netdev - Re: [PATCH 02/15] can: m_can: Wakeup net queue once tx was issued

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Date:   Fri, 2 Dec 2022 14:53:39 +0100
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Markus Schneider-Pargmann <msp@...libre.com>
Cc:     Chandrasekar Ramakrishnan <rcsekar@...sung.com>,
        Wolfgang Grandegger <wg@...ndegger.com>,
        linux-can@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 02/15] can: m_can: Wakeup net queue once tx was issued

On 16.11.2022 21:52:55, Markus Schneider-Pargmann wrote:
> Currently the driver waits to wakeup the queue until the interrupt for
> the transmit event is received and acknowledged. If we want to use the
> hardware FIFO, this is too late.
> 
> Instead release the queue as soon as the transmit was transferred into
> the hardware FIFO. We are then ready for the next transmit to be
> transferred.
> 
> Signed-off-by: Markus Schneider-Pargmann <msp@...libre.com>
> ---
>  drivers/net/can/m_can/m_can.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/can/m_can/m_can.c b/drivers/net/can/m_can/m_can.c
> index 2c01e3f7b23f..4adf03111782 100644
> --- a/drivers/net/can/m_can/m_can.c
> +++ b/drivers/net/can/m_can/m_can.c
> @@ -1097,10 +1097,9 @@ static irqreturn_t m_can_isr(int irq, void *dev_id)
>  			/* New TX FIFO Element arrived */
>  			if (m_can_echo_tx_event(dev) != 0)
>  				goto out_fail;
> -
> -			if (netif_queue_stopped(dev) &&
> -			    !m_can_tx_fifo_full(cdev))
> +			if (!cdev->tx_skb && netif_queue_stopped(dev))
>  				netif_wake_queue(dev);

Please don't start the queue if the FIFO is still full. Is this a
gamble, that it will take long enough until the work queue runs that the
FIFO is not full anymore?

> +

Nitpick: Please don't introduce an extra newline here.

>  		}
>  	}
>  
> @@ -1705,6 +1704,8 @@ static netdev_tx_t m_can_tx_handler(struct m_can_classdev *cdev)
>  		if (m_can_tx_fifo_full(cdev) ||
>  		    m_can_next_echo_skb_occupied(dev, putidx))
>  			netif_stop_queue(dev);
> +		else if (cdev->is_peripheral && !cdev->tx_skb && netif_queue_stopped(dev))
> +			netif_wake_queue(dev);

Same question as above, what happens if the FIFO is full? e.g. in case
of a slow bus or the first CAN frame in the FIFO has a low prio...

>  	}
>  
>  	return NETDEV_TX_OK;
> -- 
> 2.38.1
> 
>

Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
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