[PATCH net-next 0/2] net: stmmac: Cleanup rx coalescing computation when using RIWT

Currently when configuring interrupt coalescing on devices that relies
on the Receive Interrupt Watchdog Timer feature of dwmac, the
computation of the RIWT timings leads to off-by-one values when
reporting the timings back to userspace.

RIWT works by arming a watchdog timer upon receiving frames with the RI
bit not set in the descriptor. The timer duration is expressed in units
of 256 stmmac clock ticks, and therefore requires a bit of computation
to derive it :

riwt = (rx_usecs * n_clk_ticks_per_usec) / 256

and conversely

rx_usecs = (riwt * 256) / n_clk_ticks_per_usec

This computation as-is leads to a consistent off-by-one when setting
then getting back the rx-usecs value due to rounding errors (by truncation):

ethtool -C eth1 rx-usecs 42
ethtool -c eth1
 -> reports rx-usecs: 41

Let's use DIV_ROUND_CLOSEST instead for the computations. It does have
one side effect, the accepted boundaries for rx-usecs also shifts by one
now, going from [16us, 246us] to [15us, 245us]. For that reason, I'm not
targeting the net tree here, and it's overall a very small issue.

Maxime

Maxime Chevallier (2):
  net: stmmac: ethtool: Comment the magic numbers in RIWT computation
  net: stmmac: ethtool: Address off-by-one when reading the coal
    rx-usecs

 drivers/net/ethernet/stmicro/stmmac/stmmac_ethtool.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

-- 
2.55.0