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Merge branch 'net-stmmac-cleanup-rx-coalescing-computation-when-using-riwt'

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Maxime Chevallier says:

====================
net: stmmac: Cleanup rx coalescing computation when using RIWT

Currently when configuring interrupt coalescing on devices that relies
on the Receive Interrupt Watchdog Timer feature of dwmac, the
computation of the RIWT timings leads to off-by-one values when
reporting the timings back to userspace.

RIWT works by arming a watchdog timer upon receiving frames with the RI
bit not set in the descriptor. The timer duration is expressed in units
of 256 stmmac clock ticks, and therefore requires a bit of computation
to derive it :

riwt = (rx_usecs * n_clk_ticks_per_usec) / 256

and conversely

rx_usecs = (riwt * 256) / n_clk_ticks_per_usec

This computation as-is leads to a consistent off-by-one when setting
then getting back the rx-usecs value due to rounding errors (by truncation):

ethtool -C eth1 rx-usecs 42
ethtool -c eth1
 -> reports rx-usecs: 41

Let's use DIV_ROUND_CLOSEST instead for the computations. It does have
one side effect, the accepted boundaries for rx-usecs also shifts by one
now, going from [16us, 246us] to [15us, 245us]. For that reason, I'm not
targeting the net tree here, and it's overall a very small issue.
====================

Link: https://patch.msgid.link/20260802114015.214212-1-maxime.chevallier@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Jakub Kicinski 2026-08-03 18:35:47 -07:00
commit 63638bc3e2
1 changed files with 5 additions and 2 deletions
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7
drivers/net/ethernet/stmicro/stmmac/stmmac_ethtool.c
View File

@ -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 DIV_ROUND_CLOSEST(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 DIV_ROUND_CLOSEST(riwt * 256, clk / USEC_PER_SEC);
}
static int __stmmac_get_coalesce(struct net_device *dev,