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

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

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.

> 
> Signed-off-by: Aniroop Mathur <a.mathur@samsung.com>
> ---
>  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)) {
> +			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