linux-kernel - Re: [PATCH v2 1/9] vgacon: rework Kconfig dependencies

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Message-Id: <19631e74-415e-4dcb-b79d-33dcf03d2dfc@app.fastmail.com>
Date:   Fri, 21 Jul 2023 10:26:30 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Michael Ellerman" <mpe@...erman.id.au>,
        "Arnd Bergmann" <arnd@...nel.org>, linux-fbdev@...r.kernel.org,
        "Thomas Zimmermann" <tzimmermann@...e.de>,
        "Helge Deller" <deller@....de>,
        "Javier Martinez Canillas" <javierm@...hat.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        "Ard Biesheuvel" <ardb@...nel.org>,
        "Borislav Petkov" <bp@...en8.de>, "Brian Cain" <bcain@...cinc.com>,
        "Catalin Marinas" <catalin.marinas@....com>,
        "Christophe Leroy" <christophe.leroy@...roup.eu>,
        "Daniel Vetter" <daniel@...ll.ch>,
        "Dave Hansen" <dave.hansen@...ux.intel.com>,
        "Dave Airlie" <airlied@...il.com>,
        "Deepak Rawat" <drawat.floss@...il.com>,
        "Dexuan Cui" <decui@...rosoft.com>,
        "Dinh Nguyen" <dinguyen@...nel.org>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        guoren <guoren@...nel.org>,
        "Haiyang Zhang" <haiyangz@...rosoft.com>,
        "Huacai Chen" <chenhuacai@...nel.org>,
        "Ingo Molnar" <mingo@...hat.com>,
        "John Paul Adrian Glaubitz" <glaubitz@...sik.fu-berlin.de>,
        "Khalid Aziz" <khalid@...ehiking.org>,
        "Linus Walleij" <linus.walleij@...aro.org>,
        "Matt Turner" <mattst88@...il.com>,
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        "Nicholas Piggin" <npiggin@...il.com>,
        "Palmer Dabbelt" <palmer@...belt.com>,
        "Russell King" <linux@...linux.org.uk>,
        "Thomas Bogendoerfer" <tsbogend@...ha.franken.de>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "WANG Xuerui" <kernel@...0n.name>, "Wei Liu" <wei.liu@...nel.org>,
        "Will Deacon" <will@...nel.org>, x86@...nel.org,
        linux-alpha@...r.kernel.org, linux-kernel@...r.kernel.org,
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        linux-hyperv@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v2 1/9] vgacon: rework Kconfig dependencies

On Fri, Jul 21, 2023, at 06:59, Michael Ellerman wrote:
> Arnd Bergmann <arnd@...nel.org> writes:
>> From: Arnd Bergmann <arnd@...db.de>
>>
>> The list of dependencies here is phrased as an opt-out, but this is missing
>> a lot of architectures that don't actually support VGA consoles, and some
>> of the entries are stale:
>>
>>  - powerpc used to support VGA consoles in the old arch/ppc codebase, but
>>    the merged arch/powerpc never did
>
> Not disputing this, but how did you come to that conclusion? I grepped
> around and couldn't convince myself whether it can work on powerpc or
> not. ie. currently it's possible to enable CONFIG_VGA_CONSOLE and
> powerpc does have a struct screen_info defined which seems like it would
> allow vgacon_startup() to complete.

The VGA console needs both screen_info and vga_con to work. In arch/ppc
we had both, but in arch/powerpc we only retained the screen_info:

$ git grep vga_con v2.6.26 -- arch/ppc arch/ppc64 arch/powerpc
v2.6.26:arch/ppc/platforms/pplus.c:     conswitchp = &vga_con;
v2.6.26:arch/ppc/platforms/prep_setup.c:        conswitchp = &vga_con;

so after arch/ppc was removed, this became impossible to use on both
pplus and prep. These two platforms were also (as far as I can tell)
the only ones to support vga16fb as an alternative to vgacon, but
both platforms were removed later on.

> My only concern is that someone could be using it with Qemu?

I have not yet ruled out anyone using vga16fb on qemu before
commit 0db5b61e0dc07 ("fbdev/vga16fb: Create EGA/VGA devices
in sysfb code"), but I can see that this has been broken for
12 months without anyone complaining about it, since vga16fb
no longer works with the "orig_video_isVGA == 1" setting
in arch/powerpc (the device is not created).

In the qemu sources, I see five powerpc machines that intialize
VGA support: mac_newworld, mac_oldworld, pegasos2, prep, and spapr.
I think we can exclude prep (which was removed from the kernel)
and spapr (64-bit VGA_MAP_MEM() looks broken). I think the
macs always come up in graphical mode and only use
offb/atifb/rivafb/matroxfb but not vga16fb that would require
running the x86 VGA BIOS initialization.

I suppose it's possible to use vga16fb (not vgacon) with
"qemu-system-ppc -M pegasos2 -vga std" if that still boots
at all. Support for pegasos2 hardware appears to have been
removed with commit 04debf21fa174 ("powerpc: Remove core
support for Marvell mv64x60 hostbridges"), but it's possible
that this did not break qemu support if that only uses
devices under arch/powerpc/platforms/chrp/pci.c. I could
not get it to boot, but did not try very hard.

      Arnd