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

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Date:   Thu, 16 Nov 2023 15:40:16 +0100
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Rob Herring <robh@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Ard Biesheuvel <ardb@...nel.org>,
        Sima Vetter <daniel.vetter@...ll.ch>,
        dri-devel@...ts.freedesktop.org,
        Andrew Worsley <amworsley@...il.com>,
        Hector Martin <marcan@...can.st>,
        Sergio Lopez <slp@...hat.com>,
        Frank Rowand <frowand.list@...il.com>,
        devicetree@...r.kernel.org
Subject: Re: [RFC PATCH] of/platform: Disable sysfb if a simple-framebuffer
 node is found

Rob Herring <robh@...nel.org> writes:

> On Thu, Nov 16, 2023 at 3:36 AM Javier Martinez Canillas
> <javierm@...hat.com> wrote:

[...]

>> >
>> > 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

That's correct, and the reason why I thought that the DTB would be the
single source of truth for the firmware provided framebuffer.

For example, in some arm platforms that u-boot does provide an EFI-GOP,
you need to boot with clk_ignore_unused or the system framebuffer just
goes away once the unused clocks are gated. Same for PD, regulators, etc.

> 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?
>

I don't think grub can use the simple-fb, it can use the EFI-GOP if is
available though. And things work because of course grub won't try to
disable unused resources like Linux does.

> Rob
>

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat