[PATCH 2/2] drm/amdgpu: Allow dma_map_sg() coalescing

[Robin Murphy]

On 11/04/18 19:28, Christian König wrote:
> Am 11.04.2018 um 19:11 schrieb Robin Murphy:
>> Now that drm_prime_sg_to_page_addr_arrays() understands the case where
>> dma_map_sg() has coalesced segments and returns 0 < count < nents, we
>> can relax the check to only consider genuine failure.
> 
> That pattern is repeated in pretty much all drivers using TTM.

AFAICS from a look through drivers/gpu/ only 3 drivers consider the 
actual value of what dma_map_sg() returns - others only handle the 
failure case of 0, and some don't check it at all - and of those it's 
only amdgpu and radeon barfing on it being different from nents (vmwgfx 
appears to just stash it to use slightly incorrectly later).

> So you would need to fix all of them, but as I said I don't think that 
> this approach is a good idea.
> 
> We essentially wanted to get rid of the dma_address array in the mid 
> term and that change goes into the exactly opposite direction.

But this patch bears no intention of making any fundamental change to 
the existing behaviour of this particular driver, it simply permits said 
behaviour to work at all on more systems than it currently does. I would 
consider that entirely orthogonal to future TTM-wide development :/

Robin.

> 
> Regards,
> Christian.
> 
>>
>> Signed-off-by: Robin Murphy <robin.murphy at arm.com>
>> ---
>>   drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c 
>> b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
>> index 205da3ff9cd0..f81e96a4242f 100644
>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
>> @@ -813,7 +813,7 @@ static int amdgpu_ttm_tt_pin_userptr(struct ttm_tt 
>> *ttm)
>>       r = -ENOMEM;
>>       nents = dma_map_sg(adev->dev, ttm->sg->sgl, ttm->sg->nents, 
>> direction);
>> -    if (nents != ttm->sg->nents)
>> +    if (nents == 0)
>>           goto release_sg;
>>       drm_prime_sg_to_page_addr_arrays(ttm->sg, ttm->pages,
>