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

[Christian König]

Am 12.04.2018 um 19:53 schrieb 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 :/

That's a really good point. Please try to fix radeon as well and maybe 
ping the vmwgfx maintainers.

With that in place I'm perfectly ok with going ahead with that approach.

Christian.

>
> 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,
>>