linux-kernel - Re: [PATCH v2 6/6] net: ocelot: add jumbo frame support for FDMA

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Date:   Wed, 3 Nov 2021 12:43:20 +0000
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Clément Léger <clement.leger@...tlin.com>
CC:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Claudiu Manoil <claudiu.manoil@....com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        "UNGLinuxDriver@...rochip.com" <UNGLinuxDriver@...rochip.com>,
        Andrew Lunn <andrew@...n.ch>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH v2 6/6] net: ocelot: add jumbo frame support for FDMA

On Wed, Nov 03, 2021 at 10:19:43AM +0100, Clément Léger wrote:
> When using the FDMA, using jumbo frames can lead to a large performance
> improvement. When changing the MTU, the RX buffer size must be
> increased to be large enough to receive jumbo frame. Since the FDMA is
> shared amongst all interfaces, all the ports must be down before
> changing the MTU. Buffers are sized to accept the maximum MTU supported
> by each port.
> 
> Signed-off-by: Clément Léger <clement.leger@...tlin.com>
> ---

Instead of draining buffers and refilling with a different size, which
impacts the user experience, can you not just use scatter/gather RX
processing for frames larger than the fixed buffer size, like a normal
driver would?

>  drivers/net/ethernet/mscc/ocelot_fdma.c | 61 +++++++++++++++++++++++++
>  drivers/net/ethernet/mscc/ocelot_fdma.h |  1 +
>  drivers/net/ethernet/mscc/ocelot_net.c  |  7 +++
>  3 files changed, 69 insertions(+)
> 
> diff --git a/drivers/net/ethernet/mscc/ocelot_fdma.c b/drivers/net/ethernet/mscc/ocelot_fdma.c
> index d8cdf022bbee..bee1a310caa6 100644
> --- a/drivers/net/ethernet/mscc/ocelot_fdma.c
> +++ b/drivers/net/ethernet/mscc/ocelot_fdma.c
> @@ -530,6 +530,67 @@ static void fdma_free_skbs_list(struct ocelot_fdma *fdma,
>  	}
>  }
>  
> +int ocelot_fdma_change_mtu(struct net_device *dev, int new_mtu)
> +{
> +	struct ocelot_port_private *priv = netdev_priv(dev);
> +	struct ocelot_port *port = &priv->port;
> +	struct ocelot *ocelot = port->ocelot;
> +	struct ocelot_fdma *fdma = ocelot->fdma;
> +	struct ocelot_fdma_dcb *dcb, *dcb_temp;
> +	struct list_head tmp = LIST_HEAD_INIT(tmp);
> +	size_t old_rx_buf_size = fdma->rx_buf_size;
> +	bool all_ports_down = true;
> +	u8 port_num;
> +
> +	/* The FDMA RX list is shared amongst all the port, get the max MTU from
> +	 * all of them
> +	 */
> +	for (port_num = 0; port_num < ocelot->num_phys_ports; port_num++) {
> +		port = ocelot->ports[port_num];
> +		if (!port)
> +			continue;
> +
> +		priv = container_of(port, struct ocelot_port_private, port);
> +
> +		if (READ_ONCE(priv->dev->mtu) > new_mtu)
> +			new_mtu = READ_ONCE(priv->dev->mtu);
> +
> +		/* All ports must be down to change the RX buffer length */
> +		if (netif_running(priv->dev))
> +			all_ports_down = false;
> +	}
> +
> +	fdma->rx_buf_size = fdma_rx_compute_buffer_size(new_mtu);
> +	if (fdma->rx_buf_size == old_rx_buf_size)
> +		return 0;
> +
> +	if (!all_ports_down)
> +		return -EBUSY;
> +
> +	priv = netdev_priv(dev);
> +
> +	fdma_stop_channel(fdma, MSCC_FDMA_INJ_CHAN);
> +
> +	/* Discard all pending RX software and hardware descriptor */
> +	fdma_free_skbs_list(fdma, &fdma->rx_hw, DMA_FROM_DEVICE);
> +	fdma_free_skbs_list(fdma, &fdma->rx_sw, DMA_FROM_DEVICE);
> +
> +	/* Move all DCBs to a temporary list that will be injected in sw list */
> +	if (!list_empty(&fdma->rx_hw))
> +		list_splice_tail_init(&fdma->rx_hw, &tmp);
> +	if (!list_empty(&fdma->rx_sw))
> +		list_splice_tail_init(&fdma->rx_sw, &tmp);
> +
> +	list_for_each_entry_safe(dcb, dcb_temp, &tmp, node) {
> +		list_del(&dcb->node);
> +		ocelot_fdma_rx_add_dcb_sw(fdma, dcb);
> +	}
> +
> +	ocelot_fdma_rx_refill(fdma);
> +
> +	return 0;
> +}
> +
>  static int fdma_init_tx(struct ocelot_fdma *fdma)
>  {
>  	int i;
> diff --git a/drivers/net/ethernet/mscc/ocelot_fdma.h b/drivers/net/ethernet/mscc/ocelot_fdma.h
> index 6c5c5872abf5..74514a0b291a 100644
> --- a/drivers/net/ethernet/mscc/ocelot_fdma.h
> +++ b/drivers/net/ethernet/mscc/ocelot_fdma.h
> @@ -55,5 +55,6 @@ int ocelot_fdma_start(struct ocelot_fdma *fdma);
>  int ocelot_fdma_stop(struct ocelot_fdma *fdma);
>  int ocelot_fdma_inject_frame(struct ocelot_fdma *fdma, int port, u32 rew_op,
>  			     struct sk_buff *skb, struct net_device *dev);
> +int ocelot_fdma_change_mtu(struct net_device *dev, int new_mtu);
>  
>  #endif
> diff --git a/drivers/net/ethernet/mscc/ocelot_net.c b/drivers/net/ethernet/mscc/ocelot_net.c
> index 3971b810c5b4..d5e88d7b15c7 100644
> --- a/drivers/net/ethernet/mscc/ocelot_net.c
> +++ b/drivers/net/ethernet/mscc/ocelot_net.c
> @@ -492,6 +492,13 @@ static int ocelot_change_mtu(struct net_device *dev, int new_mtu)
>  	struct ocelot_port_private *priv = netdev_priv(dev);
>  	struct ocelot_port *ocelot_port = &priv->port;
>  	struct ocelot *ocelot = ocelot_port->ocelot;
> +	int ret;
> +
> +	if (ocelot->fdma) {
> +		ret = ocelot_fdma_change_mtu(dev, new_mtu);
> +		if (ret)
> +			return ret;
> +	}
>  
>  	ocelot_port_set_maxlen(ocelot, priv->chip_port, new_mtu);
>  	WRITE_ONCE(dev->mtu, new_mtu);
> -- 
> 2.33.0
>