linux-kernel - Re: [PATCH v3 3/8] mailbox: Add transmit done by blocking option

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Date:   Mon, 2 Jul 2018 15:09:52 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Mikko Perttunen <mperttunen@...dia.com>
Cc:     jassisinghbrar@...il.com, gregkh@...uxfoundation.org,
        jonathanh@...dia.com, devicetree@...r.kernel.org,
        linux-serial@...r.kernel.org, linux-tegra@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/8] mailbox: Add transmit done by blocking option

On Mon, Jul 02, 2018 at 02:40:28PM +0300, Mikko Perttunen wrote:
> Add a new TXDONE option, TXDONE_BY_BLOCK. With this option, the
> send_data function of the mailbox driver is expected to block until
> the message has been sent. The new option is used with the Tegra
> Combined UART driver to minimize unnecessary overhead when transmitting
> data.
> 
> Signed-off-by: Mikko Perttunen <mperttunen@...dia.com>
> ---
>  drivers/mailbox/mailbox.c | 30 +++++++++++++++++++++---------
>  drivers/mailbox/mailbox.h |  1 +
>  2 files changed, 22 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/mailbox/mailbox.c b/drivers/mailbox/mailbox.c
> index 674b35f402f5..5c76b70e673c 100644
> --- a/drivers/mailbox/mailbox.c
> +++ b/drivers/mailbox/mailbox.c
> @@ -53,6 +53,8 @@ static int add_to_rbuf(struct mbox_chan *chan, void *mssg)
>  	return idx;
>  }
>  
> +static void tx_tick(struct mbox_chan *chan, int r, bool submit_next);
> +
>  static void msg_submit(struct mbox_chan *chan)
>  {
>  	unsigned count, idx;
> @@ -60,10 +62,13 @@ static void msg_submit(struct mbox_chan *chan)
>  	void *data;
>  	int err = -EBUSY;
>  
> +next:
>  	spin_lock_irqsave(&chan->lock, flags);
>  
> -	if (!chan->msg_count || chan->active_req)
> -		goto exit;
> +	if (!chan->msg_count || chan->active_req) {
> +		spin_unlock_irqrestore(&chan->lock, flags);
> +		return;
> +	}
>  
>  	count = chan->msg_count;
>  	idx = chan->msg_free;
> @@ -82,15 +87,21 @@ static void msg_submit(struct mbox_chan *chan)
>  		chan->active_req = data;
>  		chan->msg_count--;
>  	}
> -exit:
> +
>  	spin_unlock_irqrestore(&chan->lock, flags);
>  
>  	if (!err && (chan->txdone_method & TXDONE_BY_POLL))
>  		/* kick start the timer immediately to avoid delays */
>  		hrtimer_start(&chan->mbox->poll_hrt, 0, HRTIMER_MODE_REL);
> +
> +	if (chan->txdone_method & TXDONE_BY_BLOCK) {
> +		tx_tick(chan, err, false);
> +		if (!err)
> +			goto next;
> +	}

This bit is slightly confusing: if check for err, but the call to the
tx_tick() function doesn't actually change err (it's passed by value).
Now, I don't have any great ideas on how to improve this, but perhaps
a simple comment would help clarify what's going on here, or maybe by
adding an extra blank line between the tx_tick() call and the
conditional would make it more obvious that these aren't related.

Also, it looks to me like this is actually modifying msg_submit() to
actually do more than just submit the next data from a message. This
effectively flushes the complete message via the mailbox, right?

Perhaps a slightly clearer implmentation would be to turn this into a
separate function that repeatedly calls msg_submit() in a loop or some
such.

>  }
>  
> -static void tx_tick(struct mbox_chan *chan, int r)
> +static void tx_tick(struct mbox_chan *chan, int r, bool submit_next)
>  {
>  	unsigned long flags;
>  	void *mssg;
> @@ -101,7 +112,8 @@ static void tx_tick(struct mbox_chan *chan, int r)
>  	spin_unlock_irqrestore(&chan->lock, flags);
>  
>  	/* Submit next message */
> -	msg_submit(chan);
> +	if (submit_next)
> +		msg_submit(chan);
>  
>  	if (!mssg)
>  		return;
> @@ -127,7 +139,7 @@ static enum hrtimer_restart txdone_hrtimer(struct hrtimer *hrtimer)
>  		if (chan->active_req && chan->cl) {
>  			txdone = chan->mbox->ops->last_tx_done(chan);
>  			if (txdone)
> -				tx_tick(chan, 0);
> +				tx_tick(chan, 0, true);
>  			else
>  				resched = true;
>  		}
> @@ -176,7 +188,7 @@ void mbox_chan_txdone(struct mbox_chan *chan, int r)
>  		return;
>  	}
>  
> -	tx_tick(chan, r);
> +	tx_tick(chan, r, true);
>  }
>  EXPORT_SYMBOL_GPL(mbox_chan_txdone);
>  
> @@ -196,7 +208,7 @@ void mbox_client_txdone(struct mbox_chan *chan, int r)
>  		return;
>  	}
>  
> -	tx_tick(chan, r);
> +	tx_tick(chan, r, true);
>  }
>  EXPORT_SYMBOL_GPL(mbox_client_txdone);
>  
> @@ -275,7 +287,7 @@ int mbox_send_message(struct mbox_chan *chan, void *mssg)
>  		ret = wait_for_completion_timeout(&chan->tx_complete, wait);
>  		if (ret == 0) {
>  			t = -ETIME;
> -			tx_tick(chan, t);
> +			tx_tick(chan, t, true);
>  		}
>  	}
>  
> diff --git a/drivers/mailbox/mailbox.h b/drivers/mailbox/mailbox.h
> index 456ba68513bb..ec68e2e28cd6 100644
> --- a/drivers/mailbox/mailbox.h
> +++ b/drivers/mailbox/mailbox.h
> @@ -10,5 +10,6 @@
>  #define TXDONE_BY_IRQ	BIT(0) /* controller has remote RTR irq */
>  #define TXDONE_BY_POLL	BIT(1) /* controller can read status of last TX */
>  #define TXDONE_BY_ACK	BIT(2) /* S/W ACK recevied by Client ticks the TX */
> +#define TXDONE_BY_BLOCK	BIT(3) /* mailbox driver send_data blocks until done */

Perhaps clarify here that it blocks until all data in the message has
been transmitted. As it is, this could mean just block until the current
datum is done transmitting.

Thierry

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