On Sun, Aug 10, 2014 at 05:21:47PM +0200, David Herrmann wrote:
> Hi
>
> On Fri, Aug 8, 2014 at 7:47 PM, Dmitry Torokhov
> <dmitry.torokhov@xxxxxxxxx> wrote:
> > On Fri, Aug 08, 2014 at 03:26:56PM +0200, David Herrmann wrote:
> >> Hi
> >>
> >> On Wed, Aug 6, 2014 at 3:35 AM, Peter Hutterer <peter.hutterer@xxxxxxxxx> wrote:
> >> >> +
> >> >> +/**
> >> >> + * EVIOCGABSRANGE - Fetch range of ABS values
> >> >> + *
> >> >> + * This fetches the current values of a range of ABS codes atomically. The range
> >> >> + * of codes to fetch and the buffer-types are passed as "struct input_absrange",
> >> >> + * which has the following fields:
> >> >> + * slots: Number of MT slots to fetch data for.
> >> >> + * code: First ABS axis to query.
> >> >> + * count: Number of ABS axes to query starting at @code.
> >> >> + * buffer: Pointer to a receive buffer where to store the fetched ABS
> >> >> + * values. This buffer must be an array of __s32 with at least
> >> >> + * (@slots * @code) elements. The buffer is interpreted as two
> >> >> + * dimensional __s32 array, declared as: __s32[slots][codes]
> >> >
> >> > tbh this seems more complicated than necessary. Have you thought about
> >> > just dumping the events into the client buffer as if they came fresh in from
> >> > the device? So to sync, the client calls the ioctl with a buffer and a
> >> > buffer size, and the kernel simply writes a series of struct input_events
> >> > into that buffer, with ABS_MT_SLOT as required for all slots, (optionally?)
> >> > followed by a SYN_DROPPED. So the buffer afterwards could look like this:
> >> > EV_ABS ABS_X 30
> >> > EV_ABS ABS_X 1202
> >> > EV_ABS ABS_MT_SLOT 0
> >> > EV_ABS ABS_MT_POSITION_X 30
> >> > EV_ABS ABS_MT_POSITION_Y 1202
> >> > EV_ABS ABS_MT_SLOT 1
> >> > EV_ABS ABS_MT_POSITION_X 80
> >> > EV_ABS ABS_MT_POSITION_Y 1800
> >> > EV_SYN SYN_REPORT 0
> >> >
> >> > the client can then go through and just process the events on-by-one as it
> >> > would otherwise with real events.
> >> >
> >> > This approach could be even extended to include EV_KEY, etc. providing a
> >> > single ioctl to sync the whole state of the device atomically.
> >> >
> >> > comments?
> >
> > I like it.
> >
> >>
> >> So you mean instead of passing a __32 array we pass a "struct
> >> input_event" array and write it there? So bypassing the receive-queue?
> >> That does sound quite nice, indeed. We could replace all the other
> >> "sync" ioctls and just provide a way to receive all the events
> >> directly.
> >>
> >> Something like:
> >>
> >> EVIOCQUERY(struct input_query)
> >>
> >> struct input_query {
> >> __u16 type;
> >> __u16 start_code;
> >> __u16 end_code;
> >> __u16 slots;
> >>
> >> struct input_event buffer[];
> >> };
> >
> > No, it is more like EVIOCRESYNC(void) which makes input core to dump all
> > existing state into the client's standard event queue so that here is no
> > need to reconcile/reconstruct anything. We could give a new SYN marker
> > to indicate end-of-state boundary.
>
> This doesn't make sense to me, for two reasons:
> * Events received during re-sync are usually handled differently than
> normal events. For instance, you *must* never handle key events
> from re-syncs for shortcuts, because you don't know the order
> they were pressed in.
I think you're vastly overestimating what the majority of userspace does ;)
both the xorg and the libinput-based stacks pretty much ignore this at the
moment and by the time we handle shortcuts we're already one or more APIs
away from the SYN_DROPPED handling.
also, I haven't seen SYN_DROPPED issues from keyboard events. They usually
happen only on absolute touch devices and ironically enough that's the one
case where the kernel currently doesn't allow for a race-less resync.
> If we put sync events in the queue, users cannot know where an event
> from.
> Inserting a SYNC marker is ridiculous, because following normal
> events may drop those (via SYN_DROPPED). We'd have to
> protect it. Furthermore, it's awful to handle this from user-space,
> because you have no idea how many events are sent as part
> of the sync.
I suspect Dmitry meant a new EV_SYN code. From a userspace POV, I'd know
that after the ioctl() all events up until SYN_SYNC_DONE are the syncing
events. That would work well and is coincidentally almost identical to the
libevdev public API.
In libevdev we already have code to calculate the maximum number of events
by a device for the current SYN_DROPPED handling. And you can reduce the
number of events by only sending those in a non-zero state for EV_KEY,
EV_LED, etc.
IMO the real issue preventing this approach is:
> * The receive-queue is usually too small to hold all those events.
> I mean, evdev uses EVDEV_BUF_PACKETS as number of buffer
> slots (defined as 8!). You cannot even hold a whole keyboard
> state there. Yes, usually it should be enough, but re-syncing
> is about handling corner-cases. If we make SYN_DROPPED
> handling cause SYN_DROPPED, we can just ignore it.
yep, that too was my first thought. with a plain resync ioctl you're pretty
much guaranteed to get SYN_DROPPED before the client manages to handle the
resync. Even if you reduce the number of events as above because the most
common occurance for SYN_DROPPED is in the ABS_MT range which we cannot
meaningfully reduce.
> I understand why EVIORESYNC(void) is tempting. It avoids all the
> complexity of my patch and makes all the other sync-ioctls we have so
> far obsolete. But the reason we want it, is to avoid shortcomings of
> the limited receive-queue. I don't think any solution involving the
> receive-queue is going to work out. Even if we always make sure the
> receive-queue is big enough, we might still get new events coming in
> before user-space can drain it.
>
> How about this:
>
> struct input_resync {
> __u64 count;
> struct input_event buffer[];
> };
>
> EVIOCSYNC(struct input_resync) copies the current state of all
> available event-types of the device into the buffer. It returns the
> number of events that it wants to write (so user-space can pass
> count==0 the first time to figure out the required buffer-size). The
> ioctl then flushed the _whole_ input queue.
if you go with the suggestion above of only putting events in with a
non-zero state then a count of 0 is guaranteed to return the wrong number
since you may get events between the two ioctl calls. I don't think
the double ioctl() call is that a case we need. as I said above the case of
all events must already be handled in userspace anyway.
what's the reason you don't use a EVIOCSYNC(len) approach btw? quick check
shows at 13 or 14 bits for len which should be enough for pretty much any
device for a long time.
> Note that even if we used the client's receive buffer, we'd have to
> copy huge amounts of events while holding the event lock. So both
> ideas have to lock the buffers during the operation. The downside of
> my solution is that we have to allocate temporary buffers as we cannot
> copy to user-space while holding spin-locks. However, my code already
> makes sure we don't do high-order allocations, so I think this is
> fine.
>
> This ioctl would also allow us to get rid of all the other QUERY
> ioctls. User-space can use it to initialize it's state and then update
> it according to incoming events. This way, we simplify the API
> considerably! I really like this idea. I know, we only need this to
> handle corner-cases. But as Peter already said, it's really easy to
> trigger and ignoring it causes awful bugs in user-space.
summary from my side: I'd prefer EVIOCSYNC() with a buffer parameter, only
sending nonzero state for key/led/sw/etc. and the full state for abs. no
need for a new EV_SYN code, and provided it empties the whole queue we
should be race-free.
Cheers,
Peter
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