netdev - Re: [RFC PATCH 1/2] smsc911x: add support for sh3 RX DMA

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Date:	Mon, 24 Nov 2008 14:46:03 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	lethal@...ux-sh.org
Cc:	steve.glendinning@...c.com, netdev@...r.kernel.org,
	linux-sh@...r.kernel.org, ian.saturley@...c.com
Subject: Re: [RFC PATCH 1/2] smsc911x: add support for sh3 RX DMA

From: Paul Mundt <lethal@...ux-sh.org>
Date: Mon, 24 Nov 2008 12:42:37 +0900

> On Sun, Nov 23, 2008 at 04:04:33PM +0000, Steve Glendinning wrote:
> > I've been working on adding DMA support to the smsc911x driver.  As this 
> > family of devices is non-pci, DMA transfers must be initiated and 
> > controlled by the host CPU.  Unfortunately this makes some of the code 
> > necessarily platform-specific.
> > 
> > This patch adds RX DMA support for the sh architecture.  Tested on 
> > SH7709S (sh3), where it gives a small (~10%) iperf tcp throughput 
> > increase.  DMA or PIO is selected at compile-time.
> > 
> > My first attempt stopped NAPI polling during a DMA transfer, then used 
> > DMA completion interrupts to pass the packet up and re-enable polling.
> > Obviously this defeats the interrupt-mitigation of NAPI, and on my test 
> > platform actually *reduced* performance!
> > 
> > This patch leaves NAPI polling enabled, so a later poll completes the 
> > transfer.  I'm concerned this is essentially busy-waiting on the 
> > transfer, but it does show a small performance gain.  Is this a good or 
> > bad idea?
> > 
> > I'd be interested to hear if anyone has advice on how to make this 
> > patch more generic.  There's definitely been interest from arm pxa
> > users in adding DMA, and some of this code must be re-usable for this.
> > 
> > Signed-off-by: Steve Glendinning <steve.glendinning@...c.com>
> 
> The intent was to move everything over to the dmaengine framework and
> have drivers (especially generic ones) opt for using that instead. This
> hasn't happened yet, but it doesn't seem like there is much point in
> adding hacks to the smsc911x driver at present given the overhead
> involved in the interrupt handling. While this is something that can
> easily be improved, I would rather put more effort in to getting things
> moved over to the generic frameworks now that they exist, and optimize
> later.

I'll therefore drop these two patches for now.
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