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

From: Robert Jarzmik
Date: Sat Oct 24 2015 - 11:36:10 EST

Robert Jarzmik <robert.jarzmik@xxxxxxx> 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@xxxxxxx>
> ---
> 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 ?


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