linux-kernel - Re: [PATCH 00/12] arch,fbdev: Move screen

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Message-ID: <f9185435-74bb-a325-8fe6-3beb51a66e0a@suse.de>
Date:   Thu, 29 Jun 2023 16:15:44 +0200
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Arnd Bergmann <arnd@...db.de>, Helge Deller <deller@....de>,
        Daniel Vetter <daniel@...ll.ch>,
        Dave Airlie <airlied@...il.com>
Cc:     Linux-Arch <linux-arch@...r.kernel.org>,
        linux-hyperv@...r.kernel.org, linux-efi@...r.kernel.org,
        linux-ia64@...r.kernel.org, linux-sh@...r.kernel.org,
        linux-hexagon@...r.kernel.org, linux-staging@...ts.linux.dev,
        linux-kernel@...r.kernel.org,
        "linux-csky@...r.kernel.org" <linux-csky@...r.kernel.org>,
        linux-mips@...r.kernel.org, linux-fbdev@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, loongarch@...ts.linux.dev,
        linux-alpha@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-riscv@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 00/12] arch,fbdev: Move screen_info into arch/

Hi

Am 29.06.23 um 15:31 schrieb Arnd Bergmann:
> On Thu, Jun 29, 2023, at 13:45, Thomas Zimmermann wrote:
>> The variables screen_info and edid_info provide information about
>> the system's screen, and possibly EDID data of the connected display.
>> Both are defined and set by architecture code. But both variables are
>> declared in non-arch header files. Dependencies are at bease loosely
>> tracked. To resolve this, move the global state screen_info and its
>> companion edid_info into arch/. Only declare them on architectures
>> that define them. List dependencies on the variables in the Kconfig
>> files. Also clean up the callers.
>>
>> Patch 1 to 4 resolve a number of unnecessary include statements of
>> <linux/screen_info.h>. The header should only be included in source
>> files that access struct screen_info.
>>
>> Patches 5 to 7 move the declaration of screen_info and edid_info to
>> <asm-generic/screen_info.h>. Architectures that provide either set
>> a Kconfig token to enable them.
>>
>> Patches 8 to 9 make users of screen_info depend on the architecture's
>> feature.
>>
>> Finally, patches 10 to 12 rework fbdev's handling of firmware EDID
>> data to make use of existing helpers and the refactored edid_info.
>>
>> Tested on x86-64. Built for a variety of platforms.
> 
> This all looks like a nice cleanup!

I guess that patches 1 to 4 are uncontroversial and could be landed 
quickly. Patches 10 to 12 are probably as well.

> 
>> Future directions: with the patchset in place, it will become possible
>> to provide screen_info and edid_info only if there are users. Some
>> architectures do this by testing for CONFIG_VT, CONFIG_DUMMY_CONSOLE,
>> etc. A more uniform approach would be nice. We should also attempt
>> to minimize access to the global screen_info as much as possible. To
>> do so, some drivers, such as efifb and vesafb, would require an update.
>> The firmware's EDID data could possibly made available outside of fbdev.
>> For example, the simpledrm and ofdrm drivers could provide such data
>> to userspace compositors.
> 
> I suspect that most architectures that provide a screen_info only
> have this in order to compile the framebuffer drivers, and provide
> hardcoded data that does not even reflect any real hardware.

That's quite possible. Only x86's bootparam and EFI code sets 
screen_info from external data. The rest is hardcoded. A number of 
architectures protect screen_info with CONFIG_VT, CONFIG_DUMMY_CONSOLE, 
etc. In a later patchset, I wanted to change this such that these users 
of screen_info would enable the feature via select in their Kconfig.

Do you know the reason for this branch in dummycon:

https://elixir.bootlin.com/linux/v6.4/source/drivers/video/console/dummycon.c#L21

What is special about arm that dummycon uses the screeninfo?

> 
> We can probably reduce the number of architectures that do this
> a lot, especially if we get EFI out of the picture.

Can you elaborate?

Best regards
Thomas

> 
>        Arnd

-- 
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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