Re: [net-next,v3,1/3] net: dsa: tag_yt921x: fix priority support

[Simon Horman <horms@kernel.org>]

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net: dsa: yt921x: Add DCB/QoS support

This commit adds DCB/QoS support to the yt921x DSA switch driver,
setting up global DSCP/PCP priority mappings and adding related DCB
methods.

> diff --git a/drivers/net/dsa/yt921x.c b/drivers/net/dsa/yt921x.c
> index af8464c42b51..87e33d0df4ac 100644
> --- a/drivers/net/dsa/yt921x.c
> +++ b/drivers/net/dsa/yt921x.c

[ ... ]

> +/* Not "port" - DSCP mapping is global */
> +static int __maybe_unused
> +yt921x_dsa_port_get_dscp_prio(struct dsa_switch *ds, int port, u8 dscp)
> +{
> +	struct yt921x_priv *priv = to_yt921x_priv(ds);
> +	u32 val;
> +	int res;
> +
> +	mutex_lock(&priv->reg_lock);
> +	res = yt921x_reg_read(priv, YT921X_IPM_DSCPn(dscp), &val);
> +	mutex_unlock(&priv->reg_lock);
> +
> +	if (res)
> +		return res;
> +
> +	return FIELD_GET(YT921X_IPM_PRIO_M, val);
> +}
> +
> +static int __maybe_unused
> +yt921x_dsa_port_del_dscp_prio(struct dsa_switch *ds, int port, u8 dscp, u8 prio)
> +{
> +	struct yt921x_priv *priv = to_yt921x_priv(ds);
> +	int res;
> +
> +	mutex_lock(&priv->reg_lock);
> +	res = yt921x_reg_write(priv, YT921X_IPM_DSCPn(dscp),
> +			       YT921X_IPM_PRIO(IEEE8021Q_TT_BK));
> +	mutex_unlock(&priv->reg_lock);
> +
> +	return res;
> +}

Should yt921x_dsa_port_del_dscp_prio() check the prio parameter before
resetting the DSCP mapping? The ocelot driver (ocelot_port_del_dscp_prio
in drivers/net/ethernet/mscc/ocelot.c) has a comment explaining that
during a "dcb app replace" command, the new entry is added first, then
the old entry is deleted. If the delete operation ignores the prio
parameter, it will incorrectly reset the newly-added mapping instead
of only deleting when the current priority matches the old one.

The ksz driver also checks this condition in ksz_port_del_dscp_prio().

Without this check, the sequence for "dcb app replace ... dscp X prio N"
followed by "dcb app replace ... dscp X prio M" would be:
  1. add dscp X -> prio M (correct)
  2. del dscp X, prio N (but code ignores prio N and resets to BK)
  3. result: dscp X -> BK instead of prio M

Perhaps something like this is needed:

  if (yt921x_dsa_port_get_dscp_prio(ds, port, dscp) != prio)
      return 0;