Re: [PATCH] x86/video: Preserve the default VGA fallback for legacy modes
From: Thomas Zimmermann
Date: Fri Oct 30 2026 - 04:48:46 EST
Hi
Am 29.10.26 um 17:23 schrieb Hengyu Liang:
Commit 596b3678326d ("x86/video: Only fall back to vga_default_device()
without screen info") made video_is_primary_device() rely exclusively
on screen_info resources whenever any are available.
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. :)
On a SeaBIOS QEMU guest with bochs and VKMS, the boot VGA loses its DRM
boot_display attribute and fbcon leaves the console on VKMS. This can
be checked with:
cat /sys/class/drm/card1/boot_display
stty -F /dev/tty1 size
Without the fix, boot_display is missing and tty1 is 48 rows by 128
columns. With it, boot_display contains 1 and tty1 is 50 rows by 160
columns.
Use resource-based selection only for linear framebuffers. Fall back to
vga_default_device() for legacy display modes, while keeping EFI and
VESA framebuffer ownership authoritative.
Fixes: 596b3678326d ("x86/video: Only fall back to vga_default_device() without screen info")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Hengyu Liang <hengyul@xxxxxxxxxx>
---
Tested in QEMU/KVM with identical KASAN configs: SeaBIOS VGA text mode
(with and without VKMS), VESA, and OVMF with one and two virtual GPUs.
All 26 guest-only helper checks pass, including EFI/VESA framebuffer
ownership, missing framebuffer resources, and addresses above 4 GiB.
Also build-tested x86-32 with/without CONFIG_SCREEN_INFO, and x86-64
without CONFIG_SCREEN_INFO.
arch/x86/video/video-common.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
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.
[1] https://elixir.bootlin.com/linux/v7.3/source/arch/x86/video/video-common.c#L53
Best regards
Thomas
base-commit: c339f4010dea8b0d754f84ace70cd1b089878ef8
--
--
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)