373,6 +374,19 @@ static inline void get_dma_buf(struct dma_buf *dmabuf)
        get_file(dmabuf->file);
  }
  
+/**
+ * dma_buf_is_dynamic - check if a DMA-buf uses dynamic mappings.
+ * @dmabuf: the DMA-buf to check
+ *
+ * Returns true if a DMA-buf exporter wants to create dynamic sg table mappings
+ * for each attachment. False if only a single static sg table should be used.
+ */
+static inline bool dma_buf_is_dynamic(struct dma_buf *dmabuf)
+{
+       /* Always use a static mapping for now */
+       return false;
Hm I still expect that later on we'll want this to be decided by the
attachment: It's only dynamic if both the exporter and the importer
support dynamic dma-buf management, otherwise we need to pin.

Yeah, essentially we need to handle the following cases:

1. Static DMA-buf and the exporter doesn't want caching.
2. Static DMA-buf and the exporter does want caching.
3. Dynamic DMA-buf and we use caching as a workaround for smooth transition.
4. Dynamic DMA-buf and caching doesn't make much sense.

So far we only support 3 & 4 and not 1 & 2. Could be that this gets much 
more complicated, but I would only want to add this complicated code if 
we really find that we need it.

But anyway, I feel like we need to go over the entire thing anyway once
more when p2p has landed, on this:

Reviewed-by: Daniel Vetter <daniel.vet...@ffwll.ch>

Thanks for the review, next question how do we merge this? Through drm-misc?

Regards,
Christian.


+}
+
  struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
                                                        struct device *dev);
  void dma_buf_detach(struct dma_buf *dmabuf,
--
2.17.1

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