[PATCH] fb: fix lost console when the user unplugs a USB adapter

[Mikulas Patocka]

On Wed, 4 Jul 2018, Noralf Trønnes wrote:

> AFAIU calling unregister_framebuffer() with open fd's is just fine as
> long as fb_info with buffers stay intact. All it does is to remove the
> fbX from userspace. Cleanup can be done in fb_ops->fb_destroy.
> 
> I have been working on generic fbdev emulation for DRM [1] and I did a
> test now to see what would happen if I did unbind the driver from the
> device. It worked as expected if I didn't have another fbdev present,
> but if there is an fb0 and I remove fb1 with a console on it, I would
> sometimes get crashes, often with a call to cursor_timer_handler() in
> the traceback.
> 
> I think there's index mixup in fbcon_fb_unbind(), at least this change
> seems to solve the immediate problem:
> 
> diff --git a/drivers/video/fbdev/core/fbcon.c
> b/drivers/video/fbdev/core/fbcon.c
> index 5fb156bdcf4e..271b9b988b73 100644
> --- a/drivers/video/fbdev/core/fbcon.c
> +++ b/drivers/video/fbdev/core/fbcon.c
> @@ -3066,7 +3072,7 @@ static int fbcon_fb_unbind(int idx)
>         for (i = first_fb_vc; i <= last_fb_vc; i++) {
>                 if (con2fb_map[i] != idx &&
>                     con2fb_map[i] != -1) {
> -                       new_idx = i;
> +                       new_idx = con2fb_map[i];
>                         break;
>                 }
>         }

Reviewed-by: Mikulas Patocka <mpatocka at redhat.com>

Yes - that's a good point. i is virtual console index and it is assigned 
to new_idx and new_idx is interpreted as a framebuffer device index.

> I haven't got time to follow up on this now, but making sure DRM generic
> fbdev emulation device unplug works is on my TODO.
> 
> BTW, I believe the udl drm driver should be able to use the generic fbdev
> emulation if it had a drm_driver->gem_prime_vmap hook. It uses a shadow
> buffer which would also make fbdev mmap work for udl. (shmem buffers and
> fbdev deferred I/O doesn't work together since they both use
> page->lru/mapping)

I have sent patch for this:
https://patchwork.kernel.org/patch/10445393/

Mikulas