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

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Date:   Wed, 14 Dec 2022 19:15:25 +0900
From:   Vincent MAILHOL <mailhol.vincent@...adoo.fr>
To:     Marc Kleine-Budde <mkl@...gutronix.de>
Cc:     Markus Schneider-Pargmann <msp@...libre.com>,
        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 Wed. 14 Dec. 2022 at 18:28, Marc Kleine-Budde <mkl@...gutronix.de> wrote:
> On 14.12.2022 10:18:20, Marc Kleine-Budde wrote:
> > On 14.12.2022 10:14:06, Markus Schneider-Pargmann wrote:
> > > Hi Marc,
> > >
> > > On Wed, Nov 30, 2022 at 06:21:00PM +0100, Marc Kleine-Budde wrote:
> > > > 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.
> > > >
> > > > If you want to really speed up the TX path, remove the worker and use
> > > > the spi_async() API from the xmit callback, see mcp251xfd_start_xmit().
> > > >
> > > > Extra bonus if you implement xmit_more() and transfer more than 1 skb
> > > > per SPI transfer.
> > >
> > > Just a quick question here, I mplemented a xmit_more() call and I am
> > > testing it right now, but it always returns false even under high
> > > pressure. The device has a txqueuelen set to 1000. Do I need to turn
> > > some other knob for this to work?

I was the first to use BQL in a CAN driver. It also took me time to
first figure out the existence of xmit_more() and even more to
understand how to make it so that it would return true.

> > AFAIK you need BQL support: see 0084e298acfe ("can: mcp251xfd: add BQL support").
> >
> > The etas_es58x driver implements xmit_more(), I added the Author Vincent
> > on Cc.
>
> Have a look at netdev_queue_set_dql_min_limit() in the etas driver.

The functions you need are the netdev_send_queue() and the
netdev_complete_queue():

  https://elixir.bootlin.com/linux/latest/source/include/linux/netdevice.h#L3424

For CAN, you probably want to have a look to can_skb_get_frame_len().

  https://elixir.bootlin.com/linux/latest/source/include/linux/can/length.h#L166

The netdev_queue_set_dql_min_limit() gives hints by setting a minimum
value for BQL. It is optional (and as of today I am the only user of
it).


Yours sincerely,
Vincent Mailhol