linux-kernel - Re: [PATCH 03/15] can: m_can: Cache tx putidx and transmits in flight

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Date:   Fri, 2 Dec 2022 09:37:40 +0100
From:   Markus Schneider-Pargmann <msp@...libre.com>
To:     Marc Kleine-Budde <mkl@...gutronix.de>
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 03/15] can: m_can: Cache tx putidx and transmits in flight

Hi Marc,

On Thu, Dec 01, 2022 at 12:14:50PM +0100, Marc Kleine-Budde wrote:
> On 16.11.2022 21:52:56, Markus Schneider-Pargmann wrote:
> > On peripheral chips every read/write can be costly. Avoid reading easily
> > trackable information and cache them internally. This saves multiple
> > reads.
> > 
> > Transmit FIFO put index is cached, this is increased for every time we
> > enqueue a transmit request.
> > 
> > The transmits in flight is cached as well. With each transmit request it
> > is increased when reading the finished transmit event it is decreased.
> > 
> > A submit limit is cached to avoid submitting too many transmits at once,
> > either because the TX FIFO or the TXE FIFO is limited. This is currently
> > done very conservatively as the minimum of the fifo sizes. This means we
> > can reach FIFO full events but won't drop anything.
> 
> You have a dedicated in_flight variable, which is read-modify-write in 2
> different code path, i.e. this looks racy.

True, of course, thank you. Yes I have to redesign this a bit for
concurrency.

> If you allow only power-of-two FIFO size, you can use 2 unsigned
> variables, i.e. a head and a tail pointer. You can apply a mask to get
> the index to the FIFO. The difference between head and tail is the fill
> level of the FIFO. See mcp251xfd driver for this.

Maybe that is a trivial question but what's wrong with using atomics
instead?

The tcan mram size is limited to 2048 so I would like to avoid limiting
the possible sizes of the tx fifos.

Best,
Markus

> 
> Marc
> 
> -- 
> Pengutronix e.K.                 | Marc Kleine-Budde           |
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