linux-kernel - Re: [RFC PATCH 4/4] can: bxcan: add support for ST bxCAN controller

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Message-ID: <20220818130814.z7b4rvmld6wvk4fg@pengutronix.de>
Date:   Thu, 18 Aug 2022 15:08:14 +0200
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Dario Binacchi <dario.binacchi@...rulasolutions.com>
Cc:     linux-kernel@...r.kernel.org,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Amarula patchwork <linux-amarula@...rulasolutions.com>,
        michael@...rulasolutions.com, Dario Binacchi <dariobin@...ero.it>,
        linux-can@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [RFC PATCH 4/4] can: bxcan: add support for ST bxCAN controller

On 18.08.2022 12:30:31, Marc Kleine-Budde wrote:
> One step at a time, let's look at the TX path:
> 
> On 17.08.2022 16:35:29, Dario Binacchi wrote:
> > +static netdev_tx_t bxcan_start_xmit(struct sk_buff *skb,
> > +				    struct net_device *ndev)
> > +{
> > +	struct bxcan_priv *priv = netdev_priv(ndev);
> > +	struct can_frame *cf = (struct can_frame *)skb->data;
> > +	struct bxcan_regs *regs = priv->regs;
> > +	struct bxcan_mb *mb_regs;
> 
> __iomem?
> 
> > +	unsigned int mb_id;
> > +	u32 id, tsr;
> > +	int i, j;
> > +
> > +	if (can_dropped_invalid_skb(ndev, skb))
> > +		return NETDEV_TX_OK;
> > +
> > +	tsr = readl(&regs->tsr);
> > +	mb_id = ffs((tsr & BXCAN_TSR_TME) >> BXCAN_TSR_TME_SHIFT);
> 
> We want to send the CAN frames in the exact order they are pushed into
> the driver, so don't pick the first free mailbox you find. How a
                                                                 are
> priorities for the TX mailboxes handled?
> 
> Is the mailbox with the lowest number send first? Is there a priority
> field in the mailbox?

I just had a look into the data sheet and it says that the TX mailboxes
are handled in transmit request order. This is good.

[...]

> The mcp251xfd has a proper hardware FIFO ring buffer for TX, the bxcan
> probably doesn't. The get_tx_free() check is a bit different. Look at
> c_can_get_tx_free() in:

This means you can use the simpler get_tx_free from the mcp251xfd driver.
> 
> | https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=28e86e9ab522e65b08545e5008d0f1ac5b19dad1
> 
> This patch is a good example for the relevant changes.

This patch is non the less a good example for the TX path.

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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