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

[Alex Deucher]

On Tue, Oct 27, 2026 at 9:16 AM Mario Limonciello
<mario.limonciello@xxxxxxx> wrote:
>
>
>
> 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().

Not likely.  I suspect the issue is that these systems use two display
pipes so we only reserve enough memory to cover half of the display.
On PCs the vbios never sets up MST.

Alex

>
> >
> > 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
> >>
>