[PATCH] net: mv643xx_eth: ensure coalesce settings survive read-modify-write

[PATCH] net: mv643xx_eth: ensure coalesce settings survive read-modify-write

[Russell King]

The coalesce settings behave badly when changing just one value:

... # ethtool -c eth0
rx-usecs: 249
... # ethtool -C eth0 tx-usecs 250
... # ethtool -c eth0
rx-usecs: 248

This occurs due to rounding errors when calculating the microseconds
value - the divisons round down.  This causes (eg) the rx-usecs to
decrease by one every time the tx-usecs value is set as per the above.

Fix this by making the divison round-to-nearest.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
---
 drivers/net/ethernet/marvell/mv643xx_eth.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/net/ethernet/marvell/mv643xx_eth.c b/drivers/net/ethernet/marvell/mv643xx_eth.c
index 2a85202de6da..8a472cbfda09 100644
--- a/drivers/net/ethernet/marvell/mv643xx_eth.c
+++ b/drivers/net/ethernet/marvell/mv643xx_eth.c
@@ -1429,6 +1429,7 @@ static unsigned int get_rx_coal(struct mv643xx_eth_private *mp)
 		temp = (val & 0x003fff00) >> 8;
 
 	temp *= 64000000;
+	temp += mp->t_clk / 2;
 	do_div(temp, mp->t_clk);
 
 	return (unsigned int)temp;
@@ -1465,6 +1466,7 @@ static unsigned int get_tx_coal(struct mv643xx_eth_private *mp)
 
 	temp = (rdlp(mp, TX_FIFO_URGENT_THRESHOLD) & 0x3fff0) >> 4;
 	temp *= 64000000;
+	temp += mp->t_clk / 2;
 	do_div(temp, mp->t_clk);
 
 	return (unsigned int)temp;
-- 
2.1.0

Re: [PATCH] net: mv643xx_eth: ensure coalesce settings survive read-modify-write

[Andrew Lunn]

On Tue, Nov 01, 2016 at 10:50:01AM +0000, Russell King wrote:
> The coalesce settings behave badly when changing just one value:
> 
> ... # ethtool -c eth0
> rx-usecs: 249
> ... # ethtool -C eth0 tx-usecs 250
> ... # ethtool -c eth0
> rx-usecs: 248
> 
> This occurs due to rounding errors when calculating the microseconds
> value - the divisons round down.  This causes (eg) the rx-usecs to
> decrease by one every time the tx-usecs value is set as per the above.
> 
> Fix this by making the divison round-to-nearest.
> 
> Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

Reviewed-by: Andrew Lunn <andrew@lunn.ch>

    Andrew

Re: [PATCH] net: mv643xx_eth: ensure coalesce settings survive read-modify-write

[David Miller]

From: Russell King <rmk+kernel@armlinux.org.uk>
Date: Tue, 01 Nov 2016 10:50:01 +0000

> The coalesce settings behave badly when changing just one value:
> 
> ... # ethtool -c eth0
> rx-usecs: 249
> ... # ethtool -C eth0 tx-usecs 250
> ... # ethtool -c eth0
> rx-usecs: 248
> 
> This occurs due to rounding errors when calculating the microseconds
> value - the divisons round down.  This causes (eg) the rx-usecs to
> decrease by one every time the tx-usecs value is set as per the above.
> 
> Fix this by making the divison round-to-nearest.
> 
> Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

Applied, thanks.