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    SubjectRe: [RFC PATCH 4/4] can: bxcan: add support for ST bxCAN controller
    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
    priorities for the TX mailboxes handled?

    Is the mailbox with the lowest number send first? Is there a priority
    field in the mailbox?

    If the mail with the lowest number is transmitted first, it's best to
    have a tx_head and tx_tail counter, e.g:

    struct bxcan_priv {
    ...
    unsigned int tx_head;
    unsigned int tx_tail;
    ...
    };

    They both start with 0. The xmit function pushes the CAN frame into the
    "priv->tx_head % 3" mailbox. Before triggering the xmit in hardware the
    tx_head is incremented.

    In your TX complete ISR look at priv->tx_tail % 3 for completion,
    increment tx_tail, loop.

    > + if (mb_id == 0)
    > + return NETDEV_TX_BUSY;
    > +
    > + mb_id -= 1;
    > + mb_regs = &regs->tx_mb[mb_id];
    > +
    > + if (cf->can_id & CAN_EFF_FLAG)
    > + id = BXCAN_TIxR_EXID(cf->can_id & CAN_EFF_MASK) |
    > + BXCAN_TIxR_IDE;
    > + else
    > + id = BXCAN_TIxR_STID(cf->can_id & CAN_SFF_MASK);
    > +
    > + if (cf->can_id & CAN_RTR_FLAG)
    > + id |= BXCAN_TIxR_RTR;
    > +
    > + bxcan_rmw(&mb_regs->dlc, BXCAN_TDTxR_DLC_MASK,
    > + BXCAN_TDTxR_DLC(cf->len));
    > + priv->tx_dlc[mb_id] = cf->len;

    Please use can_put_echo_skb() for this.

    > +
    > + for (i = 0, j = 0; i < cf->len; i += 4, j++)
    > + writel(*(u32 *)(cf->data + i), &mb_regs->data[j]);
    > +
    > + /* Start transmission */
    > + writel(id | BXCAN_TIxR_TXRQ, &mb_regs->id);
    > + /* Stop the queue if we've filled all mailbox entries */
    > + if (!(readl(&regs->tsr) & BXCAN_TSR_TME))
    > + netif_stop_queue(ndev);

    This is racy. You have to stop the queue before triggering the
    transmission.

    Have a look at the mcp251xfd driver:

    | https://elixir.bootlin.com/linux/latest/source/drivers/net/can/spi/mcp251xfd/mcp251xfd-tx.c#L187

    The check for NETDEV_TX_BUSY is a bit more complicated, too:

    | https://elixir.bootlin.com/linux/latest/source/drivers/net/can/spi/mcp251xfd/mcp251xfd-tx.c#L178

    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:

    | 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.

    > +
    > + return NETDEV_TX_OK;
    > +}

    [...]

    > +static irqreturn_t bxcan_tx_isr(int irq, void *dev_id)
    > +{
    > + struct net_device *ndev = dev_id;
    > + struct bxcan_priv *priv = netdev_priv(ndev);
    > + struct bxcan_regs __iomem *regs = priv->regs;
    > + struct net_device_stats *stats = &ndev->stats;
    > + u32 tsr, rqcp_bit = BXCAN_TSR_RQCP0;
    > + int i;
    > +
    > + tsr = readl(&regs->tsr);
    > + for (i = 0; i < BXCAN_TX_MB_NUM; rqcp_bit <<= 8, i++) {

    This might break the order of TX completion CAN frames.

    > + if (!(tsr & rqcp_bit))
    > + continue;
    > +
    > + stats->tx_packets++;
    > + stats->tx_bytes += priv->tx_dlc[i];

    Use can_get_echo_skb() here.

    > + }
    > +
    > + writel(tsr, &regs->tsr);
    > +
    > + if (netif_queue_stopped(ndev))
    > + netif_wake_queue(ndev);

    With tx_head and tx_tail this should look like this:

    | https://elixir.bootlin.com/linux/v5.19/source/drivers/net/can/spi/mcp251xfd/mcp251xfd-tef.c#L251

    > +
    > + return IRQ_HANDLED;
    > +}

    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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