linux-kernel - Re: [Patch v3 2/2] dmaengine: qcom_bam

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Date:	Tue, 28 Jan 2014 17:02:42 +0530
From:	Vinod Koul <vinod.koul@...el.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Arnd Bergmann <arnd@...db.de>,
	Lars-Peter Clausen <lars@...afoo.de>,
	devicetree@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Andy Gross <agross@...eaurora.org>,
	dmaengine@...r.kernel.org, Dan Williams <dan.j.williams@...el.com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [Patch v3 2/2] dmaengine: qcom_bam_dma: Add device tree binding

On Tue, Jan 28, 2014 at 11:17:57AM +0000, Russell King - ARM Linux wrote:
> On Tue, Jan 28, 2014 at 10:16:53AM +0100, Arnd Bergmann wrote:
> > On Tuesday 28 January 2014 10:05:35 Lars-Peter Clausen wrote:
> > > Why does the direction needs to be specified in specifier? I see two
> > > options, either the direction per is fixed in hardware. In that case the DMA
> > > controller node should describe which channel is which direction. Or the
> > > direction is not fixed in hardware and can be changed at runtime in which
> > > case it should be set on a per descriptor basis.
> > 
> > Normally the direction is implied by dmaengine_slave_config().
> 
> No.  The direction argument in there is deprecated - we've been talking
> about removing it for some time.
> 
> DMA engine drivers should store all parameters of the configuration, and
> then select the appropriate ones when preparing a transfer (which itself
> involves a direction.)

Right all the prep_ calls for slave cases have explcit direction argument so
sending it using slave config makes no sense. So will remove it after the merge
window closes and fix :)

--
~Vinod
> 
> Not doing this implies that if you have a half-duplex device, you have to
> repeatedly issue a dmaengine_slave_config() call, a prepare call, and a
> submit call to the DMA engine code for every segment you want to transfer.
> We don't need that kind of DMA engine specific behaviour in DMA engine
> users.
> 
> -- 
> FTTC broadband for 0.8mile line: 5.8Mbps down 500kbps up.  Estimation
> in database were 13.1 to 19Mbit for a good line, about 7.5+ for a bad.
> Estimate before purchase was "up to 13.2Mbit".
> --
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