[Bug 89549] DisplayLink (udl) Linux kernel memory fault on USB hot unplug, proposed patch

[bugzilla-daemon@freedesktop.org]

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https://bugs.freedesktop.org/show_bug.cgi?id=89549

            Bug ID: 89549
           Summary: DisplayLink (udl)  Linux kernel memory fault on USB
                    hot unplug, proposed patch
           Product: DRI
           Version: unspecified
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: DRM/other
          Assignee: dri-devel@lists.freedesktop.org
          Reporter: adam_richter2004@yahoo.com

Created attachment 114235
  --> https://bugs.freedesktop.org/attachment.cgi?id=114235&action=edit
Add ".get_vblank_counter = drm_vblank_count," to DisplayLink drm_driver
functions

[This is an edit of an email that I sent yesterday to linux-fbdev at
vger.kernel.org and the Linux kernel udlfb maintainer.]

In Linux-4.0-rc3, the DisplayLink driver gets a kernel memory fault when its
USB device is unplugged, specifically when vblank_disable_and_save in
linux-4.0-rc3/drivers/gpu/drm/drm_irq.c attempts to call the NULL function
pointer dev->driver_get_vblank_counter.

linux-4.0-rc3/Documentation/DocBook/drm.tmpl says of that field, "Otherwise
drivers can use the drm_vblank_count helper function to handle this operation."
 Trying that seems to have eliminated the oops when I unplug the device,
although I do see these lines in the console log at device driver
initialization time that I think are probably fine, but which I should mention
in case the indicate that I did the wrong the thing:

[drm] Supports vblank timestamp caching Rev 2 (21.10.2013).
[drm] No driver support for vblank timestamp query.

If that doesn't look like a problem, then please apply the attached patch and
push it upstream.  The patch just adds one line that appears in six other
framebuffer drivers to initialize the driver_get_vblank_counter drm_driver
function.

Thanks in advance for your attention to this problem.

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