netdev - Re: [RFC PATCH 1/2] net: Add new network device function to allow for MMIO batching

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Date:	Thu, 12 Jul 2012 09:14:19 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexander Duyck <alexander.h.duyck@...el.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net,
	jeffrey.t.kirsher@...el.com, edumazet@...gle.com,
	bhutchings@...arflare.com, therbert@...gle.com,
	alexander.duyck@...il.com
Subject: Re: [RFC PATCH 1/2] net: Add new network device function to allow
 for MMIO batching

On Wed, 2012-07-11 at 17:26 -0700, Alexander Duyck wrote:
> This change adds capabilities to the driver for batching the MMIO write
> involved with transmits.  Most of the logic is based off of the code for
> the qdisc scheduling.
> 
> What I did is break the transmit path into two parts.  We already had the
> ndo_start_xmit function which has been there all along.  The part I added
> was ndo_complete_xmit which is meant to handle notifying the hardware that
> frames are ready for delivery.
> 
> To control all of this I added a net sysfs value for the Tx queues called
> dispatch_limit.  When 0 it indicates that all frames will notify hardware
> immediately.  When 1 or more the netdev_complete_xmit call will queue up to
> that number of packets, and when the value is exceeded it will notify the
> hardware and reset the pending frame dispatch count.
> 
> Signed-off-by: Alexander Duyck <alexander.h.duyck@...el.com>
> ---

The idea is good, but do we really need so complex schem ?

Most of the transmits are done from __qdisc_run()

We could add logic in __qdisc_run()/qdisc_restart()

qdisc_run_end() would then have to call ndo_complete_xmit() to make
sure the MMIO is done.



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