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SubjectRe: [PATCH 02/15] can: m_can: Wakeup net queue once tx was issued
Hi Vincent,

On Wed, Dec 14, 2022 at 07:15:25PM +0900, Vincent MAILHOL wrote:
> On Wed. 14 Dec. 2022 at 18:28, Marc Kleine-Budde <mkl@pengutronix.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).

Thank you for this summary, great that you already invested the time to
make it work with a CAN driver. I will give it a try in the m_can
driver.

Best,
Markus

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