linux-kernel - Re: [PATCH 3/4] input: evdev: Indicate buffer overrun with SYN

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Date:	Fri, 25 Mar 2011 00:47:50 -0700
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	Jeff Brown <jeffbrown@...roid.com>
Cc:	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jeff Brown <jeffbrown@...gle.com>
Subject: Re: [PATCH 3/4] input: evdev: Indicate buffer overrun with
 SYN_DROPPED.

On Tue, Mar 22, 2011 at 06:04:03PM -0700, Jeff Brown wrote:
> From: Jeff Brown <jeffbrown@...gle.com>
> 
> Add a new EV_SYN code, SYN_DROPPED, to inform the client when input
> events have been dropped from the evdev input buffer due to a
> buffer overrun.  The client should use this event as a hint to
> reset its state or ignore all following events until the next
> packet begins.
> 

I like it.

> Signed-off-by: jeffbrown@...roid.com
> ---
>  drivers/input/evdev.c |   18 ++++++++++++------
>  include/linux/input.h |    1 +
>  2 files changed, 13 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/input/evdev.c b/drivers/input/evdev.c
> index 7f42d3a..203ed70 100644
> --- a/drivers/input/evdev.c
> +++ b/drivers/input/evdev.c
> @@ -57,14 +57,20 @@ static void evdev_pass_event(struct evdev_client *client,
>  {
>  	/*
>  	 * Interrupts are disabled, just acquire the lock.
> -	 * Make sure we don't leave with the client buffer
> -	 * "empty" by having client->head == client->tail.
> +	 * When the client buffer is full, replace the tail with SYN_DROPPED
> +	 * to let the client know that events were dropped.  Ensure that the
> +	 * head and tail never coincide so the buffer does not appear "empty".
>  	 */
>  	spin_lock(&client->buffer_lock);
> -	do {
> -		client->buffer[client->head++] = *event;
> -		client->head &= client->bufsize - 1;
> -	} while (client->head == client->tail);
> +	client->buffer[client->head++] = *event;
> +	client->head &= client->bufsize - 1;
> +	if (client->head == client->tail) {

Do you think this should be unlikely()?

> +		client->tail = (client->tail + 1) & (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;
> +	}
>  	spin_unlock(&client->buffer_lock);
>  
>  	if (event->type == EV_SYN)
> diff --git a/include/linux/input.h b/include/linux/input.h
> index 056ae8a..65d253e 100644
> --- a/include/linux/input.h
> +++ b/include/linux/input.h
> @@ -167,6 +167,7 @@ struct input_keymap_entry {
>  #define SYN_REPORT		0
>  #define SYN_CONFIG		1
>  #define SYN_MT_REPORT		2
> +#define SYN_DROPPED		3
>  
>  /*
>   * Keys and buttons

Thanks.

-- 
Dmitry
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