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

Den 03.07.2018 19.18, skrev Mikulas Patocka:

quoted

On Tue, 3 Jul 2018, Bartlomiej Zolnierkiewicz wrote:

quoted

Hi,

On Sunday, June 03, 2018 11:46:29 AM Mikulas Patocka wrote:

quoted

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.

quoted

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;
                 }
         }