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

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Message-ID: <20221201111450.fpadmwscjyhefs2u@pengutronix.de>
Date:   Thu, 1 Dec 2022 12:14:50 +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 03/15] can: m_can: Cache tx putidx and transmits in flight

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.

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.

Marc

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