Re: 2.6.36/2.6.37: broken compatibility with userspace input-utils ?

Mauro Carvalho Chehab
2011-01-27T18:59:37Z

Em 27-01-2011 15:21, Dmitry Torokhov escreveu:
> On Thu, Jan 27, 2011 at 08:30:00AM -0200, Mauro Carvalho Chehab wrote:
>>
>> On my tests here, this is working fine, with Fedora and RHEL 6, on my
>> usual test devices, so I don't believe that the tool itself is broken,
>> nor I think that the issue is due to the fix patch.
>>
>> I remember that when Kay added a persistence utility tool that opens a V4L
>> device in order to read some capabilities, this caused a race condition
>> into a number of drivers that use to register the video device too early.
>> The result is that udev were opening the device before the end of the
>> register process, causing OOPS and other problems.
>
> Well, this is quite possible. The usev ruls in the v4l-utils reads:
>
> ACTION=="add", SUBSYSTEM=="rc", RUN+="/usr/bin/ir-keytable -a /etc/rc_maps.cfg -s $name"
>
> So we act when we add RC device to the system. The corresponding input
> device has not been registered yet (and will not be for some time
> because before creating input ddevice we invoke request_module() to load
> initial rc map module) so the tool runs simultaneously with kernel
> registering input device and it could very well be it can't find
> something it really wants.
>
> This would explain why Mark sees the segfault only when invoked via
> udev but not when ran manually.
>
> However I still do not understand why Mark does not see the same issue
> without the patch. Like I said, maybe if Mark could recompile with
> debug data and us a core we'd see what is going on.

Race conditions are hard to track... probably the new code added some delay,
and this allowed the request_module() to finish his job.

> BTW, that means that we need to redo udev rules.

If there's a race condition, then the proper fix is to lock the driver
until it is ready to receive a fops. Maybe we'll need a mutex to preventing
opening the device until it is completely initialized.

It is hard to tell, as Mark didn't provide us yet the dmesg info (at least
on the emails I was c/c), so I don't even know what device he has, and what
drivers are used.

> Maybe we should split
> the utility into 2 parts - one dealing with rcX device and for keymap
> setting reuse udev's existing utility that adjusts maps on ann input
> devices, not for RCs only.

It could be done, but then we'll need to pollute the existing input tools
with RC-specific stuff. For IR, there are some additional steps, like
the need to select the IR protocol, otherwise the keytable is useless.
Also, the keytable and persistent info is provided via /sys/class/rc/rc?/uevent.
So, the tool need to first read the RC class, check what keytable should be
associated with that device (based on a custom file), and load the proper
table.

Also, I'm currently working on a way to map media keys for remote controllers
into X11 (basically, mapping them into the keyspace between 8-255, passing
through Xorg evdev.c, and then mapping back into some X11 symbols). This way,
we don't need to violate the X11 protocol. (Yeah, I know this is hacky, but
while X11 cannot pass the full evdev keycode, at least the Remote Controllers
will work). This probably means that we may need to add some DBus logic
inside ir-keytable, when called via udev, to allow it to announce to X11.

Regards,
Mauro