[v2,1/9] vgacon: rework Kconfig dependencies

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

 - arm lists footbridge, integrator and netwinder, but netwinder is actually
   part of footbridge, and integrator does not appear to have an actual
   VGA hardware, or list it in its ATAG or DT.

 - mips has a few platforms (malta, sibyte, and sni) that initialize
   screen_info, on everything else the console is selected but cannot
   actually work.

 - csky, hexgagon, loongarch, nios2, riscv and xtensa are not listed
   in the opt-out table and declare a screen_info to allow building
   vga_con, but this cannot work because the console is never selected.

Replace this with an opt-in table that lists only the platforms that
remain. This is effectively x86, plus a couple of historic workstation
and server machines that reused parts of the x86 system architecture.

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 drivers/video/console/Kconfig | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

On 7/19/23 6:39 AM, Arnd Bergmann wrote:
> 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
> 
>   - arm lists footbridge, integrator and netwinder, but netwinder is actually
>     part of footbridge, and integrator does not appear to have an actual
>     VGA hardware, or list it in its ATAG or DT.
> 
>   - mips has a few platforms (malta, sibyte, and sni) that initialize
>     screen_info, on everything else the console is selected but cannot
>     actually work.
> 
>   - csky, hexgagon, loongarch, nios2, riscv and xtensa are not listed
>     in the opt-out table and declare a screen_info to allow building
>     vga_con, but this cannot work because the console is never selected.
> 
> Replace this with an opt-in table that lists only the platforms that
> remain. This is effectively x86, plus a couple of historic workstation
> and server machines that reused parts of the x86 system architecture.
> 
> Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
> Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>


Reviewed-by: Khalid Aziz <khalid@gonehiking.org>


> ---
>   drivers/video/console/Kconfig | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/video/console/Kconfig b/drivers/video/console/Kconfig
> index 1b5a319971ed0..6af90db6d2da9 100644
> --- a/drivers/video/console/Kconfig
> +++ b/drivers/video/console/Kconfig
> @@ -7,9 +7,9 @@ menu "Console display driver support"
>   
>   config VGA_CONSOLE
>   	bool "VGA text console" if EXPERT || !X86
> -	depends on !4xx && !PPC_8xx && !SPARC && !M68K && !PARISC &&  !SUPERH && \
> -		(!ARM || ARCH_FOOTBRIDGE || ARCH_INTEGRATOR || ARCH_NETWINDER) && \
> -		!ARM64 && !ARC && !MICROBLAZE && !OPENRISC && !S390 && !UML
> +	depends on ALPHA || IA64 || X86 || \
> +		(ARM && ARCH_FOOTBRIDGE) || \
> +		(MIPS && (MIPS_MALTA || SIBYTE_BCM112X || SIBYTE_SB1250 || SIBYTE_BCM1x80 || SNI_RM))
>   	select APERTURE_HELPERS if (DRM || FB || VFIO_PCI_CORE)
>   	default y
>   	help

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.

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

cheers

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

Hi Arnd,

On Fri, Jul 21, 2023 at 10:29 AM Arnd Bergmann <arnd@arndb.de> wrote:
> 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.

I did use vgacon and vga16fb on CHRP on a second video card
(initialized using Gabriel Paubert's x86 BIOS emulator), but that was
definitely before the advent of arch/powerpc/.

Gr{oetje,eeting}s,

                        Geert