linux-kernel - Re: [PATCH v7] can, tty: can327 CAN/ldisc driver for ELM327 based OBD-II adapters

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Date:   Sat, 11 Jun 2022 15:36:08 +0200
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Max Staudt <max@...as.org>
Cc:     Vincent Mailhol <vincent.mailhol@...il.com>,
        Wolfgang Grandegger <wg@...ndegger.com>,
        linux-can@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Oliver Neukum <oneukum@...e.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7] can, tty: can327 CAN/ldisc driver for ELM327 based
 OBD-II adapters

On 11.06.2022 15:15:51, Max Staudt wrote:
> @Marc, Wolfgang - one SocketCAN design question about bitrate setting at
> the end. I'd appreciate your opinion on my dummy do_set_bittiming().

[...]

> > > +/* Dummy needed to use bitrate_const */
> > > +static int can327_do_set_bittiming(struct net_device *netdev)
> > > +{
> > > +       return 0;
> > > +}  
> > 
> > Is this dummy function really needed? If think that access to
> > can_priv::do_set_bittiming is always garded. Setting it to NULL should
> > be OK. c.f.:
> > https://elixir.bootlin.com/linux/latest/source/drivers/net/can/dev/netlink.c#L198
> 
> drivers/net/can/dev/netlink.c - can_changelink():
> 
> 	/* Calculate bittiming parameters based on
> 	 * bittiming_const if set, otherwise pass bitrate
> 	 * directly via do_set_bitrate(). Bail out if neither
> 	 * is given.
> 	 */
> 	if (!priv->bittiming_const && !priv->do_set_bittiming)
> 		return -EOPNOTSUPP;
> 
> can327 has neither of these two, so I provide a dummy for one.
> 
> What should I do instead?

What about this change?

diff --git a/drivers/net/can/dev/netlink.c b/drivers/net/can/dev/netlink.c
index 7633d98e3912..667ddd28fcdc 100644
--- a/drivers/net/can/dev/netlink.c
+++ b/drivers/net/can/dev/netlink.c
@@ -176,7 +176,8 @@ static int can_changelink(struct net_device *dev, struct nlattr *tb[],
                 * directly via do_set_bitrate(). Bail out if neither
                 * is given.
                 */
-               if (!priv->bittiming_const && !priv->do_set_bittiming)
+               if (!priv->bittiming_const && !priv->do_set_bittiming &&
+                   !priv->bitrate_const)
                        return -EOPNOTSUPP;
 
                memcpy(&bt, nla_data(data[IFLA_CAN_BITTIMING]), sizeof(bt));

If it works I'll make a patch and apply it to net-next/master so that
you can base your series on this.

> While at it, a comment in elm327_init mentioned bittiming_const - that
> was from the pre-bitrate_const (< v4.11) times, and I've now updated it
> to mention bitrate_const instead.

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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