linux-kernel - Re: [PATCH v8 4/7] can: Add Nuvoton NCT6694 CANFD support

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Message-ID: <20250326-utopian-mega-scallop-5f6899-mkl@pengutronix.de>
Date: Wed, 26 Mar 2025 18:35:47 +0100
From: Marc Kleine-Budde <mkl@...gutronix.de>
To: Ming Yu <a0282524688@...il.com>
Cc: tmyu0@...oton.com, lee@...nel.org, linus.walleij@...aro.org, 
	brgl@...ev.pl, andi.shyti@...nel.org, mailhol.vincent@...adoo.fr, 
	andrew+netdev@...n.ch, davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, 
	pabeni@...hat.com, wim@...ux-watchdog.org, linux@...ck-us.net, jdelvare@...e.com, 
	alexandre.belloni@...tlin.com, linux-kernel@...r.kernel.org, linux-gpio@...r.kernel.org, 
	linux-i2c@...r.kernel.org, linux-can@...r.kernel.org, netdev@...r.kernel.org, 
	linux-watchdog@...r.kernel.org, linux-hwmon@...r.kernel.org, linux-rtc@...r.kernel.org, 
	linux-usb@...r.kernel.org
Subject: Re: [PATCH v8 4/7] can: Add Nuvoton NCT6694 CANFD support

On 26.03.2025 10:37:11, Ming Yu wrote:
> Marc Kleine-Budde <mkl@...gutronix.de> 於 2025年3月17日 週一 下午8:01寫道：
> > > +static int nct6694_can_start(struct net_device *ndev)
> > > +{
> > > +     struct nct6694_can_priv *priv = netdev_priv(ndev);
> > > +     const struct can_bittiming *d_bt = &priv->can.data_bittiming;
> > > +     const struct can_bittiming *n_bt = &priv->can.bittiming;
> > > +     struct nct6694_can_setting *setting __free(kfree) = NULL;
> > > +     const struct nct6694_cmd_header cmd_hd = {
> > > +             .mod = NCT6694_CAN_MOD,
> > > +             .cmd = NCT6694_CAN_SETTING,
> > > +             .sel = ndev->dev_port,
> > > +             .len = cpu_to_le16(sizeof(*setting))
> > > +     };
> > > +     int ret;
> > > +
> > > +     setting = kzalloc(sizeof(*setting), GFP_KERNEL);
> > > +     if (!setting)
> > > +             return -ENOMEM;
> > > +
> > > +     setting->nbr = cpu_to_le32(n_bt->bitrate);
> > > +     setting->dbr = cpu_to_le32(d_bt->bitrate);
> >
> > I just noticed one thing that needs clarification/documentation.
> >
> > You have nct6694_can_bittiming_nominal_const and
> > nct6694_can_bittiming_data_const, but only pass the bit rates to your
> > device.
> >
> > Do the bit timing const really reflect the HW limitations of your
> > device?
> >
> > Are you sure your device uses the same algorithm as the kernel and
> > calculates the same bit timing parameters as the kernel, so that the
> > values given to the user space reflects the bit timing parameter chosen
> > by your device?
> >
> 
> Originally, I only intended to provide NBR and DBR for user
> configuration. In the next patch, I will add code to configure
> NBTP(Nominal Bit Timing Prescaler) and DBTP(Data Bit Timing Prescaler)
> based on the setting of nct6694_can_bittiming_nominal_const and
> nct6694_can_bittiming_data_const.

Sounds good, but this doesn't answer my questions:

You have nct6694_can_bittiming_nominal_const and
nct6694_can_bittiming_data_const, but only pass the bit rates and the
prescaler to your device.

Do the bit timing const really reflect the HW limitations of your
device?

Are you sure your device uses the same algorithm as the kernel and
calculates the same bit timing parameters as the kernel, so that the
values given to the user space reflects the bit timing parameter chosen
by your device?

Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde          |
Embedded Linux                   | https://www.pengutronix.de |
Vertretung Nürnberg              | Phone: +49-5121-206917-129 |
Amtsgericht Hildesheim, HRA 2686 | Fax:   +49-5121-206917-9   |

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