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

[Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>]

On Wednesday, July 04, 2018 05:31:19 PM Noralf Trønnes wrote:
> 
> Den 03.07.2018 19.18, skrev Mikulas Patocka:
> >
> > On Tue, 3 Jul 2018, Bartlomiej Zolnierkiewicz wrote:
> >
> >> Hi,
> >>
> >> On Sunday, June 03, 2018 11:46:29 AM Mikulas Patocka wrote:
> >>> I have a USB display adapter using the udlfb driver and I use it on an ARM
> >>> board that doesn't have any graphics card. When I plug the adapter in, the
> >>> console is properly displayed, however when I unplug and re-plug the
> >>> adapter, the console is not displayed and I can't access it until I reboot
> >>> the board.
> >>>
> >>> The reason is this:
> >>> When the adapter is unplugged, dlfb_usb_disconnect calls
> >>> unlink_framebuffer, then it waits until the reference count drops to zero
> >>> and then it deallocates the framebuffer. However, the console that is
> >>> attached to the framebuffer device keeps the reference count non-zero, so
> >>> the framebuffer device is never destroyed. When the USB adapter is plugged
> >>> again, it creates a new device /dev/fb1 and the console is not attached to
> >>> it.
> >>>
> >>> This patch fixes the bug by unbinding the console from unlink_framebuffer.
> >>> The code to unbind the console is moved from do_unregister_framebuffer to
> >>> a function unbind_console. When the console is unbound, the reference
> >>> count drops to zero and the udlfb driver frees the framebuffer. When the
> >>> adapter is plugged back, a new framebuffer is created and the console is
> >>> attached to it.
> >>>
> >>> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> >>> Cc: stable@vger.kernel.org
> >> After this change unbind_console() will be called twice in the standard
> >> framebuffer unregister path:
> >>
> >> - first time, directly by do_unregister_framebuffer()
> >>
> >> - second time, indirectly by do_unregister_framebuffer()->unlink_framebuffer()
> >>
> >> This doesn't look correctly.
> > unbind_console calls the FB_EVENT_FB_UNBIND notifier, FB_EVENT_FB_UNBIND
> > goes to the function fbcon_fb_unbind and fbcon_fb_unbind checks if the
> > console is bound to the framebuffer for which unbind is requested. So a
> > double call won't cause any trouble.
> >
> >> Also why can't udlfb just use unregister_framebuffer() like all other
> >> drivers (it uses unlink_framebuffer() and it is the only user of this
> >> helper)?
> > It uses unregister_framebuffer() - but - unregister_framebuffer() may only
> > be called when the open count of the framebuffer is zero.
> 
> 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;
>                  }
>          }

Looks fine, could you please submit this as a proper patch
(with S-o-b tag and patch description)? Thanks.

Best regards,
--
Bartlomiej Zolnierkiewicz
Samsung R&D Institute Poland
Samsung Electronics