Re: [PATCH] Input: evdev - drop partial events after emptying the buffer

[Peter Hutterer]

On Sat, Jan 02, 2016 at 08:39:21PM -0800, Dmitry Torokhov wrote:
> On Thu, Dec 31, 2015 at 03:36:47AM +0530, Aniroop Mathur wrote:
> > This patch introduces concept to drop partial events in evdev handler
> > itself after emptying the buffer which are dropped by all evdev
> > clients in userspace after SYN_DROPPED occurs.
> > This in turn saves space of partial events in evdev handler buffer
> > and reduces evdev client reading requests.
> 
> Let's add a few people who write consumer code to see if this is
> something that they consider useful.

yeah, it's useful though we already have the code in libevdev to work around
this. Still, it reduces the number of events discarde by the client, so it'll be a net
plus. but, afaict, there's a bug in this implementation.
The doc states: "Client should ignore all events up to and including next
SYN_REPORT event". If you drop partial events, you need to have an empty
SYN_REPORT after the SYN_DROPPED before you start with full events again.
This patch skips that, so after the SYN_DROPPED you have a valid full event
that will be ignored by any client currently capable of handling
SYN_DROPPED.
Example: let's assume a device sending ABS_X/ABS_Y fast enough to cause a
SYN_DROPPED, you may have this queue:
    ABS_X
    ABS_Y
    SYN_REPORT
    ...
    SYN_DROPPED
    ABS_Y      <---- partial event     
    SYN_REPORT <---- client discards up to here, sync state
    ABS_X
    ABS_Y
    SYN_REPORT <---- first full event after sync

With this patch this sequence becomes:
    ABS_X
    ABS_Y
    SYN_REPORT
    ...
    SYN_DROPPED
        [kernel discards ABS_Y + SYN_REPORT as partial event]
    ABS_X
    ABS_Y
    SYN_REPORT <--- client discards up to here, sync state
               <--- there is no event after sync

That's a change in kernel behaviour and will make all current clients
potentially buggy, you'll really need the empty SYN_REPORT here.

> > 
> > Signed-off-by: Aniroop Mathur <a.mathur@xxxxxxxxxxx>
> > ---
> >  drivers/input/evdev.c | 49 +++++++++++++++++++++++++++++++++++++++++++++----
> >  1 file changed, 45 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/input/evdev.c b/drivers/input/evdev.c
> > index e9ae3d5..e7b612e 100644
> > --- a/drivers/input/evdev.c
> > +++ b/drivers/input/evdev.c
> > @@ -58,6 +58,7 @@ struct evdev_client {
> >  	struct list_head node;
> >  	unsigned int clk_type;
> >  	bool revoked;
> > +	bool drop_pevent; /* specifies whether partial events need to be dropped */
> >  	unsigned long *evmasks[EV_CNT]; > >  	unsigned int bufsize;
> >  	struct input_event buffer[];
> > @@ -192,6 +193,7 @@ static int evdev_set_clk_type(struct evdev_client *client, unsigned int clkid)
> >  {
> >  	unsigned long flags;
> >  	unsigned int clk_type;
> > +	struct input_event *ev;
> >  
> >  	switch (clkid) {
> >  
> > @@ -218,6 +220,17 @@ static int evdev_set_clk_type(struct evdev_client *client, unsigned int clkid)
> >  		spin_lock_irqsave(&client->buffer_lock, flags);
> >  
> >  		if (client->head != client->tail) {
> > +
> > +			/*
> > +			 * Set drop_pevent to true if last event packet is
> > +			 * not stored completely in buffer.
> > +			 */
> > +			client->head--;
> > +			client->head &= client->bufsize - 1;
> > +			ev = &client->buffer[client->head];
> > +			if (!(ev->type == EV_SYN && ev->code == SYN_REPORT))
> > +				client->drop_pevent = true;
> > +
> >  			client->packet_head = client->head = client->tail;
> >  			__evdev_queue_syn_dropped(client);
> >  		}
> > @@ -228,31 +241,51 @@ static int evdev_set_clk_type(struct evdev_client *client, unsigned int clkid)
> >  	return 0;
> >  }
> >  
> > -static void __pass_event(struct evdev_client *client,
> > +static bool __pass_event(struct evdev_client *client,
> >  			 const struct input_event *event)
> >  {
> > +	struct input_event *prev_ev;
> > +
> >  	client->buffer[client->head++] = *event;
> >  	client->head &= client->bufsize - 1;
> >  
> >  	if (unlikely(client->head == client->tail)) {
> >  		/*
> > -		 * This effectively "drops" all unconsumed events, leaving
> > -		 * EV_SYN/SYN_DROPPED plus the newest event in the queue.
> > +		 * This effectively "drops" all unconsumed events, storing
> > +		 * EV_SYN/SYN_DROPPED and the newest event in the queue but
> > +		 * only if it is not part of partial packet.
> > +		 * Set drop_pevent to true if last event packet is not stored
> > +		 * completely in buffer and set to false if SYN_REPORT occurs.
> >  		 */
> > +
> >  		client->tail = (client->head - 2) & (client->bufsize - 1);
> >  
> > +		prev_ev = &client->buffer[client->tail];
> > +		if (!(prev_ev->type == EV_SYN && prev_ev->code == SYN_REPORT)) {

IMO a (prev_ev->type != EV_SYN || prev_ev->code != SYN_REPORT) would be
easier to read than this (!(a && b)).

Cheers,
   Peter

> > +			client->drop_pevent = true;
> > +			client->head--;
> > +			client->head &= client->bufsize - 1;
> > +		}
> > +
> >  		client->buffer[client->tail].time = event->time;
> >  		client->buffer[client->tail].type = EV_SYN;
> >  		client->buffer[client->tail].code = SYN_DROPPED;
> >  		client->buffer[client->tail].value = 0;
> >  
> >  		client->packet_head = client->tail;
> > +
> > +		if (event->type == EV_SYN && event->code == SYN_REPORT) {
> > +			client->drop_pevent = false;
> > +			return true;
> > +		}
> >  	}
> >  
> >  	if (event->type == EV_SYN && event->code == SYN_REPORT) {
> >  		client->packet_head = client->head;
> >  		kill_fasync(&client->fasync, SIGIO, POLL_IN);
> >  	}
> > +
> > +	return false;
> >  }
> >  
> >  static void evdev_pass_values(struct evdev_client *client,
> > @@ -284,10 +317,18 @@ static void evdev_pass_values(struct evdev_client *client,
> >  			wakeup = true;
> >  		}
> >  
> > +		/* drop partial events until SYN_REPORT occurs */
> > +		if (client->drop_pevent) {
> > +			if (v->type == EV_SYN && v->code == SYN_REPORT)
> > +				client->drop_pevent = false;
> > +			continue;
> > +		}
> > +
> >  		event.type = v->type;
> >  		event.code = v->code;
> >  		event.value = v->value;
> > -		__pass_event(client, &event);
> > +		if (__pass_event(client, &event))
> > +			wakeup = false;
> >  	}
> >  
> >  	spin_unlock(&client->buffer_lock);
> > -- 
> > 2.6.2
> > 
> 
> -- 
> Dmitry
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