[PATCH net-next 2/2] net: stmmac: ethtool: Address off-by-one when reading the coal rx-usecs

[Maxime Chevallier <maxime.chevallier@bootlin.com>]

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>
---
 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