Re: [PATCH] drm/radeon: fix TOPDOWN handling for bo_create

["Christian König" <deathsimple@vodafone.de>]

On 12.03.2015 10:30, Oded Gabbay wrote:
>
> On 03/12/2015 11:23 AM, Christian König wrote:
>> On 12.03.2015 10:02, Michel Dänzer wrote:
>>> On 12.03.2015 06:14, Alex Deucher wrote:
>>>> On Wed, Mar 11, 2015 at 4:51 PM, Alex Deucher <alexdeucher@gmail.com> wrote:
>>>>> On Wed, Mar 11, 2015 at 2:21 PM, Christian König
>>>>> <deathsimple@vodafone.de> wrote:
>>>>>> On 11.03.2015 16:44, Alex Deucher wrote:
>>>>>>> radeon_bo_create() calls radeon_ttm_placement_from_domain()
>>>>>>> before ttm_bo_init() is called.  radeon_ttm_placement_from_domain()
>>>>>>> uses the ttm bo size to determine when to select top down
>>>>>>> allocation but since the ttm bo is not initialized yet the
>>>>>>> check is always false.
>>>>>>>
>>>>>>> Noticed-by: Oded Gabbay <oded.gabbay@amd.com>
>>>>>>> Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
>>>>>>> Cc: stable@vger.kernel.org
>>>>>> And I was already wondering why the heck the BOs always made this
>>>>>> ping/pong
>>>>>> in memory after creation.
>>>>>>
>>>>>> Patch is Reviewed-by: Christian König <christian.koenig@amd.com>
>>>>> And fixing that promptly broke VCE due to vram location requirements.
>>>>> Updated patch attached.  Thoughts?
>>>> And one more take to make things a bit more explicit for static kernel
>>>> driver allocations.
>>> struct ttm_place::lpfn is honoured even with TTM_PL_FLAG_TOPDOWN, so
>>> latter should work with RADEON_GEM_CPU_ACCESS. It sounds like the
>>> problem is really that some BOs are expected to be within a certain
>>> range from the beginning of VRAM, but lpfn isn't set accordingly. It
>>> would be better to fix that by setting lpfn directly than indirectly via
>>> RADEON_GEM_CPU_ACCESS.
>> Yeah, agree. We should probably try to find the root cause of this instead.
>>
>> As far as I know VCE has no documented limitation on where buffers are
>> placed (unlike UVD). So this is a bit strange. Are you sure that it isn't
>> UVD which breaks here?
>>
>> Regards,
>> Christian.
> I noticed this bug when trying to allocate very large BOs (385MB) from the
> other side of VRAM.
> However, even with this fix, the following scenario still fails:
> 1. Allocate BO of 385MB on VRAM with no CPU access.
> 2. Map it to VRAM
> 3. Allocate second BO of 385MB on VRAM with no CPU access
>
> The last step fails as the ttm can't find a place to put this second BO. I
> suspect the Top-Down thing isn't being respected at all by the
> creation/pinning of BO.
>
> I think that what happens is that the first BO is pinned right after the
> first 256 MB, instead of pinning it at the end of the VRAM.
> Then, when trying to create the second BO, there is no room for it, as there
> is only 256MB before the first BO, and 383MB after the first BO.
>
> I need to debug it further, but will probably only do that on Sunday.

What is the content of radeon_vram_mm (in debugfs) after you allocated 
the first BO?

The placement should be visible there pretty fine.

Regards,
Christian.

>
> 	Oded
>
>>>
>>> Anyway, since this isn't the first bug which prevents
>>> TTM_PL_FLAG_TOPDOWN from working as intended in the radeon driver, I
>>> wonder if its performance impact should be re-evaluated. Lauri?
>>>
>>>
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