Re: [PATCH] PCI/VGA: Don't assume the only VGA device on a system is `boot_vga`

[Bjorn Helgaas <helgaas@kernel.org>]

On Mon, Nov 17, 2025 at 08:25:05AM +0100, Thomas Zimmermann wrote:
> Am 17.11.25 um 06:20 schrieb Mario Limonciello:
> > On 10/29/25 2:38 PM, Mario Limonciello wrote:
> > > On 10/29/25 2:34 PM, Bjorn Helgaas wrote:
> > > > On Wed, Oct 29, 2025 at 01:59:33PM -0500, Mario Limonciello
> > > > (AMD) wrote:
> > > > > Some systems ship with multiple display class devices but not all
> > > > > of them are VGA devices. If the "only" VGA device on the system is not
> > > > > used for displaying the image on the screen marking it as `boot_vga`
> > > > > because nothing was found is totally wrong.
> > > > > 
> > > > > This behavior actually leads to mistakes of the wrong device being
> > > > > advertised to userspace and then userspace can make
> > > > > incorrect decisions.
> > > > > 
> > > > > As there is an accurate `boot_display` sysfs file stop lying about
> > > > > `boot_vga` by assuming if nothing is found it's the right device.
> > > > > 
> > > > > Reported-by: Aaron Erhardt <aer@tuxedocomputers.com>
> > > > > Closes: https://bugzilla.kernel.org/show_bug.cgi?id=220712
> > > > > Tested-by: Aaron Erhardt <aer@tuxedocomputers.com>
> > > > > Signed-off-by: Mario Limonciello (AMD) <superm1@kernel.org>
> > > > 
> > > > Do we need a Fixes: here?  A stable cc?
> > > > 
> > > > The bugzilla suggests this might be a regression and hence v6.18
> > > > material?
> > > 
> > > Yeah I think you're right, we should add these two tags and this
> > > should go to 6.18 if it's a reasonable change.
> > > 
> > > Cc: stable@vger.kernel.org
> > > Fixes: ad90860bd10ee ("fbcon: Use screen info to find primary device")
> > > 
> > > But let's also make sure Thomas Zimmermann agrees with this change.
> > 
> > ping?
> 
> Sorry, this fell though the cracks. I can see that this backfires, but still
> 
> Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
> 
> because the overall logic of detecting the boot-up VGA is questionable. It
> picks the first possible candidate instead of looking at all devices before
> making the decision. If we see regressions from the patch here, this would
> be the place to fix it.

One problem is that it's hard to know when we've looked at all the
devices.  I don't think we currently have a special hook for VGA after
boot-time enumeration.

> > > > > ---
> > > > >   drivers/pci/vgaarb.c | 7 -------
> > > > >   1 file changed, 7 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/pci/vgaarb.c b/drivers/pci/vgaarb.c
> > > > > index 436fa7f4c3873..baa242b140993 100644
> > > > > --- a/drivers/pci/vgaarb.c
> > > > > +++ b/drivers/pci/vgaarb.c
> > > > > @@ -652,13 +652,6 @@ static bool vga_is_boot_device(struct
> > > > > vga_device *vgadev)
> > > > >           return true;
> > > > >       }
> > > > > -    /*
> > > > > -     * Vgadev has neither IO nor MEM enabled.  If we
> > > > > haven't found any
> > > > > -     * other VGA devices, it is the best candidate so far.
> > > > > -     */
> > > > > -    if (!boot_vga)
> > > > > -        return true;
> > > > > -
> > > > >       return false;
> > > > >   }
> > > > > -- 
> > > > > 2.43.0
> > > > > 
> > > 
> > > 
> > 
> 
> -- 
> --
> Thomas Zimmermann
> Graphics Driver Developer
> SUSE Software Solutions Germany GmbH
> Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
> GF: Jochen Jaser, Andrew McDonald, Werner Knoblich, (HRB 36809, AG Nürnberg)
> 
>