netdev - Re: [PATCH v3 04/13] can: slcan: use CAN network device driver API

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Date:   Mon, 13 Jun 2022 09:01:05 +0200
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Dario Binacchi <dario.binacchi@...rulasolutions.com>
Cc:     linux-kernel@...r.kernel.org, michael@...rulasolutions.com,
        Amarula patchwork <linux-amarula@...rulasolutions.com>,
        Oliver Hartkopp <socketcan@...tkopp.net>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Wolfgang Grandegger <wg@...ndegger.com>,
        linux-can@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH v3 04/13] can: slcan: use CAN network device driver API

On 12.06.2022 23:39:18, Dario Binacchi wrote:
> As suggested by commit [1], now the driver uses the functions and the
> data structures provided by the CAN network device driver interface.
> 
> Currently the driver doesn't implement a way to set bitrate for SLCAN
> based devices via ip tool, so you'll have to do this by slcand or
> slcan_attach invocation through the -sX parameter:
> 
> - slcan_attach -f -s6 -o /dev/ttyACM0
> - slcand -f -s8 -o /dev/ttyUSB0
> 
> where -s6 in will set adapter's bitrate to 500 Kbit/s and -s8 to
> 1Mbit/s.
> See the table below for further CAN bitrates:
> - s0 ->   10 Kbit/s
> - s1 ->   20 Kbit/s
> - s2 ->   50 Kbit/s
> - s3 ->  100 Kbit/s
> - s4 ->  125 Kbit/s
> - s5 ->  250 Kbit/s
> - s6 ->  500 Kbit/s
> - s7 ->  800 Kbit/s
> - s8 -> 1000 Kbit/s
> 
> In doing so, the struct can_priv::bittiming.bitrate of the driver is not
> set and since the open_candev() checks that the bitrate has been set, it
> must be a non-zero value, the bitrate is set to a fake value (-1U)
> before it is called.

The patch description doesn't mention the change of the locking from
rtnl to a spin_lock. Please test your code with a kernel that has
CONFIG_PROVE_LOCKING enabled.

Please move patch
| [PATCH v3 05/13] can: netlink: dump bitrate 0 if can_priv::bittiming.bitrate is -1U
in front of this one.

> @@ -374,7 +375,7 @@ static netdev_tx_t slc_xmit(struct sk_buff *skb, struct net_device *dev)
>  	spin_unlock(&sl->lock);
>  
>  out:
> -	kfree_skb(skb);
> +	can_put_echo_skb(skb, dev, 0, 0);

Where's the corresponding can_get_echo_skb()?

If you convert the driver to can_put_echo_skb()/can_get_echo_skb(), you
must set "netdev->flags |= IFF_ECHO;". And you should do the put()
before triggering the send, the corresponding get() needs to be added
where you have the TX completion from the hardware. If you don't get a
response the best you can do is moving it after the triggering of the
send.

If you want to convert the driver to can_put/get_echo_skb(), please make
it a separate patch.

>  	return NETDEV_TX_OK;
>  }
>  
> @@ -394,6 +395,8 @@ static int slc_close(struct net_device *dev)
>  		clear_bit(TTY_DO_WRITE_WAKEUP, &sl->tty->flags);
>  	}
>  	netif_stop_queue(dev);
> +	close_candev(dev);
> +	sl->can.state = CAN_STATE_STOPPED;
>  	sl->rcount   = 0;
>  	sl->xleft    = 0;
>  	spin_unlock_bh(&sl->lock);
> @@ -405,21 +408,36 @@ static int slc_close(struct net_device *dev)
>  static int slc_open(struct net_device *dev)
>  {
>  	struct slcan *sl = netdev_priv(dev);
> +	int err;
>  
>  	if (sl->tty == NULL)
>  		return -ENODEV;
>  
> +	/* The baud rate is not set with the command
> +	 * `ip link set <iface> type can bitrate <baud>' and therefore
> +	 * can.bittiming.bitrate is 0, causing open_candev() to fail.
> +	 * So let's set to a fake value.
> +	 */
> +	sl->can.bittiming.bitrate = -1;
> +	err = open_candev(dev);
> +	if (err) {
> +		netdev_err(dev, "failed to open can device\n");
> +		return err;
> +	}
> +
> +	sl->can.state = CAN_STATE_ERROR_ACTIVE;
>  	sl->flags &= BIT(SLF_INUSE);
>  	netif_start_queue(dev);
>  	return 0;
>  }
>  
> -/* Hook the destructor so we can free slcan devs at the right point in time */
> -static void slc_free_netdev(struct net_device *dev)
> +static void slc_dealloc(struct slcan *sl)
>  {
> -	int i = dev->base_addr;
> +	int i = sl->dev->base_addr;
>  
> -	slcan_devs[i] = NULL;
> +	free_candev(sl->dev);
> +	if (slcan_devs)

Why have you added this check?

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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