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net: stmmac: ethtool: Comment the magic numbers in RIWT computation

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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>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20260802114015.214212-2-maxime.chevallier@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Maxime Chevallier
2026-08-02 13:40:13 +02:00
committed by Jakub Kicinski
parent c98610c2eb
commit b6a89c8ef3
1 changed files with 5 additions and 2 deletions
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7
drivers/net/ethernet/stmicro/stmmac/stmmac_ethtool.c
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@@ -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,