net: stmmac: ethtool: Address off-by-one when reading the coal rx-usecs

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When reading the rx-usecs coalescing parameters on a dwmac variant that
uses the RIWT for RX interrupt coalescing, we convert the riwt value to
usecs :
 - One riwt cycle is 256 clock ticks, we compute how many ticks in $riwt
   cycles
 - divide that by how many ticks in a microsecond, and we get the
   rx-usecs.

The opposite computation is done when setting the rx-usecs param.

Because of the 256 ratio, we're subjected to off-by-one errors in the
value read-back, which can be reliably measured on i.mx8MP :

$ ethtool -C eth1 rx-usecs 102

$ ethtool -c eth1
  Coalesce parameters for eth1:
  [...]
  rx-usecs: 101

Let's be more explicit about the rounding for the riwt to usec
computations by using DIV_ROUND_CLOSEST, which solves the off-by-one.

This does change the boundaries of accepted rx-usecs parameters, as the
previously accepted values were in the 16-246 us range, and now fall
into the 15-245 range on imx8mp.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20260802114015.214212-3-maxime.chevallier@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

...

This commit is contained in:

Maxime Chevallier 2026-08-02 13:40:14 +02:00 • committed by Jakub Kicinski

parent b6a89c8ef3

commit ae88f78bc4

1 changed files with 2 additions and 2 deletions

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drivers/net/ethernet/stmicro/stmmac/stmmac_ethtool.c

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