linux-kernel - Re: Ideas/suggestions to avoid repeated locking and reducing too many lists with dmaengine?

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Date:	Tue, 25 Feb 2014 12:24:24 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Andy Gross <agross@...eaurora.org>
Cc:	Joel Fernandes <joelf@...com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	linux-rt-users@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Vinod Koul <vinod.koul@...el.com>,
	Lars-Peter Clausen <lars@...afoo.de>
Subject: Re: Ideas/suggestions to avoid repeated locking and reducing too
	many lists with dmaengine?

On Mon, Feb 24, 2014 at 02:50:28PM -0600, Andy Gross wrote:
> On Mon, Feb 24, 2014 at 01:03:32PM -0600, Joel Fernandes wrote:
> > Hi folks,
> > 
> > Just wanted your thoughts/suggestions on how we can avoid overhead in the EDMA
> > dmaengine driver. I am seeing a lots of performance drop specially for small
> > transfers with EDMA versus before raw EDMA was moved to DMAEngine framework
> > (atleast 25%).
> 
> I've seen roughly the same drop in my testing.  In my case it had to do
> with the nature of how work is done using virt-dma.  The virt-dma is
> predicated on only letting one transaction be active at a time and it
> increases the latency for getting the next transaction off.  For large
> transactions, it's negligible.  But for small transactions, it is pretty
> evident.

Wrong.  virt-dma allows you to fire off the next transaction in the queue
immediately that the previous transaction has finished.  I know this,
because sa11x0-dma does exactly that.

You don't need to wait for the tasklet to be called before starting the
next transaction.

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