Re: [PATCH 03/14] drm/amdgpu/gmc6: Add GART space for VCPU BO

["Timur Kristóf" <timur.kristof@gmail.com>]

On Wed, 2025-10-29 at 11:05 +0100, Christian König wrote:
> On 10/28/25 23:06, Timur Kristóf wrote:
> > Add an extra 16M (4096 pages) to the GART before GTT.
> > This space is going to be used for the VCE VCPU BO.
> > 
> > Split this into	a separate patch to make it easier to bisect,
> > in case	there are any errors in	the future.
> > 
> > Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
> > ---
> >  drivers/gpu/drm/amd/amdgpu/gmc_v6_0.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/amd/amdgpu/gmc_v6_0.c
> > b/drivers/gpu/drm/amd/amdgpu/gmc_v6_0.c
> > index 499dfd78092d..bfeb60cfbf62 100644
> > --- a/drivers/gpu/drm/amd/amdgpu/gmc_v6_0.c
> > +++ b/drivers/gpu/drm/amd/amdgpu/gmc_v6_0.c
> > @@ -214,6 +214,9 @@ static void gmc_v6_0_vram_gtt_location(struct
> > amdgpu_device *adev,
> >  	amdgpu_gmc_set_agp_default(adev, mc);
> >  	amdgpu_gmc_vram_location(adev, mc, base);
> >  	amdgpu_gmc_gart_location(adev, mc,
> > AMDGPU_GART_PLACEMENT_LOW);
> > +
> > +	/* Add space for VCE's VCPU BO so that VCE1 can access it.
> > */
> > +	mc->num_gart_pages_before_gtt += 4096;
> 
> 4096*4KiB=16MiB. Do we really need so much?

Is it enough to have just enough space for the VCPU BO?
In that case, I think we can use just 512 KiB (rounded up) if I
understand the VCPU BO size correctly. That would be 128 pages.

> 
> >  }
> >  
> >  static void gmc_v6_0_mc_program(struct amdgpu_device *adev)
> > @@ -338,7 +341,7 @@ static int gmc_v6_0_mc_init(struct
> > amdgpu_device *adev)
> >  		case CHIP_TAHITI:   /* UVD, VCE do not support
> > GPUVM */
> >  		case CHIP_PITCAIRN: /* UVD, VCE do not support
> > GPUVM */
> >  		case CHIP_OLAND:    /* UVD, VCE do not support
> > GPUVM */
> > -			adev->gmc.gart_size = 1024ULL << 20;
> > +			adev->gmc.gart_size = 1040ULL << 20;
> 
> Ideally that should be a power of two.
> 
> We can in theory increase it in units of 2MiB without wasting memory,
> but I'm not 100% sure if that is actually tested everywhere.
> 
> Regards,
> Christian.
> 
> >  			break;
> >  		}
> >  	} else {