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

[Rob Herring]

On Thu, Nov 16, 2023 at 3:36 AM Javier Martinez Canillas
<javierm at redhat.com> wrote:
>
> Rob Herring <robh at kernel.org> writes:
>
> Hello Rob,
>
> > On Mon, Nov 13, 2023 at 2:53 AM Javier Martinez Canillas
> > <javierm at 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?

Shrug. I don't really know anything more about EFI FB, but I would
guess it can't support handling resources like clocks, power domains,
regulators, etc. that simple-fb can. So if a platform needs those, do
we say they should not setup EFI-GOP? Or is there a use case for
having both? Clients that don't muck with resources can use EFI-GOP
and those that do use simple-fb. For example, does/can grub use
EFI-GOP, but not simple-fb?

Rob