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

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Date:   Wed, 4 Aug 2021 11:34:23 +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: [RESEND PATCH 4/4] can: c_can: cache frames to operate as a true
 FIFO

On 25.07.2021 18:11:50, 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>
> 
> ---
> 
>  drivers/net/can/c_can/c_can.h      |  6 +++++
>  drivers/net/can/c_can/c_can_main.c | 42 +++++++++++++++++-------------
>  2 files changed, 30 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/net/can/c_can/c_can.h b/drivers/net/can/c_can/c_can.h
> index 8fe7e2138620..fc499a70b797 100644
> --- a/drivers/net/can/c_can/c_can.h
> +++ b/drivers/net/can/c_can/c_can.h
> +static inline u8 c_can_get_tx_free(const struct c_can_tx_ring *ring)
> +{
> +	return ring->obj_num - (ring->head - ring->tail);
> +}
> +
>  #endif /* C_CAN_H */
> diff --git a/drivers/net/can/c_can/c_can_main.c b/drivers/net/can/c_can/c_can_main.c
> index 451ac9a9586a..4c061fef002c 100644
> --- a/drivers/net/can/c_can/c_can_main.c
> +++ b/drivers/net/can/c_can/c_can_main.c
> @@ -427,20 +427,6 @@ static void c_can_setup_receive_object(struct net_device *dev, int iface,
>  	c_can_object_put(dev, iface, obj, IF_COMM_RCV_SETUP);
>  }
>  
> -static u8 c_can_get_tx_free(const struct c_can_tx_ring *ring)
> -{
> -	u8 head = c_can_get_tx_head(ring);
> -	u8 tail = c_can_get_tx_tail(ring);
> -
> -	/* This is not a FIFO. C/D_CAN sends out the buffers
> -	 * prioritized. The lowest buffer number wins.
> -	 */
> -	if (head < tail)
> -		return 0;
> -
> -	return ring->obj_num - head;
> -}
> -

Can you move that change into patch 3?

Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
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