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

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Message-Id: <dd5aa01e-afad-48d2-bf4c-4a58b74f1644@app.fastmail.com>
Date:   Fri, 30 Jun 2023 13:53:24 +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 Fri, Jun 30, 2023, at 09:46, Thomas Zimmermann wrote:
> Am 29.06.23 um 15:21 schrieb Arnd Bergmann:
>> 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:
>
>>>
>>> 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.
>
> The idea here is that ARCH_HAS_ signals the architecture's support for 
> the feature.  Drivers set 'depends on' in their Kconfig.
>
> Another Kconfig token, VIDEO_SCREEN_INFO or FIRMWARE_EDID, would then 
> actually enable the feature.  Drivers select VIDEO_SCREEN_INFO or 
> FIRMWARE_EDID and the architectures contains code like

Fair enough. In that case, I guess FIRMWARE_EDID will just depend on
ARCH_HAS_EDID_INFO, or possibly "depends on FIRMWARE_EDID || EFI"
after it starts calling into an EFI specific function, right?

> #ifdef VIDEO_SCREEN_INFO
> struct screen_info screen_info = {
> 	/* set values here */
> }
> #endif
>
> This allows us to disable code that requires screen_info/edid_info, but 
> also disable screen_info/edid_info unless such code has been enabled in 
> the kernel config.
>
> Some architectures currently mimic this by guarding screen_info with 
> ifdef CONFIG_VT or similar. I'd like to make this more flexible. The 
> cost of a few more internal Kconfig tokens seems negligible.

I definitely get it for the screen_info, which needs the complexity.
For ARCHARCH_HAS_EDID_INFO I would hope that it's never selected by
anything other than x86, so I would still go with just a dependency
on x86 for simplicity, but I don't mind having the extra symbol if that
keeps it more consistent with how the screen_info is handled.

>> 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.
>
> The current code is build on top of screen_info and edid_info. I'd 
> preferably not replace that, if possible.

One way I could imagine this looking in the end would be
something like

struct screen_info *fb_screen_info(struct device *dev)
{
      struct screen_info *si = NULL;

      if (IS_ENABLED(CONFIG_EFI))
            si = efi_get_screen_info(dev);

      if (IS_ENABLED(CONFIG_ARCH_HAS_SCREEN_INFO) && !si)
            si = screen_info;

      return si;
}

corresponding to fb_firmware_edid(). With this, any driver
that wants to access screen_info would call this function
instead of using the global pointer, plus either NULL pointer
check or a CONFIG_ARCH_HAS_SCREEN_INFO dependency.

This way we could completely eliminate the global screen_info
on arm64, riscv, and loongarch but still use the efi and
hyperv framebuffer/drm drivers.

    Arnd