Re: [PATCH 1/2] drm/prime: Iterate SG DMA addresses separately

[okaya]

On 2018-04-12 06:33, Christian König wrote:

Am 12.04.2018 um 11:49 schrieb Lucas Stach:

Am Donnerstag, den 12.04.2018, 11:35 +0200 schrieb Christian König:

Am 12.04.2018 um 11:11 schrieb Lucas Stach:

Am Mittwoch, den 11.04.2018, 20:26 +0200 schrieb Christian König:

Am 11.04.2018 um 19:11 schrieb Robin Murphy:
For dma_map_sg(), DMA API implementations are free to merge 
consecutive
segments into a single DMA mapping if conditions are suitable, 
thus the

resulting DMA addresses may be packed into fewer entries than
ttm->sg->nents implies.

drm_prime_sg_to_page_addr_arrays() does not account for this, 
meaning
its callers either have to reject the 0 < count < nents case or 
risk
getting bogus addresses back later. Fortunately this is relatively 
easy
to deal with having to rejig structures to also store the mapped 
count,
since the total DMA length should still be equal to the total 
buffer
length. All we need is a separate scatterlist cursor to iterate 
the DMA

addresses separately from the CPU addresses.

Mhm, I think I like Sinas approach better.

See the hardware actually needs the dma_address on a page by page 
basis.


Joining multiple consecutive pages into one entry is just 
additional

overhead which we don't need.
But setting MAX_SEGMENT_SIZE will probably prevent an IOMMU that 
might
be in front of your GPU to map large pages as such, causing 
additional
overhead by means of additional TLB misses and page walks on the 
IOMMU

side.

And wouldn't you like to use huge pages at the GPU side, if the 
IOMMU

already provides you the service of producing one large consecutive
address range, rather than mapping them via a number of small pages?

No, I wouldn't like to use that. We're already using that :)

But we use huge pages by allocating consecutive chunks of memory so 
that

both the CPU as well as the GPU can increase their TLB hit rate.
I'm not convinced that this is a universal win. Many GPU buffers 
aren't

accessed by the CPU and allocating huge pages puts much more strain on
the kernel MM subsystem.


Yeah, we indeed see the extra overhead during allocation.


What currently happens is that the DMA subsystem tries to coalesce
multiple pages into on SG entry and we de-coalesce that inside the
driver again to create our random access array.

I guess the right thing would be to have a flag that tells the the DMA
implementation to not coalesce the entries. But (ab-)using max segment
size tells the DMA API to split the segments if they are larger than
the given size, which is probably not what you want either as you now
need to coalesce the segments again when you are mapping real huge
pages.


No, even with huge pages I need an array with every single dma address
for a 4K pages (or whatever the normal page size is).

The problem is that I need a random access array for the DMA
addresses, even when they are continuously.

I agree that max segment size is a bit ugly here, but at least for
radeon, amdgpu and pretty much TTM in general it is exactly what I
need.

I could fix TTM to not need that, but for radeon and amdgpu it is the
hardware which needs this.


I can implement i915 approach as Robin suggested as an alternative. Is 
that better?





Christian.



That is a huge waste of memory and CPU cycles and I actually wanted 
to
get rid of it for quite some time now. Sinas approach seems to be a 
good

step into the right direction to me to actually clean that up.

Detecting such a condition is much easier if the DMA map 
implementation

gives you the coalesced scatter entries.
A way which preserves both path would be indeed nice to have, but 
that

only allows for the TLB optimization for the GPU and not the CPU any
more. So I actually see that as really minor use case.

Some of the Tegras have much larger TLBs and better page-walk
performance on the system IOMMU compared to the GPU MMU, so they would
probably benefit a good deal even if the hugepage optimization only
targets the GPU side.

Regards,
Lucas

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Re: [PATCH 1/2] drm/prime: Iterate SG DMA addresses separately

[Robin Murphy]

On 12/04/18 11:33, Christian König wrote:

Am 12.04.2018 um 11:49 schrieb Lucas Stach:

Am Donnerstag, den 12.04.2018, 11:35 +0200 schrieb Christian König:

Am 12.04.2018 um 11:11 schrieb Lucas Stach:

Am Mittwoch, den 11.04.2018, 20:26 +0200 schrieb Christian König:

Am 11.04.2018 um 19:11 schrieb Robin Murphy:
For dma_map_sg(), DMA API implementations are free to merge 
consecutive
segments into a single DMA mapping if conditions are suitable, 
thus the

resulting DMA addresses may be packed into fewer entries than
ttm->sg->nents implies.

drm_prime_sg_to_page_addr_arrays() does not account for this, meaning
its callers either have to reject the 0 < count < nents case or risk
getting bogus addresses back later. Fortunately this is relatively 
easy
to deal with having to rejig structures to also store the mapped 
count,

since the total DMA length should still be equal to the total buffer
length. All we need is a separate scatterlist cursor to iterate 
the DMA

addresses separately from the CPU addresses.

Mhm, I think I like Sinas approach better.

See the hardware actually needs the dma_address on a page by page 
basis.


Joining multiple consecutive pages into one entry is just additional
overhead which we don't need.


Note that the merging done inside dma_map_sg() is pretty trivial in 
itself (it's effectively just the inverse of the logic in this patch). 
The "overhead" here is inherent in calling sg_alloc_table_from_pages() 
and dma_map_sg() at all.



But setting MAX_SEGMENT_SIZE will probably prevent an IOMMU that might
be in front of your GPU to map large pages as such, causing additional
overhead by means of additional TLB misses and page walks on the IOMMU
side.

And wouldn't you like to use huge pages at the GPU side, if the IOMMU
already provides you the service of producing one large consecutive
address range, rather than mapping them via a number of small pages?

No, I wouldn't like to use that. We're already using that :)

But we use huge pages by allocating consecutive chunks of memory so that
both the CPU as well as the GPU can increase their TLB hit rate.

I'm not convinced that this is a universal win. Many GPU buffers aren't
accessed by the CPU and allocating huge pages puts much more strain on
the kernel MM subsystem.


Yeah, we indeed see the extra overhead during allocation.


What currently happens is that the DMA subsystem tries to coalesce
multiple pages into on SG entry and we de-coalesce that inside the
driver again to create our random access array.

I guess the right thing would be to have a flag that tells the the DMA
implementation to not coalesce the entries. But (ab-)using max segment
size tells the DMA API to split the segments if they are larger than
the given size, which is probably not what you want either as you now
need to coalesce the segments again when you are mapping real huge
pages.


No, even with huge pages I need an array with every single dma address 
for a 4K pages (or whatever the normal page size is).


The problem is that I need a random access array for the DMA addresses, 
even when they are continuously.


OK, that makes me wonder if you even need dma_map_sg() in this case. 
From the sound of that it would be a lot simpler to just call 
dma_map_page() in a loop over the pair of arrays. AFAICS that's what TTM 
already does in most places.


I agree that max segment size is a bit ugly here, but at least for 
radeon, amdgpu and pretty much TTM in general it is exactly what I need.


I could fix TTM to not need that, but for radeon and amdgpu it is the 
hardware which needs this.


Sorry, I don't follow - how does the hardware care about the format of 
an intermediate data structure used to *generate* the dma_address array? 
That's all that I'm proposing to fix here.


Robin.



Christian.




That is a huge waste of memory and CPU cycles and I actually wanted to
get rid of it for quite some time now. Sinas approach seems to be a good
step into the right direction to me to actually clean that up.


Detecting such a condition is much easier if the DMA map implementation
gives you the coalesced scatter entries.

A way which preserves both path would be indeed nice to have, but that
only allows for the TLB optimization for the GPU and not the CPU any
more. So I actually see that as really minor use case.

Some of the Tegras have much larger TLBs and better page-walk
performance on the system IOMMU compared to the GPU MMU, so they would
probably benefit a good deal even if the hugepage optimization only
targets the GPU side.

Regards,
Lucas



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Re: [PATCH 1/2] drm/prime: Iterate SG DMA addresses separately

[Christian König]

Am 12.04.2018 um 11:49 schrieb Lucas Stach:

Am Donnerstag, den 12.04.2018, 11:35 +0200 schrieb Christian König:

Am 12.04.2018 um 11:11 schrieb Lucas Stach:

Am Mittwoch, den 11.04.2018, 20:26 +0200 schrieb Christian König:

Am 11.04.2018 um 19:11 schrieb Robin Murphy:

For dma_map_sg(), DMA API implementations are free to merge consecutive
segments into a single DMA mapping if conditions are suitable, thus the
resulting DMA addresses may be packed into fewer entries than
ttm->sg->nents implies.

drm_prime_sg_to_page_addr_arrays() does not account for this, meaning
its callers either have to reject the 0 < count < nents case or risk
getting bogus addresses back later. Fortunately this is relatively easy
to deal with having to rejig structures to also store the mapped count,
since the total DMA length should still be equal to the total buffer
length. All we need is a separate scatterlist cursor to iterate the DMA
addresses separately from the CPU addresses.

Mhm, I think I like Sinas approach better.

See the hardware actually needs the dma_address on a page by page basis.

Joining multiple consecutive pages into one entry is just additional
overhead which we don't need.

But setting MAX_SEGMENT_SIZE will probably prevent an IOMMU that might
be in front of your GPU to map large pages as such, causing additional
overhead by means of additional TLB misses and page walks on the IOMMU
side.

And wouldn't you like to use huge pages at the GPU side, if the IOMMU
already provides you the service of producing one large consecutive
address range, rather than mapping them via a number of small pages?

No, I wouldn't like to use that. We're already using that :)

But we use huge pages by allocating consecutive chunks of memory so that
both the CPU as well as the GPU can increase their TLB hit rate.

I'm not convinced that this is a universal win. Many GPU buffers aren't
accessed by the CPU and allocating huge pages puts much more strain on
the kernel MM subsystem.


Yeah, we indeed see the extra overhead during allocation.


What currently happens is that the DMA subsystem tries to coalesce
multiple pages into on SG entry and we de-coalesce that inside the
driver again to create our random access array.

I guess the right thing would be to have a flag that tells the the DMA
implementation to not coalesce the entries. But (ab-)using max segment
size tells the DMA API to split the segments if they are larger than
the given size, which is probably not what you want either as you now
need to coalesce the segments again when you are mapping real huge
pages.


No, even with huge pages I need an array with every single dma address 
for a 4K pages (or whatever the normal page size is).


The problem is that I need a random access array for the DMA addresses, 
even when they are continuously.


I agree that max segment size is a bit ugly here, but at least for 
radeon, amdgpu and pretty much TTM in general it is exactly what I need.


I could fix TTM to not need that, but for radeon and amdgpu it is the 
hardware which needs this.


Christian.




That is a huge waste of memory and CPU cycles and I actually wanted to
get rid of it for quite some time now. Sinas approach seems to be a good
step into the right direction to me to actually clean that up.


Detecting such a condition is much easier if the DMA map implementation
gives you the coalesced scatter entries.

A way which preserves both path would be indeed nice to have, but that
only allows for the TLB optimization for the GPU and not the CPU any
more. So I actually see that as really minor use case.

Some of the Tegras have much larger TLBs and better page-walk
performance on the system IOMMU compared to the GPU MMU, so they would
probably benefit a good deal even if the hugepage optimization only
targets the GPU side.

Regards,
Lucas


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Re: [PATCH 1/2] drm/prime: Iterate SG DMA addresses separately

[Lucas Stach]

Am Donnerstag, den 12.04.2018, 11:35 +0200 schrieb Christian König:
> Am 12.04.2018 um 11:11 schrieb Lucas Stach:
> > Am Mittwoch, den 11.04.2018, 20:26 +0200 schrieb Christian König:
> > > Am 11.04.2018 um 19:11 schrieb Robin Murphy:
> > > > For dma_map_sg(), DMA API implementations are free to merge consecutive
> > > > segments into a single DMA mapping if conditions are suitable, thus the
> > > > resulting DMA addresses may be packed into fewer entries than
> > > > ttm->sg->nents implies.
> > > > 
> > > > drm_prime_sg_to_page_addr_arrays() does not account for this, meaning
> > > > its callers either have to reject the 0 < count < nents case or risk
> > > > getting bogus addresses back later. Fortunately this is relatively easy
> > > > to deal with having to rejig structures to also store the mapped count,
> > > > since the total DMA length should still be equal to the total buffer
> > > > length. All we need is a separate scatterlist cursor to iterate the DMA
> > > > addresses separately from the CPU addresses.
> > > 
> > > Mhm, I think I like Sinas approach better.
> > > 
> > > See the hardware actually needs the dma_address on a page by page basis.
> > > 
> > > Joining multiple consecutive pages into one entry is just additional
> > > overhead which we don't need.
> > 
> > But setting MAX_SEGMENT_SIZE will probably prevent an IOMMU that might
> > be in front of your GPU to map large pages as such, causing additional
> > overhead by means of additional TLB misses and page walks on the IOMMU
> > side.
> > 
> > And wouldn't you like to use huge pages at the GPU side, if the IOMMU
> > already provides you the service of producing one large consecutive
> > address range, rather than mapping them via a number of small pages?
> 
> No, I wouldn't like to use that. We're already using that :)
> 
> But we use huge pages by allocating consecutive chunks of memory so that 
> both the CPU as well as the GPU can increase their TLB hit rate.

I'm not convinced that this is a universal win. Many GPU buffers aren't
accessed by the CPU and allocating huge pages puts much more strain on
the kernel MM subsystem.

> What currently happens is that the DMA subsystem tries to coalesce 
> multiple pages into on SG entry and we de-coalesce that inside the 
> driver again to create our random access array.

I guess the right thing would be to have a flag that tells the the DMA
implementation to not coalesce the entries. But (ab-)using max segment
size tells the DMA API to split the segments if they are larger than
the given size, which is probably not what you want either as you now
need to coalesce the segments again when you are mapping real huge
pages.

> That is a huge waste of memory and CPU cycles and I actually wanted to 
> get rid of it for quite some time now. Sinas approach seems to be a good 
> step into the right direction to me to actually clean that up.
> 
> > Detecting such a condition is much easier if the DMA map implementation
> > gives you the coalesced scatter entries.
> 
> A way which preserves both path would be indeed nice to have, but that 
> only allows for the TLB optimization for the GPU and not the CPU any 
> more. So I actually see that as really minor use case.

Some of the Tegras have much larger TLBs and better page-walk
performance on the system IOMMU compared to the GPU MMU, so they would
probably benefit a good deal even if the hugepage optimization only
targets the GPU side.

Regards,
Lucas
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Re: [PATCH 1/2] drm/prime: Iterate SG DMA addresses separately

[Christian König]

Am 12.04.2018 um 11:11 schrieb Lucas Stach:

Am Mittwoch, den 11.04.2018, 20:26 +0200 schrieb Christian König:

Am 11.04.2018 um 19:11 schrieb Robin Murphy:

For dma_map_sg(), DMA API implementations are free to merge consecutive
segments into a single DMA mapping if conditions are suitable, thus the
resulting DMA addresses may be packed into fewer entries than
ttm->sg->nents implies.

drm_prime_sg_to_page_addr_arrays() does not account for this, meaning
its callers either have to reject the 0 < count < nents case or risk
getting bogus addresses back later. Fortunately this is relatively easy
to deal with having to rejig structures to also store the mapped count,
since the total DMA length should still be equal to the total buffer
length. All we need is a separate scatterlist cursor to iterate the DMA
addresses separately from the CPU addresses.

Mhm, I think I like Sinas approach better.

See the hardware actually needs the dma_address on a page by page basis.

Joining multiple consecutive pages into one entry is just additional
overhead which we don't need.

But setting MAX_SEGMENT_SIZE will probably prevent an IOMMU that might
be in front of your GPU to map large pages as such, causing additional
overhead by means of additional TLB misses and page walks on the IOMMU
side.

And wouldn't you like to use huge pages at the GPU side, if the IOMMU
already provides you the service of producing one large consecutive
address range, rather than mapping them via a number of small pages?


No, I wouldn't like to use that. We're already using that :)

But we use huge pages by allocating consecutive chunks of memory so that 
both the CPU as well as the GPU can increase their TLB hit rate.


What currently happens is that the DMA subsystem tries to coalesce 
multiple pages into on SG entry and we de-coalesce that inside the 
driver again to create our random access array.


That is a huge waste of memory and CPU cycles and I actually wanted to 
get rid of it for quite some time now. Sinas approach seems to be a good 
step into the right direction to me to actually clean that up.



Detecting such a condition is much easier if the DMA map implementation
gives you the coalesced scatter entries.


A way which preserves both path would be indeed nice to have, but that 
only allows for the TLB optimization for the GPU and not the CPU any 
more. So I actually see that as really minor use case.


Regards,
Christian.



Regards,
Lucas


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Re: [PATCH 1/2] drm/prime: Iterate SG DMA addresses separately

[Lucas Stach]

Am Mittwoch, den 11.04.2018, 20:26 +0200 schrieb Christian König:
> Am 11.04.2018 um 19:11 schrieb Robin Murphy:
> > For dma_map_sg(), DMA API implementations are free to merge consecutive
> > segments into a single DMA mapping if conditions are suitable, thus the
> > resulting DMA addresses may be packed into fewer entries than
> > ttm->sg->nents implies.
> > 
> > drm_prime_sg_to_page_addr_arrays() does not account for this, meaning
> > its callers either have to reject the 0 < count < nents case or risk
> > getting bogus addresses back later. Fortunately this is relatively easy
> > to deal with having to rejig structures to also store the mapped count,
> > since the total DMA length should still be equal to the total buffer
> > length. All we need is a separate scatterlist cursor to iterate the DMA
> > addresses separately from the CPU addresses.
> 
> Mhm, I think I like Sinas approach better.
> 
> See the hardware actually needs the dma_address on a page by page basis.
> 
> Joining multiple consecutive pages into one entry is just additional 
> overhead which we don't need.

But setting MAX_SEGMENT_SIZE will probably prevent an IOMMU that might
be in front of your GPU to map large pages as such, causing additional
overhead by means of additional TLB misses and page walks on the IOMMU
side.

And wouldn't you like to use huge pages at the GPU side, if the IOMMU
already provides you the service of producing one large consecutive
address range, rather than mapping them via a number of small pages?
Detecting such a condition is much easier if the DMA map implementation
gives you the coalesced scatter entries.

Regards,
Lucas
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Re: [PATCH 1/2] drm/prime: Iterate SG DMA addresses separately

[Christian König]

Am 11.04.2018 um 19:11 schrieb Robin Murphy:

For dma_map_sg(), DMA API implementations are free to merge consecutive
segments into a single DMA mapping if conditions are suitable, thus the
resulting DMA addresses may be packed into fewer entries than
ttm->sg->nents implies.

drm_prime_sg_to_page_addr_arrays() does not account for this, meaning
its callers either have to reject the 0 < count < nents case or risk
getting bogus addresses back later. Fortunately this is relatively easy
to deal with having to rejig structures to also store the mapped count,
since the total DMA length should still be equal to the total buffer
length. All we need is a separate scatterlist cursor to iterate the DMA
addresses separately from the CPU addresses.


Mhm, I think I like Sinas approach better.

See the hardware actually needs the dma_address on a page by page basis.

Joining multiple consecutive pages into one entry is just additional 
overhead which we don't need.


Regards,
Christian.



Signed-off-by: Robin Murphy 
---

Off the back of Sinan's proposal for a workaround, I took a closer look
and this jumped out - I have no hardware to test it, nor do I really
know my way around this code, so I'm probably missing something, but at
face value this seems like the only obvious problem, and worth fixing
either way.

These patches are based on drm-next, and compile-tested (for arm64) only.

Robin.

  drivers/gpu/drm/drm_prime.c | 14 +++---
  1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
index 7856a9b3f8a8..db3dc8489afc 100644
--- a/drivers/gpu/drm/drm_prime.c
+++ b/drivers/gpu/drm/drm_prime.c
@@ -933,16 +933,18 @@ int drm_prime_sg_to_page_addr_arrays(struct sg_table 
*sgt, struct page **pages,
 dma_addr_t *addrs, int max_entries)
  {
unsigned count;
-   struct scatterlist *sg;
+   struct scatterlist *sg, *dma_sg;
struct page *page;
-   u32 len, index;
+   u32 len, dma_len, index;
dma_addr_t addr;
  
  	index = 0;

+   dma_sg = sgt->sgl;
+   dma_len = sg_dma_len(dma_sg);
+   addr = sg_dma_address(dma_sg);
for_each_sg(sgt->sgl, sg, sgt->nents, count) {
len = sg->length;
page = sg_page(sg);
-   addr = sg_dma_address(sg);
  
  		while (len > 0) {

if (WARN_ON(index >= max_entries))
@@ -957,6 +959,12 @@ int drm_prime_sg_to_page_addr_arrays(struct sg_table *sgt, 
struct page **pages,
len -= PAGE_SIZE;
index++;
}
+
+   if (dma_len == 0) {
+   dma_sg = sg_next(dma_sg);
+   dma_len = sg_dma_len(dma_sg);
+   addr = sg_dma_address(dma_sg);
+   }
}
return 0;
  }


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[PATCH 1/2] drm/prime: Iterate SG DMA addresses separately

[Robin Murphy]

For dma_map_sg(), DMA API implementations are free to merge consecutive
segments into a single DMA mapping if conditions are suitable, thus the
resulting DMA addresses may be packed into fewer entries than
ttm->sg->nents implies.

drm_prime_sg_to_page_addr_arrays() does not account for this, meaning
its callers either have to reject the 0 < count < nents case or risk
getting bogus addresses back later. Fortunately this is relatively easy
to deal with having to rejig structures to also store the mapped count,
since the total DMA length should still be equal to the total buffer
length. All we need is a separate scatterlist cursor to iterate the DMA
addresses separately from the CPU addresses.

Signed-off-by: Robin Murphy 
---

Off the back of Sinan's proposal for a workaround, I took a closer look
and this jumped out - I have no hardware to test it, nor do I really
know my way around this code, so I'm probably missing something, but at
face value this seems like the only obvious problem, and worth fixing
either way.

These patches are based on drm-next, and compile-tested (for arm64) only.

Robin.

 drivers/gpu/drm/drm_prime.c | 14 +++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
index 7856a9b3f8a8..db3dc8489afc 100644
--- a/drivers/gpu/drm/drm_prime.c
+++ b/drivers/gpu/drm/drm_prime.c
@@ -933,16 +933,18 @@ int drm_prime_sg_to_page_addr_arrays(struct sg_table 
*sgt, struct page **pages,
 dma_addr_t *addrs, int max_entries)
 {
unsigned count;
-   struct scatterlist *sg;
+   struct scatterlist *sg, *dma_sg;
struct page *page;
-   u32 len, index;
+   u32 len, dma_len, index;
dma_addr_t addr;
 
index = 0;
+   dma_sg = sgt->sgl;
+   dma_len = sg_dma_len(dma_sg);
+   addr = sg_dma_address(dma_sg);
for_each_sg(sgt->sgl, sg, sgt->nents, count) {
len = sg->length;
page = sg_page(sg);
-   addr = sg_dma_address(sg);
 
while (len > 0) {
if (WARN_ON(index >= max_entries))
@@ -957,6 +959,12 @@ int drm_prime_sg_to_page_addr_arrays(struct sg_table *sgt, 
struct page **pages,
len -= PAGE_SIZE;
index++;
}
+
+   if (dma_len == 0) {
+   dma_sg = sg_next(dma_sg);
+   dma_len = sg_dma_len(dma_sg);
+   addr = sg_dma_address(dma_sg);
+   }
}
return 0;
 }
-- 
2.16.1.dirty

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