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

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Date:	Tue, 5 Feb 2013 10:37:45 -0500
From:	Cyril Chemparathy <cyril@...com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Arnd Bergmann <arnd@...db.de>,
	Linus Walleij <linus.walleij@...aro.org>,
	Mark Brown <broonie@...nsource.wolfsonmicro.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>,
	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 02/05/2013 07:38 AM, Russell King - ARM Linux wrote:
> On Mon, Feb 04, 2013 at 09:47:38PM +0000, Arnd Bergmann wrote:
>> On Monday 04 February 2013, Linus Walleij wrote:
>>> So I think the above concerns are moot. The callback we can
>>> set on cookies is entirely optional, and it's even implemented by
>>> each DMA engine, and some may not even support it but require
>>> polling, and then it won't even be implemented by the driver.
>>
>> Just to ensure that everybody is talking about the same thing here:
>> Is it just the callback that is optional, or also the interrupt
>> coming from the hardware?
>
> If everyone implements stuff correctly, both.  The callback most certainly
> is optional as things stand.  The interrupt - that depends on the DMA
> engine.
>
> Some DMA engines you can't avoid it because you need to reprogram the
> hardware with the next+1 transfer upon completion of an existing transfer.
> Others may allow you to chain transfers in hardware.  That's all up to
> how the DMA engine driver is implemented and how the hardware behaves.
>
> Now, there's another problem here: that is, people abuse the API.  People
> don't pass DMA_CTRL_ACK | DMA_PREP_INTERRUPT into their operations by
> default.  People like typing '0'.
>
> The intention of the "DMA_PREP_INTERRUPT" is significant here: it means
> "ask the hardware to send an interrupt upon completion of this transfer".
>
> Because soo many people like to type '0' instead in their DMA engine
> clients, it means that this flag is utterly useless today - you have to
> ignore it.  So there's _no_ way for client drivers to actually tell the
> a DMA engine driver which _doesn't_ need to signal interrupts at the end
> of every transfer not to do so.
>
> So yes, the DMA engine API supports it.  Whether the _implementations_
> themselves do is very much hit and miss (and in reality is much more
> miss than hit.)
>

Don't these assume that the driver can determine the need for an 
interrupt upfront at prep/submit time?  AFAICT, this assumption doesn't 
hold true with NAPI.

Thanks
-- Cyril.
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