Re: [PATCH 1/3] drm/amdgpu: reserve 64M stolen memory for Apple 5K iMac

[Mario Limonciello]

On 10/27/26 07:57, Alex Deucher wrote:
> On Tue, Oct 27, 2026 at 8:21 AM Christian König
> <christian.koenig@xxxxxxx> wrote:
> > On 10/26/26 23:46, Richard Lane wrote:
> > > Apple 27" 5K iMacs report a single 2560x2880 tile (28.125 MiB) in
> > > VIEWPORT_SIZE while scanning out up to 64 MiB across the relocated EFI
> > > framebuffer.
> > >
> > > Override the stolen VGA memory allocation size to 64 MiB when matching
> > > PCI_VENDOR_ID_APPLE with a 2560x2880x4 viewport size to prevent
> > > framebuffer corruption during boot handover.
> >
> > Hacks like those are usually a big no-go.
> >
> > Why is that framebuffer handling not working with DC out of the box?
>
> The GMC IP code reserves the memory used by the display hardware from
> the pre-OS console.  The gmc get_vbios_fb_size() callback looks to see
> what display viewport was set by the pre-OS environment.  If the Macs
> need a workaround, that's where it should go.  Since these displays
> require multiple display pipes, my guess is you need to double the
> allocation in that case since the current code just checks the first
> pipe.


Could this also be a logic error in that callback?  Would it be worth 
double checking all the register values in that callback for this system?

I know for example there are a few different IP specific implementations 
in gmc_v9_0_get_vbios_fb_size().

> Alex
>
> > Regards,
> > Christian.
> >
> > > Signed-off-by: Richard Lane <richardlanedev@xxxxxxxxx>
> > > ---
> > >   drivers/gpu/drm/amd/amdgpu/amdgpu_gmc.c | 10 +++++++++-
> > >   1 file changed, 9 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_gmc.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_gmc.c
> > > index 4204c3c750a8..8b196fbfa724 100644
> > > --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_gmc.c
> > > +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_gmc.c
> > > @@ -1232,7 +1232,15 @@ void amdgpu_gmc_init_vga_resv_regions(struct amdgpu_device *adev)
> > >          } else {
> > >                  size = amdgpu_gmc_get_vbios_fb_size(adev);
> > >
> > > -               if (adev->mman.keep_stolen_vga_memory)
> > > +               /*
> > > +                * Apple 5K iMacs report one 2560x2880 tile in VIEWPORT_SIZE
> > > +                * while scanning out up to 64M across the relocated EFI
> > > +                * framebuffer.
> > > +                */
> > > +               if (adev->pdev->subsystem_vendor == PCI_VENDOR_ID_APPLE &&
> > > +                   size == (2560 * 2880 * 4))
> > > +                       size = 64U * 1024 * 1024;
> > > +               else if (adev->mman.keep_stolen_vga_memory)
> > >                          size = max(size, (unsigned)AMDGPU_VBIOS_VGA_ALLOCATION);
> > >          }
> > >
> > > --
> > > 2.55.0