[PATCH net-next 1/2] net: stmmac: ethtool: Comment the magic numbers in RIWT computation

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

Receive Interrupt Watchdog Timer is an RX interrupt coalescing mechanism
used by some variants of dwmac. It allows waiting a bit before
triggering the rx interrupts, allowing for batch processing.

The RIWT is configured with a granularity of 256 stmmac clk ticks. Let's
add a comment for that and wrap the raw "1000000" into USEC_PER_SEC, as
we're computing "how many clock cycles in one microsec" with that step.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
---
 drivers/net/ethernet/stmicro/stmmac/stmmac_ethtool.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_ethtool.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_ethtool.c
index 92585d27ab88..eebfd6976aaa 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_ethtool.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_ethtool.c
@@ -759,7 +759,10 @@ static u32 stmmac_usec2riwt(u32 usec, struct stmmac_priv *priv)
 			return 0;
 	}
 
-	return (usec * (clk / 1000000)) / 256;
+	/* Receive Interrupt Watchdog Timer (riwt) has a resolution of 256
+	 * ticks.
+	 */
+	return (usec * (clk / USEC_PER_SEC)) / 256;
 }
 
 static u32 stmmac_riwt2usec(u32 riwt, struct stmmac_priv *priv)
@@ -772,7 +775,7 @@ static u32 stmmac_riwt2usec(u32 riwt, struct stmmac_priv *priv)
 			return 0;
 	}
 
-	return (riwt * 256) / (clk / 1000000);
+	return (riwt * 256) / (clk / USEC_PER_SEC);
 }
 
 static int __stmmac_get_coalesce(struct net_device *dev,
-- 
2.55.0