Re: [RFC PATCH v1 4/8] amdgpu/amdgpu_ttm: Fix AMDGPU_GTT_MAX_TRANSFER_SIZE for non-4K page size

[Donet Tom <donettom@linux.ibm.com>]

[-- Attachment #1: Type: text/plain, Size: 3058 bytes --]


On 12/12/25 2:23 PM, Christian König wrote:
> On 12/12/25 07:40, Donet Tom wrote:
>> The SDMA engine has a hardware limitation of 4 MB maximum transfer
>> size per operation.
> That is not correct. This is only true on ancient HW.
>
> What problems are you seeing here?
>
>> AMDGPU_GTT_MAX_TRANSFER_SIZE was hardcoded to
>> 512 pages, which worked correctly on systems with 4K pages but fails
>> on systems with larger page sizes.
>>
>> This patch divides the max transfer size / AMDGPU_GPU_PAGES_IN_CPU_PAGE
>> to match with non-4K page size systems.
> That is actually a bad idea. The value was meant to match the PMD size.


Hi Christian, Felix, Alex and philip

Instead of hardcoding the AMDGPU_GTT_MAX_TRANSFER_SIZE value to 512,
what do you think about doing something like the change below?
This should work across all architectures and page sizes, so
AMDGPU_GTT_MAX_TRANSFER_SIZE will always correspond to the PMD
size on all architectures and with all page sizes.


diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.h b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.h
index 0be2728aa872..c594ed7dff18 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.h
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.h
@@ -37,7 +37,7 @@
  #define AMDGPU_PL_MMIO_REMAP   (TTM_PL_PRIV + 5)
  #define __AMDGPU_PL_NUM        (TTM_PL_PRIV + 6)
  
-#define AMDGPU_GTT_MAX_TRANSFER_SIZE   512
+#define AMDGPU_GTT_MAX_TRANSFER_SIZE   1 << (PMD_SHIFT - PAGE_SHIFT)
  #define AMDGPU_GTT_NUM_TRANSFER_WINDOWS

Could you please provide your thoughts on above? Is it looking ok to you?

If this looks good - here is what we were thinking:

Patches 1-4 are required to fix initial non-4k pagesize support to AMD GPU.
And since these patches are looking in good shape (since Philip has already
reviewed [1-3])- We thought it will be good to split the patch series into two.
I will send a v2 of Part-1 with patches [1-4] (will also address the review comments
in v2 for Patch-1 & 2 from Philip) and for the rest of the patches [5-8] Part-2, we
can continue the discussion till other things are sorted. That will also allow us to
get these initial fixes in Part-1 ready before the 6.20 merge window.

Thoughts?


>
> Regards,
> Christian.
>
>> Signed-off-by: Donet Tom<donettom@linux.ibm.com>
>> Signed-off-by: Ritesh Harjani (IBM)<ritesh.list@gmail.com>
>> ---
>>   drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.h | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.h b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.h
>> index 0be2728aa872..9d038feb25b0 100644
>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.h
>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.h
>> @@ -37,7 +37,7 @@
>>   #define AMDGPU_PL_MMIO_REMAP	(TTM_PL_PRIV + 5)
>>   #define __AMDGPU_PL_NUM	(TTM_PL_PRIV + 6)
>>   
>> -#define AMDGPU_GTT_MAX_TRANSFER_SIZE	512
>> +#define AMDGPU_GTT_MAX_TRANSFER_SIZE	(512 / AMDGPU_GPU_PAGES_IN_CPU_PAGE)
>>   #define AMDGPU_GTT_NUM_TRANSFER_WINDOWS	2
>>   
>>   extern const struct attribute_group amdgpu_vram_mgr_attr_group;

[-- Attachment #2: Type: text/html, Size: 4163 bytes --]