Re: [PATCH 1/3] dmaengine: add TI EDMA DMA engine driver

[Matt Porter <mporter@ti.com>]

On Thu, Aug 16, 2012 at 11:29:12PM +0100, Russell King wrote:
> On Thu, Aug 16, 2012 at 05:44:29PM -0400, Matt Porter wrote:
> > Add a DMA engine driver for the TI EDMA controller. This driver
> > is implemented as a wrapper around the existing DaVinci private
> > DMA implementation. This approach allows for incremental conversion
> > of each peripheral driver to the DMA engine API. The EDMA driver
> > supports slave transfers but does not yet support cyclic transfers.
> 
> Have you looked at the virt-dma support?  That should allow you to
> avoid some common errors, like forgetting that stuff submitted but
> not issued should not be started even if the channel is currently
> running.

I had previously skimmed it a bit and wrote it off as a helper for
certain types of controllers. On your suggestion, I looked into it
in detail and it does look to be a nice improvement on handling those
types of errors...some of which I noticed were not being handled
properly in my v1 version. I did a quick and dirty conversion of the
EDMA driver to use virt-dma now and it also results in simplified
handling of the descriptors and about a 10% code reduction. I'll do
some clean up on this virt-dma-enabled version and post v2.

One thing to note is that although I'm borrowing a lot from how you
did the omap-dma.c driver, the EDMA implementation of issue_pending()
cannot schedule a tasklet to send the descriptor to the hardware. The
client drivers have to be sure that dma_async_issue_pending() results
in the PaRAM set been written to the h/w before the driver specific
DMA reqs are enabled. Enabling the DMA req before the PaRAM set is
programmed results in an event error condition on this h/w. Ideally
there would be a dma_status state to reflect that a descriptor has
actually been written to the h/w so that client drivers can safely
enable dma reqs where this is a requirement.

Thanks,
Matt