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Message-ID: <87v89b91n2.fsf@waldekranz.com>
Date: Wed, 06 Dec 2023 09:27:29 +0100
From: Tobias Waldekranz <tobias@...dekranz.com>
To: Vladimir Oltean <olteanv@...il.com>
Cc: davem@...emloft.net, kuba@...nel.org, andrew@...n.ch,
f.fainelli@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH v2 net-next 6/6] net: dsa: mv88e6xxx: Add "rmon" counter
group support
On ons, dec 06, 2023 at 02:22, Vladimir Oltean <olteanv@...il.com> wrote:
> On Tue, Dec 05, 2023 at 05:04:18PM +0100, Tobias Waldekranz wrote:
>> +static void mv88e6xxx_get_rmon_stats(struct dsa_switch *ds, int port,
>> + struct ethtool_rmon_stats *rmon_stats,
>> + const struct ethtool_rmon_hist_range **ranges)
>> +{
>> + static const struct ethtool_rmon_hist_range rmon_ranges[] = {
>> + { 64, 64 },
>> + { 65, 127 },
>> + { 128, 255 },
>> + { 256, 511 },
>> + { 512, 1023 },
>> + { 1024, 65535 },
>> + {}
>> + };
>> + struct mv88e6xxx_chip *chip = ds->priv;
>> + int ret;
>> +
>> + ret = mv88e6xxx_stats_snapshot(chip, port);
>> + if (ret < 0)
>> + return;
>> +
>> +#define MV88E6XXX_RMON_STAT_MAP(_id, _member) \
>> + mv88e6xxx_stats_get_stat(chip, port, \
>> + &mv88e6xxx_hw_stats[MV88E6XXX_HW_STAT_ID_ ## _id], \
>> + &rmon_stats->stats._member)
>> +
>> + MV88E6XXX_RMON_STAT_MAP(in_undersize, undersize_pkts);
>> + MV88E6XXX_RMON_STAT_MAP(in_oversize, oversize_pkts);
>> + MV88E6XXX_RMON_STAT_MAP(in_fragments, fragments);
>> + MV88E6XXX_RMON_STAT_MAP(in_jabber, jabbers);
>> + MV88E6XXX_RMON_STAT_MAP(hist_64bytes, hist[0]);
>> + MV88E6XXX_RMON_STAT_MAP(hist_65_127bytes, hist[1]);
>> + MV88E6XXX_RMON_STAT_MAP(hist_128_255bytes, hist[2]);
>> + MV88E6XXX_RMON_STAT_MAP(hist_256_511bytes, hist[3]);
>> + MV88E6XXX_RMON_STAT_MAP(hist_512_1023bytes, hist[4]);
>> + MV88E6XXX_RMON_STAT_MAP(hist_1024_max_bytes, hist[5]);
>> +
>> +#undef MV88E6XXX_RMON_STAT_MAP
>> +
>> + *ranges = rmon_ranges;
>> +}
>
> I just noticed that this doesn't populate the TX counters, just RX.
>
> I haven't tried it, but I think the Histogram Mode bits (11:10) of the
> Stats Operation Register might be able to control what gets reported for
> the Set 4 of counters. Currently AFAICS, the driver always sets it to
> MV88E6XXX_G1_STATS_OP_HIST_RX_TX, aka what gets reported to
> "rx-rmon-etherStatsPkts64to64Octets" is actually an RX+TX counter.
You have a keen eye! Yes, that is what's happening.
> What's the story behind this?
I think the story starts, and ends, with this value being the hardware
default.
Seeing as the hardware only has a single set of histogram counters, it
seems to me like we have to prioritize between:
1. Keeping Rx+Tx: Backwards-compatible, but we can't export any histogram via
the standard RMON group.
2. Move to Rx-only: We can export them via the RMON group, but we change
the behavior of the "native" counters.
3. Move to Tx-only: We can export them via the RMON group, but we change
the behavior of the "native" counters.
Looking at RFC2819, which lays out the original RMON MIB, we find this
description:
etherStatsPkts64Octets OBJECT-TYPE
SYNTAX Counter32
UNITS "Packets"
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The total number of packets (including bad
packets) received that were 64 octets in length
(excluding framing bits but including FCS octets)."
::= { etherStatsEntry 14 }
In my opinion, this gives (2) a clear edge over (3), so we're down to
choosing between (1) and (2). Personally, I lean towards (2), as I think
it is more useful because:
- Most people will tend to assume that the histogram counters refers to
those defined in RFC2819 anyway
- It means we can deliver _something_ rather than nothing to someone
building an operating system, who is looking for a hardware
independent way of providing diagnostics
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