On Tuesday, July 03, 2018 01:18:57 PM Mikulas Patocka wrote:

On Tue, 3 Jul 2018, Bartlomiej Zolnierkiewicz wrote:

...

Hi,

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

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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.

Even if it works okay currently it is not a best design to send duplicate events - especially since this can be easily avoided (for non-udlfb users) by:

- renaming "vanilla" unlink_framebuffer() to __unlink_framebuffer()

- converting do_unregister_framebuffer() to use __unlink_framebuffer()

- adding "new" unlink_framebuffer() that will also call unbind_console()

...

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. So, the udlfb driver waits until the open count drops to zero and then calls unregister_framebuffer().

But the console subsystem keeps the framebuffer open - which means that if user use unplugs the USB adapter, the open count won't drop to zero (because the console is bound to it) - which means that unregister_framebuffer() will not be called.

Is it a really the console subsystem and not the user-space keeping /dev/fb0 (with console binded to fb0) opened after the USB device vanishes? After re-plugging the USB device /dev/fb0 stays and /dev/fb1 appears, right?

I also mean that unregister_framebuffer() should be called instead unlink_framebuffer(), not additionally some time later as it is done currently.

Moreover the dlfb <-> fb_info locking scheme seems to be reversed (+racy) as it is dlfb that should control lifetime of fb_info, then in dlfb_free() we should just call framebuffer_release() etc.

BTW comment in dlfb_ops_release():

/* We can't free fb_info here - fbmem will touch it when we return */

seems to be wrong as fbmem keeps an extra reference on fb_info during ->fb_release().

You must unbind the console before calling unregister_framebuffer(). The

Hmm? The first thing that [do_]unregister_framebuffer) does seems to be unbinding the console.

PCI framebuffer drivers don't have this problem because the user is not expected to just unplug the PCI card while it is being used by the console.

PCI framebuffer drivers currently don't use .suppress_bind_attrs driver flag so the PCI devices can be unbinded at any time by using sysfs "unbind" functionality (I guess we should be using .suppress_bind_attrs flag if it doesn't work currently).

Mikulas

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---

drivers/video/fbdev/core/fbmem.c | 21 +++++++++++++++++---- 1 file changed, 17 insertions(+), 4 deletions(-)

Index: linux-4.16.12/drivers/video/fbdev/core/fbmem.c

--- linux-4.16.12.orig/drivers/video/fbdev/core/fbmem.c 2018-05-26 06:13:20.000000000 +0200 +++ linux-4.16.12/drivers/video/fbdev/core/fbmem.c 2018-05-26 06:13:20.000000000 +0200 @@ -1805,12 +1805,12 @@ static int do_register_framebuffer(struc return 0; }

-static int do_unregister_framebuffer(struct fb_info *fb_info) +static int unbind_console(struct fb_info *fb_info) { struct fb_event event;

• int i, ret = 0;

• int ret;
• int i = fb_info->node;

• i = fb_info->node; if (i < 0 || i >= FB_MAX || registered_fb[i] != fb_info) return -EINVAL;

@@ -1825,6 +1825,16 @@ static int do_unregister_framebuffer(str unlock_fb_info(fb_info); console_unlock();

• return ret;

+}

+static int do_unregister_framebuffer(struct fb_info *fb_info) +{

• struct fb_event event;
• int ret;
• ret = unbind_console(fb_info);
• if (ret) return -EINVAL;

@@ -1835,7 +1845,7 @@ static int do_unregister_framebuffer(str (fb_info->pixmap.flags & FB_PIXMAP_DEFAULT)) kfree(fb_info->pixmap.addr); fb_destroy_modelist(&fb_info->modelist);

• registered_fb[i] = NULL;

• registered_fb[fb_info->node] = NULL; num_registered_fb--; fb_cleanup_device(fb_info); event.info = fb_info;

@@ -1860,6 +1870,9 @@ int unlink_framebuffer(struct fb_info *f device_destroy(fb_class, MKDEV(FB_MAJOR, i)); fb_info->dev = NULL; }

• unbind_console(fb_info);
• return 0;

} EXPORT_SYMBOL(unlink_framebuffer);

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