Re: [PATCH v2 1/6] net: wan: Add support for QMC HDLC

[Paolo Abeni]

On Tue, 2024-01-30 at 09:40 +0100, Herve Codina wrote:
> The QMC HDLC driver provides support for HDLC using the QMC (QUICC
> Multichannel Controller) to transfer the HDLC data.
> 
> Signed-off-by: Herve Codina <herve.codina@xxxxxxxxxxx>
> Reviewed-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
> Acked-by: Jakub Kicinski <kuba@xxxxxxxxxx>
> ---
>  drivers/net/wan/Kconfig        |  12 +
>  drivers/net/wan/Makefile       |   1 +
>  drivers/net/wan/fsl_qmc_hdlc.c | 422 +++++++++++++++++++++++++++++++++
>  3 files changed, 435 insertions(+)
>  create mode 100644 drivers/net/wan/fsl_qmc_hdlc.c
> 
> diff --git a/drivers/net/wan/Kconfig b/drivers/net/wan/Kconfig
> index 7dda87756d3f..31ab2136cdf1 100644
> --- a/drivers/net/wan/Kconfig
> +++ b/drivers/net/wan/Kconfig
> @@ -197,6 +197,18 @@ config FARSYNC
>  	  To compile this driver as a module, choose M here: the
>  	  module will be called farsync.
>  
> +config FSL_QMC_HDLC
> +	tristate "Freescale QMC HDLC support"
> +	depends on HDLC
> +	depends on CPM_QMC
> +	help
> +	  HDLC support using the Freescale QUICC Multichannel Controller (QMC).
> +
> +	  To compile this driver as a module, choose M here: the
> +	  module will be called fsl_qmc_hdlc.
> +
> +	  If unsure, say N.
> +
>  config FSL_UCC_HDLC
>  	tristate "Freescale QUICC Engine HDLC support"
>  	depends on HDLC
> diff --git a/drivers/net/wan/Makefile b/drivers/net/wan/Makefile
> index 8119b49d1da9..00e9b7ee1e01 100644
> --- a/drivers/net/wan/Makefile
> +++ b/drivers/net/wan/Makefile
> @@ -25,6 +25,7 @@ obj-$(CONFIG_WANXL)		+= wanxl.o
>  obj-$(CONFIG_PCI200SYN)		+= pci200syn.o
>  obj-$(CONFIG_PC300TOO)		+= pc300too.o
>  obj-$(CONFIG_IXP4XX_HSS)	+= ixp4xx_hss.o
> +obj-$(CONFIG_FSL_QMC_HDLC)	+= fsl_qmc_hdlc.o
>  obj-$(CONFIG_FSL_UCC_HDLC)	+= fsl_ucc_hdlc.o
>  obj-$(CONFIG_SLIC_DS26522)	+= slic_ds26522.o
>  
> diff --git a/drivers/net/wan/fsl_qmc_hdlc.c b/drivers/net/wan/fsl_qmc_hdlc.c
> new file mode 100644
> index 000000000000..e7b2b72a6050
> --- /dev/null
> +++ b/drivers/net/wan/fsl_qmc_hdlc.c
> @@ -0,0 +1,422 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * Freescale QMC HDLC Device Driver
> + *
> + * Copyright 2023 CS GROUP France
> + *
> + * Author: Herve Codina <herve.codina@xxxxxxxxxxx>
> + */
> +
> +#include <linux/dma-mapping.h>
> +#include <linux/hdlc.h>
> +#include <linux/module.h>
> +#include <linux/of.h>
> +#include <linux/of_platform.h>
> +#include <linux/platform_device.h>
> +#include <linux/slab.h>
> +#include <soc/fsl/qe/qmc.h>
> +
> +struct qmc_hdlc_desc {
> +	struct net_device *netdev;
> +	struct sk_buff *skb; /* NULL if the descriptor is not in use */
> +	dma_addr_t dma_addr;
> +	size_t dma_size;
> +};
> +
> +struct qmc_hdlc {
> +	struct device *dev;
> +	struct qmc_chan *qmc_chan;
> +	struct net_device *netdev;
> +	bool is_crc32;
> +	spinlock_t tx_lock; /* Protect tx descriptors */
> +	struct qmc_hdlc_desc tx_descs[8];
> +	unsigned int tx_out;
> +	struct qmc_hdlc_desc rx_descs[4];
> +};
> +
> +static inline struct qmc_hdlc *netdev_to_qmc_hdlc(struct net_device *netdev)
> +{
> +	return dev_to_hdlc(netdev)->priv;
> +}

Please, no 'inline' function in c files. You could move this function
and the struct definition into a new, local, header file.

> +static int qmc_hdlc_recv_queue(struct qmc_hdlc *qmc_hdlc, struct qmc_hdlc_desc *desc, size_t size);
> +
> +#define QMC_HDLC_RX_ERROR_FLAGS (QMC_RX_FLAG_HDLC_OVF | \
> +				 QMC_RX_FLAG_HDLC_UNA | \
> +				 QMC_RX_FLAG_HDLC_ABORT | \
> +				 QMC_RX_FLAG_HDLC_CRC)
> +
> +static void qmc_hcld_recv_complete(void *context, size_t length, unsigned int flags)
> +{
> +	struct qmc_hdlc_desc *desc = context;
> +	struct net_device *netdev = desc->netdev;
> +	struct qmc_hdlc *qmc_hdlc = netdev_to_qmc_hdlc(netdev);
> +	int ret;

Please, respect the reverse x-mas tree order for local variable
definition.

> +	dma_unmap_single(qmc_hdlc->dev, desc->dma_addr, desc->dma_size, DMA_FROM_DEVICE);
> +
> +	if (flags & QMC_HDLC_RX_ERROR_FLAGS) {
> +		netdev->stats.rx_errors++;
> +		if (flags & QMC_RX_FLAG_HDLC_OVF) /* Data overflow */
> +			netdev->stats.rx_over_errors++;
> +		if (flags & QMC_RX_FLAG_HDLC_UNA) /* bits received not multiple of 8 */
> +			netdev->stats.rx_frame_errors++;
> +		if (flags & QMC_RX_FLAG_HDLC_ABORT) /* Received an abort sequence */
> +			netdev->stats.rx_frame_errors++;
> +		if (flags & QMC_RX_FLAG_HDLC_CRC) /* CRC error */
> +			netdev->stats.rx_crc_errors++;
> +		kfree_skb(desc->skb);
> +	} else {
> +		netdev->stats.rx_packets++;
> +		netdev->stats.rx_bytes += length;
> +
> +		skb_put(desc->skb, length);
> +		desc->skb->protocol = hdlc_type_trans(desc->skb, netdev);
> +		netif_rx(desc->skb);
> +	}
> +
> +	/* Re-queue a transfer using the same descriptor */
> +	ret = qmc_hdlc_recv_queue(qmc_hdlc, desc, desc->dma_size);
> +	if (ret) {
> +		dev_err(qmc_hdlc->dev, "queue recv desc failed (%d)\n", ret);
> +		netdev->stats.rx_errors++;
> +	}
> +}

[...]

> +static netdev_tx_t qmc_hdlc_xmit(struct sk_buff *skb, struct net_device *netdev)
> +{
> +	struct qmc_hdlc *qmc_hdlc = netdev_to_qmc_hdlc(netdev);
> +	struct qmc_hdlc_desc *desc;
> +	unsigned long flags;
> +	int ret;
> +
> +	spin_lock_irqsave(&qmc_hdlc->tx_lock, flags);
> +	desc = &qmc_hdlc->tx_descs[qmc_hdlc->tx_out];
> +	if (WARN_ONCE(!desc->skb, "No tx descriptors available\n")) {
> +		/* Should never happen.
> +		 * Previous xmit should have already stopped the queue.
> +		 */
> +		netif_stop_queue(netdev);
> +		spin_unlock_irqrestore(&qmc_hdlc->tx_lock, flags);
> +		return NETDEV_TX_BUSY;
> +	}
> +	spin_unlock_irqrestore(&qmc_hdlc->tx_lock, flags);
> +
> +	desc->netdev = netdev;
> +	desc->dma_size = skb->len;
> +	desc->skb = skb;
> +	ret = qmc_hdlc_xmit_queue(qmc_hdlc, desc);
> +	if (ret) {
> +		desc->skb = NULL; /* Release the descriptor */
> +		if (ret == -EBUSY) {
> +			netif_stop_queue(netdev);
> +			return NETDEV_TX_BUSY;
> +		}
> +		dev_kfree_skb(skb);
> +		netdev->stats.tx_dropped++;
> +		return NETDEV_TX_OK;
> +	}
> +
> +	qmc_hdlc->tx_out = (qmc_hdlc->tx_out + 1) % ARRAY_SIZE(qmc_hdlc->tx_descs);
> +
> +	spin_lock_irqsave(&qmc_hdlc->tx_lock, flags);
> +	if (qmc_hdlc->tx_descs[qmc_hdlc->tx_out].skb)
> +		netif_stop_queue(netdev);
> +	spin_unlock_irqrestore(&qmc_hdlc->tx_lock, flags);

The locking schema is quite bad, as the drivers acquires and releases 3
locks for each tx packet. You could improve that using the qmc_chan-
>tx_lock to protect the whole qmc_hdlc_xmit() function, factoring out a
lockless variant of qmc_hdlc_xmit_queue(), and using it here.

In general is quite bad that the existing infra does not allow
leveraging NAPI. Have you considered expanding the QMC to accomodate
such user?

Cheers,

Paolo