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

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Date:   Tue, 13 Dec 2022 18:13:09 +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

On Fri, Dec 02, 2022 at 03:46:30PM +0100, Marc Kleine-Budde wrote:
> On 02.12.2022 09:37:40, Markus Schneider-Pargmann wrote:
> > 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?
> 
> I think it's Ok to use an atomic for the fill level. The put index
> doesn't need to be. No need to cache the get index, as it's in the same
> register as the fill level.
> 
> As the mcp251xfd benefits from caching both indexes, a head and tail
> pointer felt like the right choice. As both are only written in 1
> location, no need for atomics or locking.
> 
> > The tcan mram size is limited to 2048 so I would like to avoid limiting
> > the possible sizes of the tx fifos.
> 
> What FIFO sizes are you using currently?

I am currently using 13 for TXB, TXE and RXF0.

Best,
Markus