linux-kernel - Re: [PATCH 07/12] arch/x86: Declare edid_info in <asm/screen

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Message-Id: <0dbbdfc4-0e91-4be4-9ca0-d8ba6f18453d@app.fastmail.com>
Date:   Thu, 29 Jun 2023 15:21:39 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Thomas Zimmermann" <tzimmermann@...e.de>,
        "Helge Deller" <deller@....de>, "Daniel Vetter" <daniel@...ll.ch>,
        "Dave Airlie" <airlied@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-alpha@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-efi@...r.kernel.org,
        "linux-csky@...r.kernel.org" <linux-csky@...r.kernel.org>,
        linux-hexagon@...r.kernel.org, linux-ia64@...r.kernel.org,
        loongarch@...ts.linux.dev, linux-mips@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-hyperv@...r.kernel.org,
        linux-fbdev@...r.kernel.org, linux-staging@...ts.linux.dev,
        Linux-Arch <linux-arch@...r.kernel.org>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...hat.com>, "Borislav Petkov" <bp@...en8.de>,
        "Dave Hansen" <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        "Kees Cook" <keescook@...omium.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        "Peter Zijlstra" <peterz@...radead.org>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Frederic Weisbecker" <frederic@...nel.org>,
        "Nicholas Piggin" <npiggin@...il.com>,
        "Ard Biesheuvel" <ardb@...nel.org>,
        "Sami Tolvanen" <samitolvanen@...gle.com>,
        "Juerg Haefliger" <juerg.haefliger@...onical.com>
Subject: Re: [PATCH 07/12] arch/x86: Declare edid_info in <asm/screen_info.h>

On Thu, Jun 29, 2023, at 15:01, Thomas Zimmermann wrote:
> Am 29.06.23 um 14:35 schrieb Arnd Bergmann:
>> On Thu, Jun 29, 2023, at 13:45, Thomas Zimmermann wrote:
>>> The global variable edid_info contains the firmware's EDID information
>>> as an extension to the regular screen_info on x86. Therefore move it to
>>> <asm/screen_info.h>.
>>>
>>> Add the Kconfig token ARCH_HAS_EDID_INFO to guard against access on
>>> architectures that don't provide edid_info. Select it on x86.
>> 
>> I'm not sure we need another symbol in addition to
>> CONFIG_FIRMWARE_EDID. Since all the code behind that
>> existing symbol is also x86 specific, would it be enough
>> to just add either 'depends on X86' or 'depends on X86 ||
>> COMPILE_TEST' there?
>
> FIRMWARE_EDID is a user-selectable feature, while ARCH_HAS_EDID_INFO 
> announces an architecture feature. They do different things.

I still have trouble seeing the difference.

> Right now, ARCH_HAS_EDID_INFO only works on the old BIOS-based VESA 
> systems. In the future, I want to add support for EDID data from EFI and 
> OF as well. It would be stored in edid_info. I assume that the new 
> symbol will become useful then.

I don't see why an OF based system would have the same limitation
as legacy BIOS with supporting only a single monitor, if we need
to have a generic representation of EDID data in DT, that would
probably be in a per device property anyway.

I suppose you could use FIRMWARE_EDID on EFI or OF systems without
the need for a global edid_info structure, but that would not
share any code with the current fb_firmware_edid() function.

     Arnd