netdev - Re: [RFC 5/9] staging: dpaa2-switch: handle Rx path on control interface

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Message-ID: <20201105004516.GG933237@lunn.ch>
Date:   Thu, 5 Nov 2020 01:45:16 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Ioana Ciornei <ciorneiioana@...il.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        Ioana Ciornei <ioana.ciornei@....com>
Subject: Re: [RFC 5/9] staging: dpaa2-switch: handle Rx path on control
 interface

> +/* Manage all NAPI instances for the control interface.
> + *
> + * We only have one RX queue and one Tx Conf queue for all
> + * switch ports. Therefore, we only need to enable the NAPI instance once, the
> + * first time one of the switch ports runs .dev_open().
> + */
> +
> +static void dpaa2_switch_enable_ctrl_if_napi(struct ethsw_core *ethsw)
> +{
> +	int i;
> +
> +	/* a new interface is using the NAPI instance */
> +	ethsw->napi_users++;
> +
> +	/* if there is already a user of the instance, return */
> +	if (ethsw->napi_users > 1)
> +		return;

Does there need to be any locking here? Or does it rely on RTNL?
Maybe a comment would be nice, or a check that RTNL is actually held.

> +
> +	if (!dpaa2_switch_has_ctrl_if(ethsw))
> +		return;
> +
> +	for (i = 0; i < DPAA2_SWITCH_RX_NUM_FQS; i++)
> +		napi_enable(&ethsw->fq[i].napi);
> +}

> +static void dpaa2_switch_rx(struct dpaa2_switch_fq *fq,
> +			    const struct dpaa2_fd *fd)
> +{
> +	struct ethsw_core *ethsw = fq->ethsw;
> +	struct ethsw_port_priv *port_priv;
> +	struct net_device *netdev;
> +	struct vlan_ethhdr *hdr;
> +	struct sk_buff *skb;
> +	u16 vlan_tci, vid;
> +	int if_id = -1;
> +	int err;
> +
> +	/* prefetch the frame descriptor */
> +	prefetch(fd);

Does this actually do any good, given that the next call:

> +
> +	/* get switch ingress interface ID */
> +	if_id = upper_32_bits(dpaa2_fd_get_flc(fd)) & 0x0000FFFF;

is accessing the frame descriptor? The idea of prefetch is to let it
bring it into the cache while you are busy doing something else,
hopefully with something which is already cache hot.

	  Andrew