Re: [PATCH v2] fbdev: Prevent probing generic drivers if a FB is already registered

[Geert Uytterhoeven]

Hi Javier,

On Thu, Nov 11, 2021 at 12:13 PM Javier Martinez Canillas
<javierm@xxxxxxxxxx> wrote:
> 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>
> Signed-off-by: Javier Martinez Canillas <javierm@xxxxxxxxxx>
> Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> ---
>
> 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/efifb.c    | 11 +++++++++++
>  drivers/video/fbdev/simplefb.c | 11 +++++++++++
>  2 files changed, 22 insertions(+)
>
> diff --git drivers/video/fbdev/efifb.c drivers/video/fbdev/efifb.c
> index edca3703b964..ea42ba6445b2 100644
> --- drivers/video/fbdev/efifb.c
> +++ drivers/video/fbdev/efifb.c
> @@ -351,6 +351,17 @@ static int efifb_probe(struct platform_device *dev)
>         char *option = NULL;
>         efi_memory_desc_t md;
>
> +       /*
> +        * 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 (num_registered_fb > 0) {

Who says this registered fbdev is driving the same hardware as efifb?
This might be e.g. a small external display connected to i2c or spi.

> +               dev_err(&dev->dev,
> +                       "efifb: a framebuffer is already registered\n");
> +               return -EINVAL;
> +       }
> +
>         if (screen_info.orig_video_isVGA != VIDEO_TYPE_EFI || pci_dev_disabled)
>                 return -ENODEV;
>
> diff --git drivers/video/fbdev/simplefb.c drivers/video/fbdev/simplefb.c
> index 62f0ded70681..b63074fd892e 100644
> --- drivers/video/fbdev/simplefb.c
> +++ drivers/video/fbdev/simplefb.c
> @@ -407,6 +407,17 @@ static int simplefb_probe(struct platform_device *pdev)
>         struct simplefb_par *par;
>         struct resource *mem;
>
> +       /*
> +        * 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 (num_registered_fb > 0) {

Likewise.

> +               dev_err(&pdev->dev,
> +                       "simplefb: a framebuffer is already registered\n");
> +               return -EINVAL;
> +       }
> +
>         if (fb_get_options("simplefb", NULL))
>                 return -ENODEV;

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds