[PATCH] fbdevhw: fix sysfs path PCI device detection

[Tj]

Linux kernel v6.9 has changed the symlink to point to the parent device. This breaks fbdev_open() detection logic. Change it to use the subsystem symlink instead which will remain stable.

Kernel v6.8:

[    14.067] (II) fbdev_open() sysfs_path=/sys/class/graphics/fb0
[    14.067] (II) fbdev_open() buf=../../devices/platform/vesa-framebuffer.0/graphics/fb0

Kernel v6.9:

[    15.609] (II) fbdev_open() sysfs_path=/sys/class/graphics/fb0
[    15.609] (II) fbdev_open() buf=../../devices/pci0000:00/0000:00:01.0/vesa-framebuffer.0/graphics/fb0

Originally found in automated Debian ISO QA testing [0] and confirmed in Linux [1].

Tested on kernels v6.9.7 and v6.8.12

[0] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1075713
[1] https://lore.kernel.org/lkml/lLyvPFC_APGHNfyGNHRpQy5izBikkaTPOpHooZIT3fFAoJPquSI31ZMueA99XTdr8ysir3X7O7IMdc6za-0m79vr_claeparHhoRouVgHOI=@proton.me/

Fixes: https://gitlab.freedesktop.org/xorg/xserver/-/issues/1714
Signed-off-by: Tj <tj.iam.tj at proton.me>
---
 hw/xfree86/fbdevhw/fbdevhw.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/hw/xfree86/fbdevhw/fbdevhw.c b/hw/xfree86/fbdevhw/fbdevhw.c
index 76bc4809f..019af305b 100644
--- a/hw/xfree86/fbdevhw/fbdevhw.c
+++ b/hw/xfree86/fbdevhw/fbdevhw.c
@@ -378,9 +378,9 @@ fbdev_open(int scrnIndex, const char *dev, char **namep)
             node++;
         }
 
-        if (asprintf(&sysfs_path, "/sys/class/graphics/%s", node) < 0 ||
+        if (asprintf(&sysfs_path, "/sys/class/graphics/%s/device/subsystem", node) < 0 ||
             readlink(sysfs_path, buf, sizeof(buf) - 1) < 0 ||
-            strstr(buf, "devices/pci")) {
+            strstr(buf, "bus/pci")) {
             free(sysfs_path);
             close(fd);
             return -1;
-- 
2.39.2