Re: [PATCH net-next v15 9/9] net: dsa: lan9645x: add port statistics

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- [Medium] lan9645x_stats_get_stats64()
  (drivers/net/dsa/microchip/lan9645x/lan9645x_stats.c) counts error…

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net: dsa: lan9645x: add port statistics

This adds port statistics support to the lan9645x DSA driver. The 32 bit
hardware counters are polled into a 64 bit software cache, and the pMAC
counters are summed into the aggregate source. The cache is exposed through
ethtool -S, the standard ethtool groups and ndo_get_stats64.

> diff --git a/drivers/net/dsa/microchip/lan9645x/lan9645x_stats.c b/drivers/net/dsa/microchip/lan9645x/lan9645x_stats.c
> new file mode 100644
> index 0000000000000..a21afb4d98532
> --- /dev/null
> +++ b/drivers/net/dsa/microchip/lan9645x/lan9645x_stats.c
> @@ -0,0 +1,854 @@

[ ... ]

> +void lan9645x_stats_get_stats64(struct lan9645x *lan9645x, int port,
> +				struct rtnl_link_stats64 *stats)
> +{

[ ... ]

> +	stats->rx_packets = c[SCNT_RX_SHORT] +
> +			    c[SCNT_RX_JABBER] +
> +			    c[SCNT_RX_SZ_64] +
> +			    c[SCNT_RX_SZ_65_127] +
> +			    c[SCNT_RX_SZ_128_255] +
> +			    c[SCNT_RX_SZ_256_511] +
> +			    c[SCNT_RX_SZ_512_1023] +
> +			    c[SCNT_RX_SZ_1024_1526] +
> +			    c[SCNT_RX_SZ_JUMBO] +
> +			    c[SCNT_RX_LONG] +
> +			    c[SCNT_RX_PMAC_SHORT] +
> +			    c[SCNT_RX_PMAC_FRAG] +
> +			    c[SCNT_RX_PMAC_JABBER] +
> +			    c[SCNT_RX_PMAC_SZ_64] +
> +			    c[SCNT_RX_PMAC_SZ_65_127] +
> +			    c[SCNT_RX_PMAC_SZ_128_255] +
> +			    c[SCNT_RX_PMAC_SZ_256_511] +
> +			    c[SCNT_RX_PMAC_SZ_512_1023] +
> +			    c[SCNT_RX_PMAC_SZ_1024_1526] +
> +			    c[SCNT_RX_PMAC_SZ_JUMBO] +
> +			    c[SCNT_RX_PMAC_LONG];
> +
> +	stats->multicast = c[SCNT_RX_MC] + c[SCNT_RX_PMAC_MC];
> +
> +	stats->rx_errors = c[SCNT_RX_SHORT] +
> +			   c[SCNT_RX_JABBER] +
> +			   c[SCNT_RX_CRC] +
> +			   c[SCNT_RX_LONG] +
> +			   c[SCNT_RX_PMAC_SHORT] +
> +			   c[SCNT_RX_PMAC_JABBER] +
> +			   c[SCNT_RX_PMAC_CRC] +
> +			   c[SCNT_RX_PMAC_LONG];

[Severity: Medium]
Should short, jabber and oversize frames be counted in rx_packets in
lan9645x_stats_get_stats64()?

rx_packets and rx_errors both add SCNT_RX_SHORT, SCNT_RX_JABBER and
SCNT_RX_LONG, plus their pMAC versions. JABBER and LONG are also added to
rx_length_errors. So every malformed frame received on a user port counts
once as a good packet and once as an error.

The rtnl_link_stats64 kernel-doc in include/uapi/linux/if_link.h says:

  @rx_packets: Number of good packets received by the interface.

The v5 changelog also says CRC and symbol error frames "are already counted
in SZ_* buckets". Since those buckets feed rx_packets, frames with a bad CRC
would be counted as good packets as well.

This also gives a different answer from lan9645x_stats_get_eth_mac_stats(),
which counts good received frames as UC + MC + BC only:

	m->FramesReceivedOK = c[SCNT_RX_UC] +
			      c[SCNT_RX_MC] +
			      c[SCNT_RX_BC] +
			      ...

As a result, rx_packets is larger than FramesReceivedOK by the number of
errored frames. ocelot_port_get_stats64() uses a similar rx_packets formula,
but that does not match the uAPI description either.

Could rx_packets be derived the same way as FramesReceivedOK?

A separate point: c[SCNT_RX_PMAC_FRAG] is added to rx_packets but not to
rx_errors. The comment in lan9645x_stats_get_rmon_stats() says:

	/* ... Only SCNT_RX_PMAC_FRAG counts runts with an invalid
	 * CRC, so fragments is left unset rather than reporting the pMAC alone.
	 */

Going by that comment, a pMAC runt with a bad CRC would be counted as a
good packet and left out of the error total. Is that intended?

v12 removed the eMAC SCNT_RX_FRAG from rx_packets, but the pMAC version is
still there. Since the pMAC gets no traffic without MAC merge, this has
little effect today.

-- 
Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20261015-dsa_lan9645x_switch_driver_base-v15-0-562f316f78b9%40microchip.com