USB GPU bugs

Pekka Paalanen ppaalanen at
Wed Oct 9 10:23:58 UTC 2019

On Wed, 9 Oct 2019 09:59:29 +0200
Hans de Goede <hdegoede at> wrote:

> Hi Pekka,
> On 09-10-2019 09:47, Pekka Paalanen wrote:
> > On Tue, 8 Oct 2019 22:19:45 +0200
> > Hans de Goede <hdegoede at> wrote:
> >   
> >> My main reason for suggesting either one is that I personally am aware
> >> of at least 2 issues (both related to secondary USB GPUs handling) which
> >> are only present in master and not in the 1.20 branch and which I really
> >> would like to see fixed before a new release. I have taking a look at
> >> these on my to do list, but not at the top of it (yet).  
> > 
> > Hi Hans,
> > 
> > it so happens that I am, too, looking at some secondary DRM device
> > hotplug issues. I think I've seen three different things: not enough
> > RandR events to make Mutter take a hotplugged device & output into use,
> > Xorg crash on DRM device hotplug, and failure to start with Xorg USB
> > DRM device plugged in.
> > 
> > Do you have any public records of the issues you have on your plate?  
> No I've not filed issues yet, since I was planning on debugging them
> myself, but if are both seeing these issues then having gitlab
> issues to track them might be a good idea.
> I am using a gm12u320 based mini projector (Acer C120) for most of
> my tests, but I believe that this will reproduce with an udl (USB 2 version)
> device too. If necessary I can try to reproduce with an udl device too.

Hi Hans,

I'm working with a Dell DisplayLink dock, USB 3 version, so it's using
the proprietary userspace and the EVDI out-of-tree kernel driver (open

I'd prefer talking about DRM devices or such, because these USB things
do not have a GPU. Having a GPU or not behind a DRM device is sometimes

> Thinking more about it I'm seeing 3 things:
> 1. USB GPU is not seen (not shown in xrandr --list-providers, not usable)
>     when present before Xorg is started.

For me, this case seems to stop Xorg from starting:

MESA-LOADER: failed to retrieve device information
MESA-LOADER: failed to open evdi (search paths /usr/lib/x86_64-linux-gnu/dri:\$${ORIGIN}/dri:/usr/lib/dri)
failed to load driver: evdi
Fatal server error:
(EE) AddScreen/ScreenInit failed for gpu driver 0 -1

> 2. Hot plugging the USB GPU sometimes causes things to crash shortly
>     afterwards.

I've seen this. Looks like a bad pointer, but I couldn't yet figure out
where from. I did get Valgrind to tell me this:

==1843== Invalid free() / delete / delete[] / realloc()
==1843==    at 0x483AD4B: realloc (in /usr/lib/x86_64-linux-gnu/valgrind/
==1843==    by 0x24BA68: XNFreallocarray (utils.c:1167)
==1843==    by 0x26D4F1: xf86SetDepthBpp (xf86Helper.c:558)
==1843==    by 0x644DB72: PreInit (driver.c:951)
==1843==    by 0x27F3B6: xf86platformAddDevice (xf86platformBus.c:638)
==1843==    by 0x29E1F0: NewGPUDeviceRequest (lnx_platform.c:176)
==1843==    by 0x2C27FB: device_added (udev.c:140)
==1843==    by 0x2C2D9F: socket_handler (udev.c:375)
==1843==    by 0x249B40: ospoll_wait (ospoll.c:657)
==1843==    by 0x243682: WaitForSomething (WaitFor.c:208)
==1843==    by 0x17ADBB: Dispatch (dispatch.c:421)
==1843==    by 0x17EFF5: dix_main (main.c:274)
==1843==  Address 0x5d982d0 is 0 bytes after an unallocated block of size 0 in arena "client"

> 3. Hot unplugging the USB GPU always causes a crash shortly
>     afterwards.

Unplugging does not happen in my case, because EVDI does not remove the
DRM device. It simply marks the connector as disconnected instead, and
the DRM device is re-used for the next DisplayLink device.

> My test environment is xserver master + mutter/gnome-shell 3.34 on
> top of Xorg. Last time I tried downgrading the xserver to 1.20 made all
> 3 issues go away (*)

Good to know. I was debugging a first hotplug issue on
gnome-shell/mutter from git, a revision within the past few weeks.
Xserver was stock Ubuntu 19.04 (1.20.4), but then I wanted to add
prints in it, switched to git master without thinking, and started
seeing these crashes.

> I think I might also be seeing some variant of your "not enough hotplug
> events" bug. With the gm12u320 projector when hotplugged it is listed
> as "unknown" in gnome's display-settings until I change the settings
> once and then it becomes "Acer" as it should be. I believe that this
> issue is actually also present in the 1.20 branch.

In my case, I see an "add" and three HOTPLUG events via udev when I
hotplug the DisplayLink dock and it creates the DRM device node on the
spot. Mutter receives just one RandR event though, which it deems is
not a hotplug, and looking at the config timestamps I think the event
indeed is reflecting too old state. 'xrandr' will show everything about
the new connector just fine though, IIRC.

Knowing that there are regressions in xserver master, I will first
concentrate on my primary issue with the "not enough hotplug events".
Once I have that sorted out, I might be able to help with the other

Let's keep each other updated, so we don't do duplicate effort. :-)

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