Subject: Re: [PATCH v4 1/4] can: dev: Add support for limiting configured
 bitrate
To: Franklin S Cooper Jr <fcooper@ti.com>, linux-kernel@vger.kernel.org,
        devicetree@vger.kernel.org, netdev@vger.kernel.org,
        linux-can@vger.kernel.org, wg@grandegger.com, mkl@pengutronix.de,
        robh+dt@kernel.org, quentin.schulz@free-electrons.com,
        dev.kurt@vandijck-laurijssen.be, andrew@lunn.ch,
        socketcan@hartkopp.net
References: <20170810005916.27163-1-fcooper@ti.com>
 <20170810005916.27163-2-fcooper@ti.com>
From: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Message-ID: <9a6f4fdf-a4f6-283b-0e5a-693cc7d263ef@cogentembedded.com>
Date: Thu, 10 Aug 2017 13:05:14 +0300
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Hello!

On 8/10/2017 3:59 AM, Franklin S Cooper Jr wrote:

> Various CAN or CAN-FD IP may be able to run at a faster rate than
> what the transceiver the CAN node is connected to. This can lead to
> unexpected errors. However, CAN transceivers typically have fixed
> limitations and provide no means to discover these limitations at
> runtime. Therefore, add support for a can-transceiver node that
> can be reused by other CAN peripheral drivers to determine for both
> CAN and CAN-FD what the max bitrate that can be used. If the user
> tries to configure CAN to pass these maximum bitrates it will throw
> an error.
> 
> Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
> ---
> Version 4 changes:
> Used can-transceiver instead of fixed-transceiver.
> 
>   drivers/net/can/dev.c   | 50 +++++++++++++++++++++++++++++++++++++++++++++++++
>   include/linux/can/dev.h |  5 +++++
>   2 files changed, 55 insertions(+)
> 
> diff --git a/drivers/net/can/dev.c b/drivers/net/can/dev.c
> index 365a8cc..372108f 100644
> --- a/drivers/net/can/dev.c
> +++ b/drivers/net/can/dev.c
[...]
> @@ -814,6 +815,39 @@ int open_candev(struct net_device *dev)
>   }
>   EXPORT_SYMBOL_GPL(open_candev);
>   
> +#ifdef CONFIG_OF
> +/*
> + * Common function that can be used to understand the limitation of
> + * a transceiver when it provides no means to determine these limitations
> + * at runtime.
> + */
> +void of_can_transceiver(struct net_device *dev)
> +{
> +	struct device_node *dn;
> +	struct can_priv *priv = netdev_priv(dev);
> +	struct device_node *np;
> +	unsigned int max_bitrate;
> +	int ret;
> +
> +	np = dev->dev.parent->of_node;

    I'd do that as an initializer.

> +
> +	dn = of_get_child_by_name(np, "can-transceiver");
> +	if (!dn)
> +		return;
> +
> +	max_bitrate = 0;
> +	ret = of_property_read_u32(dn, "max-bitrate", &max_bitrate);

    I'd initialize max_bitrate to 0 as iff of_property_read_u32() fails, it'll 
leave the variable unset...

> +
> +	if (max_bitrate > 0) {

    You risk checking unset variable here.

> +		priv->max_bitrate = max_bitrate;
> +		priv->is_bitrate_limited = true;
> +	} else if (ret != -EINVAL) {
> +		netdev_warn(dev, "Invalid value for transceiver max bitrate\n");
> +	}
> +}
> +EXPORT_SYMBOL(of_can_transceiver);
> +#endif
> +
>   /*
>    * Common close function for cleanup before the device gets closed.
>    *
[...]

MBR, Sergei