netdev - Re: [PATCH net-next 2/4] net: bcmgenet: add support for ethtool tx-frames

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Date:	Mon, 21 Apr 2014 15:17:20 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	f.fainelli@...il.com
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH net-next 2/4] net: bcmgenet: add support for ethtool
 tx-frames

From: Florian Fainelli <f.fainelli@...il.com>
Date: Mon, 21 Apr 2014 08:45:22 -0700

> Configuring the ethtool tx-frames property, which translates into N
> packets before a TX interrupt is the simplest configuration scheme
> because it requires no locking neither at the softare nor hardware
> level, and is completely indepedent from the link speed. Since ethtool
> does not allow per-tx queue coalescing parameters, we apply the same
> setting to any transmit queue.
> 
> We can no longer enable the BDONE/PDONE interrupts as those would fire
> for each packet/buffer received, which would defeat the MBDONE interrupt
> purpose. The MBDONE interrupt is guaranteed to correspond to a
> PDONE/BDONE interrupt when the threshold is set to 1.
> 
> Signed-off-by: Florian Fainelli <f.fainelli@...il.com>

Does the MBDONE scheme have a timeout?

For example if you ask for a MBDONE setting where N=4, if only 2 packets
arrive will you get an interrupt or will it wait for 2 more to arrive
no matter what?

If a timeout doesn't exist, you cannot use this.

I'm very pessimistic because I see no inspection of the timeout
parameter passed into the ethtool commands.  And in fact if the
timeout does exist, and is fixed, you should error on non-zero
values specified in the timeout field(s).

So no matter what the situation is wrt. timeouts, this series needs
either change or be completely tossed.
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