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

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Date:	Mon, 20 Aug 2012 10:17:21 -0400
From:	Matt Porter <mporter@...com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	vinod.koul@...el.com, dan.j.williams@...el.com, cjb@...top.org,
	grant.likely@...retlab.ca,
	Linux DaVinci Kernel List 
	<davinci-linux-open-source@...ux.davincidsp.com>,
	Sekhar Nori <nsekhar@...com>,
	Linux MMC List <linux-mmc@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux SPI Devel List 
	<spi-devel-general@...ts.sourceforge.net>,
	Linux ARM Kernel List <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/3] dmaengine: add TI EDMA DMA engine driver

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
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