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 --- drivers/net/ethernet/stmicro/stmmac/stmmac_ethtool.c | 4 ++-- 1 file 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, -- 2.55.0