On 11/16/21 11:01, Javier Martinez Canillas wrote: > Hello Geert, > > On 11/16/21 10:43, Geert Uytterhoeven wrote: > > [snip] > >>> >>> So this is already a fragile solution and $SUBJECT doesn't make things worse >>> IMO. Since not having something like this can lead to issues as reported by: >>> >>> https://lore.kernel.org/all/20211110200253.rfudkt3edbd3nsyj@lahvuun/ >>> >>> We could probably do some smarter here by providing a function that checks >>> if the registered fbdev drivers matches the aperture base. But I'm unsure >>> if that's worth it. After all, fbdev drivers are likely to be disabled by >>> most distros soon now that we have the simpledrm driver. >> >> Checking the aperture base is what was done in all other cases of >> preventing generic (fbdev) drivers from stepping on specific drivers' >> toes... >> > > Ok, I can re-spin the patch checking if the aperture ranges overlap. There's > an apertures_overlap() function in drivers/video/fbdev/core/fbmem.c that can > be exported for fbdev drivers to use. > So I tried the following patch [0]. But when testing on a VM, the efifb driver is probed even after the virtio_gpu driver has already been probed. Being a DRM driver, it doesn't use the fbdev infra and AFAIU doesn't reserve any apertures. When the {efi,simple}fb drivers check if there's an aperture already reserved using the fb_aperture_registered() helper, this just returns false even when a driver for the same hardware was already registered. The kernel log says: [ 0.891512] checking generic (0 0) vs hw (c0000000 1d5000) That is because when DRM_FBDEV_EMULATION=y, the virtio_gpu driver registers an fbdev but without any aperture set. I discussed this with Thomas and even though $SUBJECT is just a workaround, it seems that is the best we can do as an heuristic to prevent the generic fbdev drivers to be probed after a native DRM driver. [0]: >From d962c20bc9fd90c2525d79b69e632d99e8050fc5 Mon Sep 17 00:00:00 2001 From: Javier Martinez Canillas <javierm@xxxxxxxxxx> Date: Thu, 11 Nov 2021 00:55:06 +0100 Subject: [PATCH v4] fbdev: Prevent probing generic drivers if a FB is already registered The efifb and simplefb drivers just render to a pre-allocated frame buffer and rely on the display hardware being initialized before the kernel boots. But if another driver already probed correctly and registered a fbdev, the generic drivers shouldn't be probed since an actual driver for the display hardware is already present. This is more likely to occur after commit d391c5827107 ("drivers/firmware: move x86 Generic System Framebuffers support") since the "efi-framebuffer" and "simple-framebuffer" platform devices are registered at a later time. Link: https://lore.kernel.org/r/20211110200253.rfudkt3edbd3nsyj@lahvuun/ Fixes: d391c5827107 ("drivers/firmware: move x86 Generic System Framebuffers support") Reported-by: Ilya Trukhanov <lahvuun@xxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> # 5.15.x Signed-off-by: Javier Martinez Canillas <javierm@xxxxxxxxxx> --- Changes in v4: - Only fail to probe if a registered fbdev has overlapping aperture (Geert). Changes in v3: - Cc <stable@xxxxxxxxxxxxxxx> since a Fixes: tag is not enough (gregkh). Changes in v2: - Add a Link: tag with a reference to the bug report (Thorsten Leemhuis). - Add a comment explaining why the probe fails earlier (Daniel Vetter). - Add a Fixes: tag for stable to pick the fix (Daniel Vetter). - Add Daniel Vetter's Reviewed-by: tag. - Improve the commit message and mention the culprit commit drivers/video/fbdev/core/fbmem.c | 16 ++++++++++++++++ drivers/video/fbdev/efifb.c | 11 +++++++++++ drivers/video/fbdev/simplefb.c | 11 +++++++++++ include/linux/fb.h | 1 + 4 files changed, 39 insertions(+) diff --git drivers/video/fbdev/core/fbmem.c drivers/video/fbdev/core/fbmem.c index 826175ad88a2..9906b83132cb 100644 --- drivers/video/fbdev/core/fbmem.c +++ drivers/video/fbdev/core/fbmem.c @@ -1546,6 +1546,22 @@ static bool fb_do_apertures_overlap(struct apertures_struct *gena, return false; } +bool fb_aperture_registered(struct apertures_struct *a) +{ + int i; + + for_each_registered_fb(i) { + struct apertures_struct *gen_aper; + + gen_aper = registered_fb[i]->apertures; + if (fb_do_apertures_overlap(gen_aper, a)) + return true; + } + + return false; +} +EXPORT_SYMBOL(fb_aperture_registered); + static void do_unregister_framebuffer(struct fb_info *fb_info); #define VGA_FB_PHYS 0xA0000 diff --git drivers/video/fbdev/efifb.c drivers/video/fbdev/efifb.c index edca3703b964..1ad6698b2e05 100644 --- drivers/video/fbdev/efifb.c +++ drivers/video/fbdev/efifb.c @@ -457,6 +457,17 @@ static int efifb_probe(struct platform_device *dev) info->apertures->ranges[0].base = efifb_fix.smem_start; info->apertures->ranges[0].size = size_remap; + /* + * Generic drivers must not be registered if a framebuffer exists. + * If a native driver was probed, the display hardware was already + * taken and attempting to use the system framebuffer is dangerous. + */ + if (fb_aperture_registered(info->apertures)) { + dev_err(&dev->dev, + "efifb: a framebuffer is already registered\n"); + return -EINVAL; + } + if (efi_enabled(EFI_MEMMAP) && !efi_mem_desc_lookup(efifb_fix.smem_start, &md)) { if ((efifb_fix.smem_start + efifb_fix.smem_len) > diff --git drivers/video/fbdev/simplefb.c drivers/video/fbdev/simplefb.c index 62f0ded70681..3ad0f538ca91 100644 --- drivers/video/fbdev/simplefb.c +++ drivers/video/fbdev/simplefb.c @@ -456,6 +456,17 @@ static int simplefb_probe(struct platform_device *pdev) info->apertures->ranges[0].base = info->fix.smem_start; info->apertures->ranges[0].size = info->fix.smem_len; + /* + * Generic drivers must not be registered if a framebuffer exists. + * If a native driver was probed, the display hardware was already + * taken and attempting to use the system framebuffer is dangerous. + */ + if (fb_aperture_registered(info->apertures)) { + dev_err(&pdev->dev, + "simplefb: a framebuffer is already registered\n"); + return -EINVAL; + } + info->fbops = &simplefb_ops; info->flags = FBINFO_DEFAULT | FBINFO_MISC_FIRMWARE; info->screen_base = ioremap_wc(info->fix.smem_start, diff --git include/linux/fb.h include/linux/fb.h index 6f3db99ab990..f1fbdb39932b 100644 --- include/linux/fb.h +++ include/linux/fb.h @@ -604,6 +604,7 @@ extern ssize_t fb_sys_write(struct fb_info *info, const char __user *buf, size_t count, loff_t *ppos); /* drivers/video/fbmem.c */ +extern bool fb_aperture_registered(struct apertures_struct *a); extern int register_framebuffer(struct fb_info *fb_info); extern void unregister_framebuffer(struct fb_info *fb_info); extern int remove_conflicting_pci_framebuffers(struct pci_dev *pdev, -- 2.33.1 Best regards, -- Javier Martinez Canillas Linux Engineering Red Hat
