linux-kernel - Re: [PATCH v7 01/10] ARM: davinci: move private EDMA API to arm/common

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Message-ID: <20130205174113.GJ4720@opensource.wolfsonmicro.com>
Date:	Tue, 5 Feb 2013 17:41:13 +0000
From:	Mark Brown <broonie@...nsource.wolfsonmicro.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Linus Walleij <linus.walleij@...aro.org>,
	Cyril Chemparathy <cyril@...com>, balbi@...com,
	Sergei Shtylyov <sshtylyov@...sta.com>,
	Linux Documentation List <linux-doc@...r.kernel.org>,
	Lindgren <tony@...mide.com>, Vinod Koul <vinod.koul@...el.com>,
	"Nair, Sandeep" <sandeep_n@...com>, Chris Ball <cjb@...top.org>,
	Matt Porter <mporter@...com>, Arnd Bergmann <arnd@...db.de>,
	Devicetree Discuss <devicetree-discuss@...ts.ozlabs.org>,
	Rob Herring <rob.herring@...xeda.com>,
	Linux OMAP List <linux-omap@...r.kernel.org>,
	ARM Kernel List <linux-arm-kernel@...ts.infradead.org>,
	Linux DaVinci Kernel List 
	<davinci-linux-open-source@...ux.davincidsp.com>,
	"Cousson, Benoit" <b-cousson@...com>,
	Linux MMC List <linux-mmc@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Landley <rob@...dley.net>, Dan Williams <djbw@...com>,
	Linux SPI Devel List 
	<spi-devel-general@...ts.sourceforge.net>
Subject: Re: [PATCH v7 01/10] ARM: davinci: move private EDMA API to
 arm/common

On Tue, Feb 05, 2013 at 05:06:28PM +0000, Russell King - ARM Linux wrote:
> On Tue, Feb 05, 2013 at 04:47:05PM +0000, Mark Brown wrote:
> > On Tue, Feb 05, 2013 at 05:21:48PM +0100, Linus Walleij wrote:

> > > For IRQ mode, use the completion callback to push each cookie
> > > to NAPI, and thus let the IRQ drive the traffic.

> > The whole purpose of NAPI is to avoid taking interrupts for completion
> > of transfers.  Anything that generates interrupts when NAPI is in
> > polling mode is defeating the point.

> Yes, but you're missing one very important point.  If your DMA engine
> is decoupled from the network device, and the DMA engine relies upon
> interrupts to queue further transfers to move data into RAM, then you
> have two options:

> 1. Receive these interrupts so you can update the DMA engine for
>    further data transfer.
> 2. Don't receive these interrupts, and cause the network device to
>    error out with a FIFO overrun because its DMA requests have not
>    been serviced. (which may raise an interrupt.)

There's still the third option of just implementing a non-NAPI driver,
though I guess we've now built so much infrastructure on top of NAPI
that it's still useful to pretend we can disable interrupts.

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