netdev - RE: [PATCH 3/3] net: hisilicon: new hip04 ethernet driver

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Date:	Wed, 26 Mar 2014 09:55:46 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Arnd Bergmann' <arnd@...db.de>,
	Florian Fainelli <f.fainelli@...il.com>
CC:	Zhangfei Gao <zhangfei.gao@...il.com>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	Mark Rutland <mark.rutland@....com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"Russell King - ARM Linux" <linux@....linux.org.uk>,
	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>,
	netdev <netdev@...r.kernel.org>,
	Zhangfei Gao <zhangfei.gao@...aro.org>,
	"David S. Miller" <davem@...emloft.net>
Subject: RE: [PATCH 3/3] net: hisilicon: new hip04 ethernet driver

From: Arnd Bergmann
> On Tuesday 25 March 2014 10:16:28 Florian Fainelli wrote:
> >
> > Ok, well that's really unfortunate, to achieve the best of everything,
> > the workaround should probably look like:
> >
> > - keep reclaiming TX buffers in ndo_start_xmit() in case you push more
> > packets to the NICs than your timer can free
> > - reclaim TX buffers in NAPI poll() context for "symetrical" workloads
> > where e.g: TCP ACKs received allow you to complete TX buffers
> > - have a timer like you suggest which should help with transmit only
> > workloads at a slow rate
> 
> Yes, that is what I was thinking, but with orphaning the tx skbs,
> we can probably be a little smarter. Note that in order to check
> the state of the queue, we have to do a read from uncached memory,
> since the hardware also doesn't support cache coherent DMA.
> We don't want to do that too often.

Possibly you can check for all the pending transmits having
completed - the most likely case. Instead of checking them
individually?

You should limit the number of tx bytes as well as the number
of tx frames - buffering a ring full of large frames subverts
some of the algorithms higher up the stack.

	David



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