[PATCH] drm/amdgpu: add coreboot workaround for KV/KB

[Christian König]

Am 17.01.20 um 03:01 schrieb Felix Kuehling:
> On 2020-01-16 8:09, Christian König wrote:
>> Coreboot seems to have a problem correctly setting up access to the 
>> stolen VRAM
>> on KV/KB. Use the direct access only when necessary.
>
> I'm not sure what you mean by "necessary".

Necessary for better performance.

>
>>
>> Signed-off-by: Christian König <christian.koenig at amd.com>
>> ---
>>   drivers/gpu/drm/amd/amdgpu/gmc_v7_0.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/amd/amdgpu/gmc_v7_0.c 
>> b/drivers/gpu/drm/amd/amdgpu/gmc_v7_0.c
>> index 19d5b133e1d7..9da9596a3638 100644
>> --- a/drivers/gpu/drm/amd/amdgpu/gmc_v7_0.c
>> +++ b/drivers/gpu/drm/amd/amdgpu/gmc_v7_0.c
>> @@ -381,7 +381,8 @@ static int gmc_v7_0_mc_init(struct amdgpu_device 
>> *adev)
>>       adev->gmc.aper_size = pci_resource_len(adev->pdev, 0);
>>     #ifdef CONFIG_X86_64
>> -    if (adev->flags & AMD_IS_APU) {
>> +    if (adev->flags & AMD_IS_APU &&
>> +        adev->gmc.real_vram_size > adev->gmc.aper_size) {
>
> CPU access to the whole VRAM isn't really necessary. I thought the 
> main motivation for accessing FB directly on APUs was better 
> performance. You're disabling that optimization on all APUs where the 
> FB is smaller than the aperture size.

Correct, yes. For some reason coreboot seems to explicitly setup the 
memory used for the FB as write-protected.

The GPU can still read/write it normally cause it ignores that 
protection, but the CPU can't change the FB memory any more with that setup.

No idea why they do this, most likely just an over conservative 
protection of a reserved area of memory.

Regards,
Christian.

>
> Regards,
>   Felix
>
>
>>           adev->gmc.aper_base = ((u64)RREG32(mmMC_VM_FB_OFFSET)) << 22;
>>           adev->gmc.aper_size = adev->gmc.real_vram_size;
>>       }
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