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

Robert Jarzmik [off-list ref] 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@free.fr>

Hi Jun, Lars-Peter and Vinod,

The revert of the former patch of this type, 8c8fe97b2b8a ("Revert "dmaengine:
virt-dma: don't always free descriptor upon completion"") broke pxa_dma driver.

The reason behind is that pxa_dma was designed, upon transfer submission (in
pxad_tx_submit()), to "move" the virtual descriptor to the submitted list,
rather than adding it at its tail (because it was previously on the allocated
list).

As a consequence, the list_head is not initialized, pxa_dma fails, and the
current status is that pxa_dma Oopses in linux-next, and therefore in Linus's
tree once the merge window is closed.

I'd really like to have this patch as the fix, ie. that at transfer preparation
in vchan_tx_prep(), the virtual descriptor is added on "allocated" list tail.

But to be sure there is no regression on other platforms, I need a test,
especially from someone who suffered from the former version of this patch
(where we had DMA_CTRL_ACK instead of DMA_CTRL_REUSE).

Could anybody give it a try, as I need a fix to go in the early -rc of v4.3
please ?

Cheers.

-- 
Robert