Re: [PATCH net-next v7 2/2] net: ti: icssg-prueth: Add ethtool ops for Frame Preemption MAC Merge

[Jakub Kicinski <kuba@kernel.org>]

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Subject: net: ti: icssg-prueth: Add ethtool ops for Frame Preemption MAC Merge

This patch wires up the ethtool MAC Merge ops (.get_mm, .set_mm,
.get_mm_stats) for the ICSSG PRU Ethernet driver and adds five firmware
preemption counters that feed both the new MM stats and the existing
ethtool -S table.

> diff --git a/drivers/net/ethernet/ti/icssg/icssg_ethtool.c b/drivers/net/ethernet/ti/icssg/icssg_ethtool.c
> index b715af21d23a..0620782318ab 100644
> --- a/drivers/net/ethernet/ti/icssg/icssg_ethtool.c
> +++ b/drivers/net/ethernet/ti/icssg/icssg_ethtool.c
> @@ -294,6 +294,103 @@ static int emac_set_per_queue_coalesce(struct net_device *ndev, u32 queue,

[ ... ]

> +static int emac_set_mm(struct net_device *ndev, struct ethtool_mm_cfg *cfg,
> +		       struct netlink_ext_ack *extack)
> +{
> +	struct prueth_emac *emac = netdev_priv(ndev);
> +	struct prueth_qos_iet *iet = &emac->qos.iet;
> +	int err;
> +
> +	if (emac->is_sr1)
> +		return -EOPNOTSUPP;
> +
> +	if (!cfg->pmac_enabled) {
> +		NL_SET_ERR_MSG_MOD(extack, "preemptible MAC is always enabled");
> +		return -EOPNOTSUPP;
> +	}
> +
> +	mutex_lock(&iet->fpe_lock);
> +	iet->verify_time_ms = cfg->verify_time;
> +	iet->tx_min_frag_size = cfg->tx_min_frag_size;
> +	iet->fpe_enabled = cfg->tx_enabled;
> +	iet->mac_verify_configure = cfg->verify_enabled;
> +	err = icssg_config_ietfpe(ndev, cfg->tx_enabled);
> +	mutex_unlock(&iet->fpe_lock);
> +
> +	return err;
> +}

[Medium]
Can the cached software state in iet diverge from the hardware here when
icssg_config_ietfpe() fails?

The new fields (verify_time_ms, tx_min_frag_size, fpe_enabled,
mac_verify_configure) are written to iet before icssg_config_ietfpe() is
called, but the error paths in icssg_config_ietfpe() only roll back a
subset of state:

icssg_qos.c:icssg_config_ietfpe() {
    ...
disable_tx:
        icssg_set_port_state(emac, ICSSG_EMAC_PORT_PREMPT_TX_DISABLE);
fallback:
        writeb(0, config + PRE_EMPTION_ENABLE_TX);
        writeb(0, config + PRE_EMPTION_ENABLE_VERIFY);
        iet->verify_status = ICSSG_IETFPE_STATE_DISABLED;
        iet->fpe_active =  false;
        return ret;
}

iet->fpe_enabled is left set to cfg->tx_enabled even though the hardware
TX preempt path has been disabled.

Two follow-on effects appear to come from this:

emac_get_mm() reports state->tx_enabled = iet->fpe_enabled, so userspace
sees tx_enabled=true after a set_mm that returned an error and left FPE
disabled in hardware.

icssg_qos_link_state_update() drives icssg_iet_change_preemptible_tcs(),
which calls icssg_config_ietfpe(emac->ndev, iet->fpe_enabled) on every
link transition, so the failed configuration is retried each link-up
without further user action.

Would it make sense to stage cfg into local variables, only commit them
to iet after icssg_config_ietfpe() returns success, or otherwise
restore the previous iet fields on the error return?

> +static void emac_get_mm_stats(struct net_device *ndev,
> +			      struct ethtool_mm_stats *s)
> +{
> +	struct prueth_emac *emac = netdev_priv(ndev);
> +
> +	if (emac->is_sr1)
> +		return;
> +
> +	if (!emac->prueth->pa_stats)
> +		return;
> +
> +	emac_update_hardware_stats(emac);
> +
> +	/* MACMergeHoldCount stats is not tracked by the firmware */
> +	s->MACMergeFrameAssOkCount = emac_get_stat_by_name(emac, "FW_PREEMPT_ASSEMBLY_OK");
> +	s->MACMergeFrameAssErrorCount = emac_get_stat_by_name(emac, "FW_PREEMPT_ASSEMBLY_ERR");
> +	s->MACMergeFragCountRx = emac_get_stat_by_name(emac, "FW_PREEMPT_FRAG_CNT_RX");
> +	s->MACMergeFragCountTx = emac_get_stat_by_name(emac, "FW_PREEMPT_FRAG_CNT_TX");
> +	s->MACMergeFrameSmdErrorCount = emac_get_stat_by_name(emac, "FW_PREEMPT_BAD_FRAG");
> +}

[ ... ]

> diff --git a/drivers/net/ethernet/ti/icssg/icssg_stats.h b/drivers/net/ethernet/ti/icssg/icssg_stats.h
> index 5ec0b38e0c67..8073deac35c3 100644
> --- a/drivers/net/ethernet/ti/icssg/icssg_stats.h
> +++ b/drivers/net/ethernet/ti/icssg/icssg_stats.h
> @@ -189,6 +187,11 @@ static const struct icssg_pa_stats icssg_all_pa_stats[] = {
>  	ICSSG_PA_STATS(FW_INF_DROP_PRIOTAGGED),
>  	ICSSG_PA_STATS(FW_INF_DROP_NOTAG),
>  	ICSSG_PA_STATS(FW_INF_DROP_NOTMEMBER),
> +	ICSSG_PA_STATS(FW_PREEMPT_BAD_FRAG),
> +	ICSSG_PA_STATS(FW_PREEMPT_ASSEMBLY_ERR),
> +	ICSSG_PA_STATS(FW_PREEMPT_FRAG_CNT_TX),
> +	ICSSG_PA_STATS(FW_PREEMPT_ASSEMBLY_OK),
> +	ICSSG_PA_STATS(FW_PREEMPT_FRAG_CNT_RX),
>  	ICSSG_PA_STATS(FW_RX_EOF_SHORT_FRMERR),
>  	ICSSG_PA_STATS(FW_RX_B0_DROP_EARLY_EOF),
>  	ICSSG_PA_STATS(FW_TX_JUMBO_FRM_CUTOFF),

[Medium]
Are these five new entries duplicating values that already have a
standard uAPI?

The same five firmware counters are exposed through the new
.get_mm_stats callback as the standardized MAC Merge stats
(MACMergeFrameAssOkCount, MACMergeFrameAssErrorCount, MACMergeFragCountRx,
MACMergeFragCountTx, MACMergeFrameSmdErrorCount in struct
ethtool_mm_stats), and adding them to icssg_all_pa_stats[] also
publishes them via emac_get_strings() / emac_get_ethtool_stats() as
ethtool -S strings.

Documentation/networking/statistics.rst describes ethtool -S as the
private-driver-stats interface; counters that have a standard uAPI are
expected to flow only through that uAPI.

Could the firmware-register lookup table used by emac_get_stat_by_name()
be separated from the ethtool -S string table, so the new preemption
counters feed get_mm_stats without also showing up under ethtool -S?