netdev - Re: [PATCH v2 08/18] can: m_can: Write transmit header and data in one transaction

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Date:   Thu, 26 Jan 2023 09:04:56 +0100
From:   Simon Horman <simon.horman@...igine.com>
To:     Markus Schneider-Pargmann <msp@...libre.com>
Cc:     Marc Kleine-Budde <mkl@...gutronix.de>,
        Chandrasekar Ramakrishnan <rcsekar@...sung.com>,
        Wolfgang Grandegger <wg@...ndegger.com>,
        Vincent MAILHOL <mailhol.vincent@...adoo.fr>,
        linux-can@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 08/18] can: m_can: Write transmit header and data in
 one transaction

On Wed, Jan 25, 2023 at 08:50:49PM +0100, Markus Schneider-Pargmann wrote:
> Combine header and data before writing to the transmit fifo to reduce
> the overhead for peripheral chips.
> 
> Signed-off-by: Markus Schneider-Pargmann <msp@...libre.com>
> ---
>  drivers/net/can/m_can/m_can.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/net/can/m_can/m_can.c b/drivers/net/can/m_can/m_can.c
> index 78f6ed744c36..440bc0536951 100644
> --- a/drivers/net/can/m_can/m_can.c
> +++ b/drivers/net/can/m_can/m_can.c
> @@ -1681,6 +1681,7 @@ static netdev_tx_t m_can_tx_handler(struct m_can_classdev *cdev)
>  		m_can_write(cdev, M_CAN_TXBAR, 0x1);
>  		/* End of xmit function for version 3.0.x */
>  	} else {
> +		char buf[TXB_ELEMENT_SIZE];
>  		/* Transmit routine for version >= v3.1.x */
>  
>  		txfqs = m_can_read(cdev, M_CAN_TXFQS);
> @@ -1720,12 +1721,11 @@ static netdev_tx_t m_can_tx_handler(struct m_can_classdev *cdev)
>  		fifo_header.dlc = FIELD_PREP(TX_BUF_MM_MASK, putidx) |
>  			FIELD_PREP(TX_BUF_DLC_MASK, can_fd_len2dlc(cf->len)) |
>  			fdflags | TX_BUF_EFC;
> -		err = m_can_fifo_write(cdev, putidx, M_CAN_FIFO_ID, &fifo_header, 2);
> -		if (err)
> -			goto out_fail;
> +		memcpy(buf, &fifo_header, 8);
> +		memcpy(&buf[8], &cf->data, cf->len);
>  
> -		err = m_can_fifo_write(cdev, putidx, M_CAN_FIFO_DATA,
> -				       cf->data, DIV_ROUND_UP(cf->len, 4));
> +		err = m_can_fifo_write(cdev, putidx, M_CAN_FIFO_ID,
> +				       buf, 8 + DIV_ROUND_UP(cf->len, 4));

Perhaps I am missing something here, but my reading is that:

- 8 is a length in bytes
- the 5th argument to m_can_fifo_write is the val_count parameter,
  whose unit is 4-byte long values.

  By this logic, perhaps the correct value for this argument is:

  DIV_ROUND_UP(8 + cf->len, 4)

Also:

- If cf->len is not a multiple of 4, is there a possibility
  that uninitialised trailing data in buf will be used
  indirectly by m_can_fifo_write()?

>  		if (err)
>  			goto out_fail;
>  
> -- 
> 2.39.0
>