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

Robert Jarzmik robert.jarzmik at free.fr
Sat Oct 24 02:57:53 PDT 2015

Robert Jarzmik <robert.jarzmik at free.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 at free.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 ?



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