linux-kernel - Re: [PATCH v2 1/3] dmaengine: virt-dma: don't always free descriptor upon completion

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Date:	Thu, 29 Oct 2015 10:54:13 +0900
From:	Vinod Koul <vinod.koul@...el.com>
To:	Robert Jarzmik <robert.jarzmik@...e.fr>
Cc:	Daniel Mack <daniel@...que.org>,
	Haojian Zhuang <haojian.zhuang@...il.com>,
	dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 1/3] dmaengine: virt-dma: don't always free descriptor
 upon completion

On Sat, Oct 24, 2015 at 11:57:53AM +0200, Robert Jarzmik wrote:
> Robert Jarzmik <robert.jarzmik@...e.fr> writes:
> 
> > This patch attempts to enhance the case of a transfer submitted multiple
> > times, and where the cost of creating the descriptors chain is not
> > negligible.
> >
> > This happens with big video buffers (several megabytes, ie. several
> > thousands of linked descriptors in one scatter-gather list). In these
> > cases, a video driver would want to do :
> >  - tx = dmaengine_prep_slave_sg()
> >  - dma_engine_submit(tx);
> >  - dma_async_issue_pending()
> >  - wait for video completion
> >  - read video data (or not, skipping a frame is also possible)
> >  - dma_engine_submit(tx)
> >    => here, the descriptors chain recalculation will take time
> >    => the dma coherent allocation over and over might create holes in
> >       the dma pool, which is counter-productive.
> >  - dma_async_issue_pending()
> >  - etc ...
> >
> > In order to cope with this case, virt-dma is modified to prevent freeing
> > the descriptors upon completion if DMA_CTRL_REUSE flag is set in the
> > transfer.
> >
> > This patch is a respin of the former DMA_CTRL_ACK approach, which was
> > reverted due to a regression in audio drivers.
> >
> > Signed-off-by: Robert Jarzmik <robert.jarzmik@...e.fr>
> > ---
> > Since v1: added doxygen commit to vchan_tx_desc_free
> 
> Hi Vinod,
> 
> Is this serie good for you or do you have remaining comments to be addressed ?

Hi Robert,

This series looks good, but I am afraid we are very close to merge window I
would like this to be deffered to next one for more stabililty tests and
coverage

I have pushed this to topic/test/desc_reuse now.

This will be in -next after merge window

Thanks for fixing this up

-- 
~Vinod
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