Re: [PATCH] evdev: flush ABS_* events during EVIOCGABS

[Peter Hutterer <peter.hutterer@who-t.net>]

On Tue, Apr 22, 2014 at 10:46:47PM -0700, Dmitry Torokhov wrote:
> On Wed, Apr 23, 2014 at 03:38:49PM +1000, Peter Hutterer wrote:
> > On Wed, Apr 23, 2014 at 10:21:03AM +1000, Peter Hutterer wrote:
> > > On Tue, Apr 22, 2014 at 08:21:54AM +0200, David Herrmann wrote:
> > > > Hi Peter
> > > > 
> > > > On Tue, Apr 22, 2014 at 6:15 AM, Peter Hutterer
> > > > <peter.hutterer@who-t.net> wrote:
> > > > > How are you planning to handle the slot-based events? We'd either need to
> > > > > add something similar (but more complex) to evdev_handle_mt_request or rely
> > > > > on the caller to call the whole EV_ABS range and ditch anything ABS_MT_.
> > > > > I'd prefer the former, the latter is yet more behaviour that's easy to get
> > > > > wrong.
> > > > 
> > > > This is all racy..
> > > > 
> > > > We _really_ need an ioctl to receive _all_ ABS information atomically.
> > > > I mean, there's no way we can know the user's state from the kernel.
> > > > Even if the user resyncs via EVIOCGMTSLOTS, we can never flush the
> > > > whole ABS queue. Problem is, the user has to call the ioctl for _each_
> > > > available MT code and events might get queued in between. So yeah,
> > > > this patch doesn't help much..
> > > > 
> > > > I have no better idea than adding a new EVIOCGABS call that retrieves
> > > > ABS values for all slots atomically (and for all other axes..). No
> > > > idea how to properly fix the old ioctls.
> > > 
> > > bonus points for making that ioctl fetch the state of the last SYN_DROPPED
> > > and leave the events since in the client buffer. That way we can smooth over
> > > SYN_DROPPED and lose less information.
> > 
> > to expand on this, something like the below would work from userspace:
> >  
> > 1. userspace opens fd, EVIOCGBIT for everything
> > 2. userspace calls EVIOCGABSATOMIC
> > 3. kernel empties the event queue, flags the client as capable
> > 4. kernel copies current device state into client-specific struct
> > 5. kernel replies with that device state to the ioctl
> > 6. client reads events 
> > ..
> > 7. kernel sees a SYN_DROPPED for this client. Takes a snapshot of the device
> > for the client, empties the buffer, leaves SYN_DROPPED in the buffer
> > (current behaviour)
> > 8. client reads SYN_DROPPED, calls EVIOCGABSATOMIC
> > 9. kernel replies with the snapshot state, leaves the event buffer otherwise
> > unmodified
> > 10. client reads all events after SYN_DROPPED, gets a smooth continuation
> > 11. goto 6
> > 
> > if the buffer overflows multiple times, repeat 7 so that the snapshot state
> > is always the last SYN_DROPPED state. well, technically the state should be
> > the state of the device at the first SYN_REPORT after the last SYN_DROPPED,
> > since the current API says that interrupted event is incomplete.
> > 
> > there are two oddities here:
> > 1. the first ioctl will have to flush the buffer to guarantee consistent state,
> > though you could even avoid that by taking a snapshot of the device on
> > open(). though that comes with a disadvantage, you don't know if the client
> > supports the new approach so you're wasting effort and memory here.
> > 2. I'm not quite sure how to handle multiple repeated calls short of
> > updating the client-specific snapshot with every event as it is read
> > successfully.
> > 
> > any comments?
> 
> Do we really need to optimize the case when we are dropping events?

It happens frequently, to the point where on some laptops you're pretty much
guaranteed to get SYN_DROPPED events on resume and sometimes even during
normal multi-finger user.

I don't have any measurements on how many events are dropped on average.
Could be one or two, could be several buffer sizes, I honestly don't know.

Cheers,
   Peter