[RFC PATCH 2/2] drm/amdgpu/uvd: Ensure vcpu bos are within the uvd segment

[John Olender]

On 6/2/25 6:00 AM, Christian König wrote:
> Hi John,
> 
> first of all thanks a lot for taking a look into this.
> 
>>> Okay, I did a read and compare after each write.
>>>
>>> Both writes seem to go through on both the Kaveri and s9150:
>>>
>>> Kaveri (512MB UMA Buffer):
>>> amdgpu 0000:00:01.0: amdgpu: [drm] uvd_v4_2_mc_resume: mmUVD_LMI_ADDR_EXT: gpu_addr=0xF41FA00000, addr=0x00000001, wrote 0x00001001, read 0x00001001 [same]
>>> amdgpu 0000:00:01.0: amdgpu: [drm] uvd_v4_2_mc_resume: mmUVD_LMI_EXT40_ADDR: gpu_addr=0xF41FA00000, addr=0x000000F4, wrote 0x800900F4, read 0x800900F4 [same]
>>>
>>> s9150:
>>> amdgpu 0000:41:00.0: amdgpu: [drm] uvd_v4_2_mc_resume: mmUVD_LMI_ADDR_EXT: gpu_addr=0xF7FFA00000, addr=0x0000000F, wrote 0x0000F00F, read 0x0000F00F [same]
>>> amdgpu 0000:41:00.0: amdgpu: [drm] uvd_v4_2_mc_resume: mmUVD_LMI_EXT40_ADDR: gpu_addr=0xF7FFA00000, addr=0x000000F7, wrote 0x800900F7, read 0x800900F7 [same]
>>>
>>
>> I've also confirmed the patch works fine when segments other than
>> [0, 256M) are used.
>>  
>> E.g.: Both init and VA-API playback work fine with a UVD segment of
>> [1792M, 2048M) on Kaveri with a 2G UMA buffer.
> 
> Oh, that's a very interesting find. Could you try to turn around the way the patch works?
> 
> E.g. instead of forcing the UVD FW into the first segment, change amdgpu_uvd_force_into_uvd_segment() so that the BOs are forced into the same segment as the UVD firmware.
> 
> That would resolve my concern that this could overload the first segment. The feedback and message BO are usually rather small (4 or 128k IIRC), but the firmware is a couple of megabytes in size.
> 
> When we have other FW and VGA emulation buffers in the first segment as well then that could result into clashing that segment to much.
> 
> Thanks,
> Christian.
> 

Okay, yeah, that should make for a significantly simpler fix.

>>>>> I will try to find a Kaveri system which is still working to reproduce the issue.
> 
> I unfortunately couldn't find a working box of hand. Would need to search in our HW stash for a box which still works and get that shipped to me.
> 
> And that is overhead for this issue I would rather like to avoid.
> 
> So if you can come up with a simpler patch which works for you I'm happy to take that.
> 
> Regards,
> Christian.

Sounds good.  I'll get started on this variant.

Thanks,
John