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

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

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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