netdev - RE: [net-next,PATCH] net: dsa: microchip: Do not count RX/TX Bytes and discards on KSZ87xx

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Message-ID:
 <DM3PR11MB8736FCDDE21054AC6ACCEA44EC28A@DM3PR11MB8736.namprd11.prod.outlook.com>
Date: Mon, 11 Aug 2025 22:09:22 +0000
From: <Tristram.Ha@...rochip.com>
To: <marek.vasut@...lbox.org>
CC: <andrew@...n.ch>, <Arun.Ramadoss@...rochip.com>, <edumazet@...gle.com>,
	<kuba@...nel.org>, <pabeni@...hat.com>, <UNGLinuxDriver@...rochip.com>,
	<olteanv@...il.com>, <Woojung.Huh@...rochip.com>, <netdev@...r.kernel.org>
Subject: RE: [net-next,PATCH] net: dsa: microchip: Do not count RX/TX Bytes
 and discards on KSZ87xx

> On 8/11/25 10:47 PM, Tristram.Ha@...rochip.com wrote:
> >> Unlike KSZ94xx, the KSZ87xx is missing the last four MIB counters at
> >> address 0x20..0x23, namely rx_total, tx_total, rx_discards, tx_discards.
> >>
> >> Using values from rx_total / tx_total in rx_bytes / tx_bytes calculation
> >> results in an underflow, because rx_total / tx_total returns values 0,
> >> and the "raw->rx_total - stats->rx_packets * ETH_FCS_LEN;" calculation
> >> undeflows if rx_packets / tx_packets is > 0 .
> >>
> >> Stop using the missing MIB counters on KSZ87xx .
> >>
> >> Fixes: c6101dd7ffb8 ("net: dsa: ksz9477: move get_stats64 to ksz_common.c")
> >> Signed-off-by: Marek Vasut <marek.vasut@...lbox.org>
> >> ---
> >> Cc: Andrew Lunn <andrew@...n.ch>
> >> Cc: Arun Ramadoss <arun.ramadoss@...rochip.com>
> >> Cc: Eric Dumazet <edumazet@...gle.com>
> >> Cc: Jakub Kicinski <kuba@...nel.org>
> >> Cc: Paolo Abeni <pabeni@...hat.com>
> >> Cc: Tristram Ha <tristram.ha@...rochip.com>
> >> Cc: UNGLinuxDriver@...rochip.com
> >> Cc: Vladimir Oltean <olteanv@...il.com>
> >> Cc: Woojung Huh <woojung.huh@...rochip.com>
> >> Cc: netdev@...r.kernel.org
> >> ---
> >>   drivers/net/dsa/microchip/ksz_common.c | 13 ++++++++-----
> >>   1 file changed, 8 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/drivers/net/dsa/microchip/ksz_common.c
> >> b/drivers/net/dsa/microchip/ksz_common.c
> >> index 7292bfe2f7cac..9c01526779a6d 100644
> >> --- a/drivers/net/dsa/microchip/ksz_common.c
> >> +++ b/drivers/net/dsa/microchip/ksz_common.c
> >> @@ -2239,20 +2239,23 @@ void ksz_r_mib_stats64(struct ksz_device *dev, int
> port)
> >>          /* HW counters are counting bytes + FCS which is not acceptable
> >>           * for rtnl_link_stats64 interface
> >>           */
> >> -       stats->rx_bytes = raw->rx_total - stats->rx_packets * ETH_FCS_LEN;
> >> -       stats->tx_bytes = raw->tx_total - stats->tx_packets * ETH_FCS_LEN;
> >> -
> >> +       if (!ksz_is_ksz87xx(dev)) {
> >> +               stats->rx_bytes = raw->rx_total - stats->rx_packets * ETH_FCS_LEN;
> >> +               stats->tx_bytes = raw->tx_total - stats->tx_packets * ETH_FCS_LEN;
> >> +       }
> >>          stats->rx_length_errors = raw->rx_undersize + raw->rx_fragments +
> >>                  raw->rx_oversize;
> >>
> >>          stats->rx_crc_errors = raw->rx_crc_err;
> >>          stats->rx_frame_errors = raw->rx_align_err;
> >> -       stats->rx_dropped = raw->rx_discards;
> >> +       if (!ksz_is_ksz87xx(dev))
> >> +               stats->rx_dropped = raw->rx_discards;
> >>          stats->rx_errors = stats->rx_length_errors + stats->rx_crc_errors +
> >>                  stats->rx_frame_errors  + stats->rx_dropped;
> >>
> >>          stats->tx_window_errors = raw->tx_late_col;
> >> -       stats->tx_fifo_errors = raw->tx_discards;
> >> +       if (!ksz_is_ksz87xx(dev))
> >> +               stats->tx_fifo_errors = raw->tx_discards;
> >>          stats->tx_aborted_errors = raw->tx_exc_col;
> >>          stats->tx_errors = stats->tx_window_errors + stats->tx_fifo_errors +
> >>                  stats->tx_aborted_errors;
> >
> > I am not sure why you have that problem.  In my KSZ8795 board running in
> > kernel 6.16 those counters are working.  Using "ethtool -S lan3" or
> > "ethtool -S eth0" shows rx_total and tx_total.  The "rx_discards" and
> > "tx_discards" are hard to get.  In KSZ8863 the "tx_discards" counter is
> > incremented when the port does not have link and a packet is sent to that
> > port.  In newer switches that behavior was changed.  Occasionally you may
> > get 1 or 2 at the beginning when the tail tagging function is not enabled
> > yet but the MAC sends out packets.  The "rx_discards" count typically
> > seldom happens, but somehow in my first bootup there are many from my
> > KSZ8795 board.
> >
> > Actually that many rx_discards may be a problem I need to find out.
> >
> > I think you are confused about how those MIB counters are read from
> > KSZ8795.  They are not using the code in ksz9477.c but in ksz8.c.
> 
> See [1] TABLE 4-26: PORT MIB COUNTER INDIRECT MEMORY OFFSETS (CONTINUED)
> , page 108 at the very end . Notice the table ends at 0x1F
> TxMultipleCollision .
> 
> See [2] TABLE 5-6: MIB COUNTERS (CONTINUED) , page 219 at the end of the
> table . Notice the table contains four extra entries , 0x80 RxByteCnt ,
> 0x81 TxByteCnt , 0x82 RxDropPackets , 0x83 TXDropPackets .
> 
> These entries are present on KSZ9477 and missing on KSZ8795 .
> 
> This is what this patch attempts to address.

As I said KSZ8795 MIB counters are not using the code in ksz9477.c and
their last counter locations are not the same as KSZ9477.  KSZ9477 uses
ksz9477_r_mib_pkt while KSZ8795 uses ksz8795_r_mib_pkt.  The other
KSZ8895 and KSZ8863 switches uses ksz8863_r_mib_pkt.

The 0x80 and such registers are not used in KSZ8795.  Its registers start
at 0x100, 0x101, ...

They are in table 4-28.