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

[Mario Limonciello <mario.limonciello@amd.com>]

On 10/27/26 07:57, Alex Deucher wrote:
> On Tue, Oct 27, 2026 at 8:21 AM Christian König
> <christian.koenig@amd.com> 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@gmail.com>
>>> ---
>>>   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
>>