Re: [RFC PATCH] of/platform: Disable sysfb if a simple-framebuffer node is found

[Javier Martinez Canillas <javierm@redhat.com>]

Rob Herring <robh@kernel.org> writes:

Hello Rob,

> On Mon, Nov 13, 2023 at 2:53 AM Javier Martinez Canillas
> <javierm@redhat.com> wrote:
>>
>> Some DT platforms use EFI to boot and in this case the EFI Boot Services
>> may register a EFI_GRAPHICS_OUTPUT_PROTOCOL handle, that will later be
>> queried by the Linux EFI stub to fill the global struct screen_info data.
>>
>> The data is used by the Generic System Framebuffers (sysfb) framework to
>> add a platform device with platform data about the system framebuffer.
>>
>> But if there is a "simple-framebuffer" node in the DT, the OF core will
>> also do the same and add another device for the system framebuffer.
>>
>> This could lead for example, to two platform devices ("simple-framebuffer"
>> and "efi-framebuffer") to be added and matched with their corresponding
>> drivers. So both efifb and simpledrm will be probed, leading to following:
>>
>> [    0.055752] efifb: framebuffer at 0xbd58dc000, using 16000k, total 16000k
>> [    0.055755] efifb: mode is 2560x1600x32, linelength=10240, pages=1
>> [    0.055758] efifb: scrolling: redraw
>> [    0.055759] efifb: Truecolor: size=2:10:10:10, shift=30:20:10:0
>> ...
>> [    3.295896] simple-framebuffer bd58dc000.framebuffer: [drm] *ERROR*
>> could not acquire memory range [??? 0xffff79f30a29ee40-0x2a5000001a7
>> flags 0x0]: -16
>> [    3.298018] simple-framebuffer: probe of bd58dc000.framebuffer
>> failed with error -16
>>
>> To prevent the issue, make the OF core to disable sysfb if there is a node
>> with a "simple-framebuffer" compatible. That way only this device will be
>> registered and sysfb would not attempt to register another one using the
>> screen_info data even if this has been filled.
>>
>> This seems the correct thing to do in this case because:
>>
>> a) On a DT platform, the DTB is the single source of truth since is what
>>    describes the hardware topology. Even if EFI Boot Services are used to
>>    boot the machine.
>
> This is the opposite of what we do for memory and memory reservations.
> EFI is the source of truth for those.
>
> This could also lead to an interesting scenario. As simple-framebuffer
> can define its memory in a /reserved-memory node, but that is ignored
> in EFI boot. Probably would work, but only because EFI probably
> generates its memory map table from the /reserved-memory nodes.
>

I see. So what would be the solution then? Ignoring creating a platform
device for "simple-framebuffer" if booted using EFI and have an EFI-GOP?

> Rob
>

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat