[PATCH] drm/amdgpu: don't limit gtt size on apus

[Joshua Ashton]

On 1/5/21 10:10 PM, Alex Deucher wrote:
> On Tue, Jan 5, 2021 at 5:05 PM Joshua Ashton <joshua at froggi.es> wrote:
>>
>> Since commit 24562523688b ("Revert "drm/amd/amdgpu: set gtt size
>> according to system memory size only""), the GTT size was limited by
>> 3GiB or VRAM size.
> 
> The commit in question was to fix a hang with certain tests on APUs.
> That should be tested again before we re-enable this.  If it is fixed,
> we should just revert the revert rather than special case dGPUs.
> 
> Alex
> 

I think the commit before the revert (ba851eed895c) has some fundamental 
problems:

It was always specifying max(3GiB, 3/4ths RAM) of GTT, even if that 
wouldn't fit into say, 1GiB or 2GiB of available RAM.

Limiting GTT to min(max(3GiB, VRAM), 3/4ths RAM) size on dGPUs makes 
sense also and is a sensible limit to avoid silly situations with 
overallocation and potential OOM.

This patch solves both of those issues.

- Joshie 🐸✨

> 
>>
>> This is problematic on APUs, especially with a small carveout
>> which can be as low as a fixed 128MiB, as there would be very a limited
>> 3GiB available for video memory.
>> This obviously does not meet the demands of modern applications.
>>
>> This patch makes it so the GTT size heuristic always uses 3/4ths of
>> the system memory size on APUs (limiting the size by 3GiB/VRAM size
>> only on devices with dedicated video memory).
>>
>> Fixes: 24562523688b ("Revert drm/amd/amdgpu: set gtt size according to
>> system memory size only")
>>
>> Signed-off-by: Joshua Ashton <joshua at froggi.es>
>> ---
>>   drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c |  5 +++--
>>   drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c | 12 +++++++++---
>>   2 files changed, 12 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c
>> index 72efd579ec5e..a5a41e9272d6 100644
>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c
>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c
>> @@ -192,8 +192,9 @@ module_param_named(gartsize, amdgpu_gart_size, uint, 0600);
>>
>>   /**
>>    * DOC: gttsize (int)
>> - * Restrict the size of GTT domain in MiB for testing. The default is -1 (It's VRAM size if 3GB < VRAM < 3/4 RAM,
>> - * otherwise 3/4 RAM size).
>> + * Restrict the size of GTT domain in MiB for testing. The default is -1 (On APUs this is 3/4th
>> + * of the system memory; on dGPUs this is 3GiB or VRAM sized, whichever is bigger,
>> + * with an upper bound of 3/4th of system memory.
>>    */
>>   MODULE_PARM_DESC(gttsize, "Size of the GTT domain in megabytes (-1 = auto)");
>>   module_param_named(gttsize, amdgpu_gtt_size, int, 0600);
>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
>> index 4d8f19ab1014..294f26f4f310 100644
>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
>> @@ -1865,9 +1865,15 @@ int amdgpu_ttm_init(struct amdgpu_device *adev)
>>                  struct sysinfo si;
>>
>>                  si_meminfo(&si);
>> -               gtt_size = min(max((AMDGPU_DEFAULT_GTT_SIZE_MB << 20),
>> -                              adev->gmc.mc_vram_size),
>> -                              ((uint64_t)si.totalram * si.mem_unit * 3/4));
>> +               gtt_size = (uint64_t)si.totalram * si.mem_unit * 3/4;
>> +               /* If we have dedicated memory, limit our GTT size to
>> +                * 3GiB or VRAM size, whichever is bigger
>> +                */
>> +               if (!(adev->flags & AMD_IS_APU)) {
>> +                       gtt_size = min(max(AMDGPU_DEFAULT_GTT_SIZE_MB << 20,
>> +                               adev->gmc.mc_vram_size),
>> +                               gtt_size);
>> +               }
>>          }
>>          else
>>                  gtt_size = (uint64_t)amdgpu_gtt_size << 20;
>> --
>> 2.30.0
>>
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