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

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Message-ID: <20161101111023.GA4211@lunn.ch>
Date:   Tue, 1 Nov 2016 12:10:23 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Russell King <rmk+kernel@...linux.org.uk>
Cc:     Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>,
        netdev@...r.kernel.org
Subject: Re: [PATCH] net: mv643xx_eth: ensure coalesce settings survive
 read-modify-write

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@...linux.org.uk>

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

    Andrew