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

From: Russell King - ARM Linux
Date: Tue Feb 25 2014 - 07:24:49 EST


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.

--
FTTC broadband for 0.8mile line: now at 9.7Mbps down 460kbps up... slowly
improving, and getting towards what was expected from it.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/