[PATCH 4/7] omap:mailbox-send message in process context

[Sapiens, Rene]

Hi Hari,

On Thursday, October 14, 2010 9:13 PM Kanigeri, Hari wrote:
> Schedule the Tasklet to send only when mailbox fifo is full, else
> send the message in the Process context. This would avoid
> needless scheduling of Tasklet for every message transfer
> 
> Signed-off-by: Hari Kanigeri <h-kanigeri2 at ti.com>
> ---
>  arch/arm/plat-omap/mailbox.c |    9 +++++++--
>  1 files changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm/plat-omap/mailbox.c b/arch/arm/plat-omap/mailbox.c
> index ed960c1..a4170c7 100644
> --- a/arch/arm/plat-omap/mailbox.c
> +++ b/arch/arm/plat-omap/mailbox.c
> @@ -92,20 +92,25 @@ int omap_mbox_msg_send(struct omap_mbox *mbox,
>  	mbox_msg_t msg) struct omap_mbox_queue *mq = mbox->txq;
>  	int ret = 0, len;
> 
> -	spin_lock(&mq->lock);
> +	spin_lock_bh(&mq->lock);
> 

Please check if this scenario looks valid to you, as discussed and depicted
with Fernando:

Supposing that at this point the hw fifo is full and there are messages in
the kfifo.

>  	if (kfifo_avail(&mq->fifo) < sizeof(msg)) {
>  		ret = -ENOMEM;
>  		goto out;
>  	}
> 

We reach this point.

In this scenario, the DSP or other Core reads a message from the mbox hw fifo.
The next happens:
1.- The not full interrupt is triggered to the MPU.
2.- The mbox's ISR schedules the tasklet to write the message.
3.- The ISR is left and returns to here (since we still have the bh lock the
    tasklet won't run).

> +	if (!__mbox_poll_for_space(mbox)) {\

4.- We check for mbox_fifo_full and we have space. so the message is written.

> +		mbox_fifo_write(mbox, msg);

At this point it looks that the FIFO order is lost. We write this message
before the ones in the kfifo.

A solution for this could be checking if there are messages in the kfifo
before trying to write directly to the hw fifo, if so, just continue
scheduling the tasklet.

> +		goto out;
> +	}
> +
>  	len = kfifo_in(&mq->fifo, (unsigned char *)&msg, sizeof(msg));
>  	WARN_ON(len != sizeof(msg));
> 
>  	tasklet_schedule(&mbox->txq->tasklet);
> 
>  out:
> -	spin_unlock(&mq->lock);
> +	spin_unlock_bh(&mq->lock);
>  	return ret;
>  }
>  EXPORT_SYMBOL(omap_mbox_msg_send);
> --
> 1.7.0