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

[Thomas Zimmermann]

Hi

Am 18.11.23 um 12:10 schrieb Javier Martinez Canillas:
> Ard Biesheuvel <ardb at kernel.org> writes:
> 
> Hello Ard,
> 
>> On Fri, 17 Nov 2023 at 00:09, Rob Herring <robh at kernel.org> wrote:
> 
> [...]
> 
>>>>>
>>>>> 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?
>>>
>>
>> The EFI GOP is just a dumb framebuffer, and it is not even generally
>> possible to cross reference the GOP with a particular device in the
>> device hierarchy unless you e.g., compare the BARs of each device with
>> the region described by the GOP protocol.
>>
>> GRUB for EFI will use the GOP and nothing else, but only at boot time
>> (the GOP protocol is more than a magic linear memory region, it also
>> implements a Blt() abstraction that permits the use of framebuffers
>> that are not mapped linearly into the address space at all, and GRUB
>> makes use of this)
>>
>> The EFI stub will only expose GOPs to the kernel if they are in fact
>> linear framebuffers, but has zero insight into whether the hardware
>> needs clocks and regulators, and whether or not the framebuffer needs
>> IOMMU pass through (which might be the case if the scanout is using
>> DMA into system memory)
>>
>> So calling EFI GOP 'source of truth' is rather generous, and I think
>> it makes sense to prioritize more accurate descriptions of the
>> underlying framebuffer over EFI GOP.
>>
> 
> That was my opinion as well and the reason why I called the DTB the
> single source of truth.
> 
>> However, making 'simple-framebuffer' magic in this regard doesn't seem
>> like a great approach to me. Is there a better way we could get the
>> resource conflict to be decided in a way where the EFI GOP gets
>> superseded if its resources are claimed by another device?
>>
> 
> There is an aperture [0] framework that is used by the fbdev and DRM
> subsystems to allow native drivers to remove any conflicting devices
> that share the same framebuffer aperture.
> 
> But it only makes sense for native drivers to use that I think, but
> in this case is about two drivers that attempt to use the same frame
> buffer provided by the firmware but getting it from different places.
> 
> I don't have a better idea than this patch but maybe Thomas or Sima do?

At SUSE, we've carried such a patch in our repos for some time. It works 
around the double-framebuffer problem in a similar way. [1]

As Javier mentioned, we do track the framebuffer ranges for EFI/VESA/OF 
framebuffers in the graphics aperture helpers. Fbdev has done this for 
decades, and the current codebase extends and harmonizes this 
functionality among fbdev and DRM drivers.

WRT DT vs EFI: AFAIK the EFI support on affected platforms looks at the 
DT to set up the EFI framebuffer. So IMHO the DT is the authoritative 
source on the framebuffer.

Best regards
Thomas

[1] https://bugzilla.suse.com/show_bug.cgi?id=1204315

> 
> [0]: https://elixir.bootlin.com/linux/latest/source/drivers/video/aperture.c
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)
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