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

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Date:   Fri, 2 Dec 2022 10:16:30 +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 01.12.2022 17:49:02, Markus Schneider-Pargmann wrote:
> Hi Marc,
> 
> On Thu, Dec 01, 2022 at 10:16:05AM +0100, Marc Kleine-Budde wrote:
> > On 01.12.2022 09:43:02, 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().
> > > 
> > > Good idea. I will check how regmap's async_write works and if it is
> > > suitable to do the job. I don't want to drop the regmap usage for this
> > > right now.
> > 
> > IIRC regmap async write still uses mutexes, but sleeping is not allowed
> > in the xmit handler. The mcp251xfd driver does the endianness conversion
> > (and the optional CRC) manually for the TX path.
> 
> I just saw, you can force regmap to use spinlocks as well. But it uses
> the same operation for sync operations as well.

But you cannot use sync SPI api under a spinlock.

> > Sending directly from the xmit handler basically eliminates the queuing
> > between the network stack and the worker. Getting rid of the worker
> > makes life easier and it's faster anyways.
> 
> The current implementation of the driver doesn't really queue anything
> between the network stack and the worker. It is a queue of size 1 ;).

Ok

> To be honest I would rather focus on the other things than on getting
> rid of the worker completely as this can be done in a separate patch
> later as well. Yes I agree it would be nice to get rid of the worker but
> it is also probably not a major bottleneck for the performance and in
> its current state it works. If I have time left at the end I will be
> more than happy to do that. But for the moment I would just keep the
> worker as it is. Is that OK for you?

Sure.

Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
Vertretung West/Dortmund         | Phone: +49-231-2826-924     |
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