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

[Markus Schneider-Pargmann <msp@baylibre.com>]

Hi Simon,

On Thu, Jan 26, 2023 at 09:04:56AM +0100, Simon Horman wrote:
> 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@baylibre.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)

Thank you for spotting this. You are totally right, I will fix it for
the next version.

> 
> 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()?

Good point. I think this can only happen for 1, 2, 3, 5, 6, 7 bytes,
values above have to be multiple of 4 because of the CAN-FD
specification.

With 'buf' it should read garbage from the buffer which I think is not a
problem as the chip knows how much of the data to use. Also the tx
elemnt size is hardcoded to 64 byte in the driver, so we do not overwrite
the next element with that. The chip minimum size is 8 bytes for the
data field anyways. So I think this is fine.

Thanks,
Markus

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