diff options
| author | Maxime Chevallier <maxime.chevallier@bootlin.com> | 2026-08-02 13:40:14 +0200 |
|---|---|---|
| committer | Jakub Kicinski <kuba@kernel.org> | 2026-08-03 18:35:45 -0700 |
| commit | ae88f78bc4ebae984a60ec4b4d6610cce5cfce0f (patch) | |
| tree | f5683bc22b97975ebdeab57f9f3278d8647b5a90 | |
| parent | b6a89c8ef34f84488f877e29b14f0c843bfdff51 (diff) | |
net: stmmac: ethtool: Address off-by-one when reading the coal rx-usecs
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>
| -rw-r--r-- | drivers/net/ethernet/stmicro/stmmac/stmmac_ethtool.c | 4 |
1 files changed, 2 insertions, 2 deletions
diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_ethtool.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_ethtool.c index eebfd6976aaa..154cc0c7623d 100644 --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_ethtool.c +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_ethtool.c @@ -762,7 +762,7 @@ static u32 stmmac_usec2riwt(u32 usec, struct stmmac_priv *priv) /* Receive Interrupt Watchdog Timer (riwt) has a resolution of 256 * ticks. */ - return (usec * (clk / USEC_PER_SEC)) / 256; + return DIV_ROUND_CLOSEST(usec * (clk / USEC_PER_SEC), 256); } static u32 stmmac_riwt2usec(u32 riwt, struct stmmac_priv *priv) @@ -775,7 +775,7 @@ static u32 stmmac_riwt2usec(u32 riwt, struct stmmac_priv *priv) return 0; } - return (riwt * 256) / (clk / USEC_PER_SEC); + return DIV_ROUND_CLOSEST(riwt * 256, clk / USEC_PER_SEC); } static int __stmmac_get_coalesce(struct net_device *dev, |