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>]

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

Thank you for the reply.

In svm_migrate_copy_memory_gart(), the number of bytes to copy passed to 
amdgpu_copy_buffer() is based on the PMD size. On systems with a 4K page 
size, the PMD size is calculated correctly because 
AMDGPU_GTT_MAX_TRANSFER_SIZE is 512, and 512 × 4K = 2MB.

On systems with a 64K page size, however, the calculation becomes 512 × 
64K = 32MB. As a result, amdgpu_copy_buffer() ends up copying data in 
4MB chunks instead of PMD-sized chunks. To ensure consistent behavior 
across both 64K and 4K page-size systems, we adjusted the transfer size 
so that the maximum transfer remains 2MB, matching the PMD size.

The issue we observed was that the rocr-debug-agent test triggered SDMA 
hangs. This happened because an incorrect size was being passed when 
copying the GART mapping in svm_migrate_gart_map(). That problem was 
addressed in patch 3/8. While root-causing that issue, we also 
identified this inconsistency between 4K and 64K systems, So we felt 
that this change was needed to align the behavior with 4K system page sizes.



>
> 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;