Re: Multiple clients reading from /dev/input/eventX

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

On Wed, Oct 24, 2012 at 12:26:17AM -0400, simon@mungewell.org wrote:
> >> Can anyone here confirm the behaviour under the situation that one
> >> client
> >> is busy? What is the maximum time it can be 'away' or what is the size
> >> of
> >> the inline buffer?
> >
> > Every user of event device has a buffer that can hold up at least 64
> > input events and is completely independent from other users. Newer
> > kernels generate EV_SYN/SYN_DROPPED event to indicate that user was too
> > slow reading data and there was buffer overrun.
> >
> > Hope this helps.
> 
> Yes, very helpful.... I am seeing 'SYN_DROPPED' on my system, so the next
> question is how to work around this?
> 
> In the event of an overflow is there any method of reading/polling the
> last (correct) event value for each of the axis?

EVIOCGABS() and other ioctls allow to query state of the device.

> 
> In my case on the G27 wheel when your foot transitions from the brake to
> accelerator, you get no more 'brake events' to 'refresh' the value.
> 
> I see that I could use QUIRK to enlarge the buffer for this device, would
> that be an acceptable workaround?
> http://www.spinics.net/lists/linux-input/msg15497.html

With 3.7 Henrik just put in code that would better estimate number of
events in devices event packet, so hopefully quirk is not needed.

> 
> Attached is a little log showing SDL 'missing' 68 events, which are
> correctly reported by 'evtest'. The time stamps in log will give you an
> idea about how quickly the events are coming in from this wheel.

You need to figure out why SDL takes such a long time to get around
reading device data

Thanks.

-- 
Dmitry