Re: [PATCH] x86/video: Preserve the default VGA fallback for legacy modes

[Thomas Zimmermann]

Hi

Am 30.10.26 um 10:24 schrieb Hengyu Liang:
> Hi Thomas,
>
> On Fri, Oct 30, 2026 at 09:48:19AM +0100, Thomas Zimmermann wrote:
> > Am 29.10.26 um 17:23 schrieb Hengyu Liang:
> > > However, screen_info_resources() also returns legacy I/O and memory
> > > ranges for VGA text modes. These ranges are not PCI BARs, so the resource
> > > lookup fails and the default VGA fallback is skipped.
> > If the BIOS booted in text mode or VGA graphics mode, the rsp device is
> > still the primary display device. So the current matching code is not
> > necessarily incorrect. (But also not necessarily correct either. :)
> I agree that the device is still the primary display in text mode. The
> patch gets that from the vga_default_device() fallback, since the legacy
> ranges can't be matched against BARs.
>
> > > diff --git a/arch/x86/video/video-common.c b/arch/x86/video/video-common.c
> > > index 8ed82fff7638..b8f83346dfe8 100644
> > > --- a/arch/x86/video/video-common.c
> > > +++ b/arch/x86/video/video-common.c
> > > @@ -45,7 +45,7 @@ bool video_is_primary_device(struct device *dev)
> > >
> > >   #ifdef CONFIG_SCREEN_INFO
> > >   	numres = screen_info_resources(si, res, ARRAY_SIZE(res));
> > > -	if (numres > 0) {
> > > +	if (numres > 0 && __screen_info_has_lfb(screen_info_video_type(si))) {
> > >   		for (i = 0; i < numres; ++i) {
> > >   			if (!(res[i].flags & IORESOURCE_MEM))
> > >   				continue;
> > Further below in that code is a call to pci_find_resource(). [1]  Why
> > does it match with vkms?  Vkms is not even on a PCI device.
> It doesn't match vkms. As you say, vkms isn't a PCI device, so
> video_is_primary_device() returns false for it at the dev_is_pci() check.
>
> The problem is on the bochs side. In text mode, screen_info_resources()
> returns the legacy VGA ranges, pci_find_resource() doesn't find them in
> the BARs, and the function returns false there as well. So no device is
> primary, and fbcon stays on the first registered framebuffer. In this
> setup that is vkms (fb0), so the console is never remapped to bochs (fb1).

I see. That makes sense. Thanks for clarifying.

Reviewed-by: Thomas Zimmermann <tzimmermann@xxxxxxx>

Best regards
Thomas

> vkms only makes the console part visible. Without vkms, tty1 has the same
> size with and without the patch, but boot_display is still missing without
> it.
>
> Thanks,
> Hengyu

--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
GF: Stefan Gaiser, Jochen Jaser, Abhinav Puri, (HRB 36809, AG Nürnberg)