netdev - Re: [PATCH v2 4/4] can: c_can: cache frames to operate as a true FIFO

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Date:   Fri, 6 Aug 2021 11:36:58 +0200
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Dario Binacchi <dariobin@...ero.it>
Cc:     linux-kernel@...r.kernel.org,
        Gianluca Falavigna <gianluca.falavigna@...ind.it>,
        Andrew Lunn <andrew@...n.ch>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Oliver Hartkopp <socketcan@...tkopp.net>,
        Vincent Mailhol <mailhol.vincent@...adoo.fr>,
        Wolfgang Grandegger <wg@...ndegger.com>,
        linux-can@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH v2 4/4] can: c_can: cache frames to operate as a true FIFO

On 05.08.2021 22:19:00, Dario Binacchi wrote:
> As reported by a comment in the c_can_start_xmit() this was not a FIFO.
> C/D_CAN controller sends out the buffers prioritized so that the lowest
> buffer number wins.
> 
> What did c_can_start_xmit() do if head was less tail in the tx ring ? It
> waited until all the frames queued in the FIFO was actually transmitted
> by the controller before accepting a new CAN frame to transmit, even if
> the FIFO was not full, to ensure that the messages were transmitted in
> the order in which they were loaded.
> 
> By storing the frames in the FIFO without requiring its transmission, we
> will be able to use the full size of the FIFO even in cases such as the
> one described above. The transmission interrupt will trigger their
> transmission only when all the messages previously loaded but stored in
> less priority positions of the buffers have been transmitted.
> 
> Suggested-by: Gianluca Falavigna <gianluca.falavigna@...ind.it>
> Signed-off-by: Dario Binacchi <dariobin@...ero.it>

My review from
https://lore.kernel.org/linux-can/20210806092523.hij5ejjq6wecbgfr@pengutronix.de/
applies here, too.

Please use IF_RX in c_can_do_tx(), remove the spin_lock and test. After
applying your series, I'll send a patch that changes IF_RX into IF_NAPI
to avoid any further confusion.

regards,
Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
Vertretung West/Dortmund         | Phone: +49-231-2826-924     |
Amtsgericht Hildesheim, HRA 2686 | Fax:   +49-5121-206917-5555 |

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