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

["Arnd Bergmann" <arnd@arndb.de>]

On Fri, Jul 21, 2023, at 06:59, Michael Ellerman wrote:
> Arnd Bergmann <arnd@kernel.org> writes:
>> From: Arnd Bergmann <arnd@arndb.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