[RFC PATCH 1/2] drm/amdgpu: amdgpu_vram_mgr_new(): Clamp lpfn to total vram

[Paneer Selvam, Arunpravin]

On 5/3/2025 5:53 PM, Paneer Selvam, Arunpravin wrote:
>
>
> On 5/2/2025 9:02 PM, John Olender wrote:
>> On 4/30/25 5:44 PM, Paneer Selvam, Arunpravin wrote:
>>>
>>> On 5/1/2025 2:50 AM, Alex Deucher wrote:
>>>> + Christian
>>>>
>>>> On Tue, Apr 29, 2025 at 7:24 AM John Olender <john.olender at gmail.com>
>>>> wrote:
>>>>> The drm_mm allocator tolerated being passed end > mm->size, but the
>>>>> drm_buddy allocator does not.
>>>>>
>>>>> Restore the pre-buddy-allocator behavior of allowing such placements.
>>>>>
>>>>> Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3448
>>>>> Signed-off-by: John Olender <john.olender at gmail.com>
>>>> This looks correct to me.
>>>> Reviewed-by: Alex Deucher <alexander.deucher at amd.com>
>>> I was thinking that we should return an error when lpfn > man->size.
>>>
>>> Regards,
>>> Arun.
>> This patch restores the previous behavior in the spirit of "Do not crash
>> the kernel".  The existing uvd placements are pretty clear in their
>> intent and were accepted until the switch to drm_buddy.  I think it's
>> fair to consider their style as expected.
>>
>> With that in mind, I'm not sure amdgpu_vram_mgr is the place this change
>> really belongs.  That is, I think it's worth asking:
>>
>> 1) Why does drm_mm accept end > mm->size without complaint?
>> 2) Why doesn't drm_buddy do the same?
>
> I remember that during the development of DRM buddy , we had a 
> discussion with Intel folks and decided to
> return an error in DRM buddy when end > mm->size. This was done to 
> ensure that, at the driver level,  lpfn
> has the correct value.
>
> I will modify this at drm_buddy to match with drm_mm and send the patch.
After giving it some thought, I think it is more effective to implement 
this tolerance at the VRAM manager level
and allow the DRM buddy manager to perform a strict validation, as this 
is necessary for other graphics drivers
(e.g., i915).

Regards,
Arun.
>
> Regards,
> Arun.
>>
>> Thanks,
>> John
>>
>>>>> ---
>>>>>    drivers/gpu/drm/amd/amdgpu/amdgpu_vram_mgr.c | 2 +-
>>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_vram_mgr.c b/drivers/
>>>>> gpu/drm/amd/amdgpu/amdgpu_vram_mgr.c
>>>>> index 2d7f82e98df9..abdc52b0895a 100644
>>>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_vram_mgr.c
>>>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_vram_mgr.c
>>>>> @@ -463,7 +463,7 @@ static int amdgpu_vram_mgr_new(struct
>>>>> ttm_resource_manager *man,
>>>>>           int r;
>>>>>
>>>>>           lpfn = (u64)place->lpfn << PAGE_SHIFT;
>>>>> -       if (!lpfn)
>>>>> +       if (!lpfn || lpfn > man->size)
>>>>>                   lpfn = man->size;
>>>>>
>>>>>           fpfn = (u64)place->fpfn << PAGE_SHIFT;
>>>>> -- 
>>>>> 2.47.2
>>>>>
>