Re: DT DMA channel binding for Tegra I2S

On 11/23/2011 03:25 PM, Stephen Warren wrote:

Rob, Grant,

Re: http://www.spinics.net/lists/arm-kernel/msg148899.html

Background: Tegra's DMA controller needs to be told which DMA "request
select" to use for each transfer. The identifies which peripheral to
transfer to/from.

The Tegra I2S driver needs to know its own "request select" value to
pass to the audio driver DMA path. The board files typically provide
this to the driver as an IORESOURCE_DMA. I don't think there's any
standard binding that creates such a resource when instantiating a
platform device from device tree, and the discussions I found when
looking for one didn't seem to reach conclusion that there should be
one. So, I proposed the following property in the I2S driver's DT
binding for this:

I think this is a case of every platform being different and
IORESOURCE_DMA is not too widely used. Even on PPC, there's not a
standard way.

Doc:

- dma-channel : The Tegra DMA controller's channel ID for this I2S controller

Example:

i2s@70002800 {
	compatible = "nvidia,tegra20-i2s";
	reg = <0x70002800 0x200>;
	interrupts = < 45 >;
	dma-channel = < 2 >;
};

Does that look reasonable? Or, should I pursue some more standardized
solution?

I had some discussion with Thomas Abraham about this for the pl330 dma:

http://lists.ozlabs.org/pipermail/devicetree-discuss/2011-September/008269.html

So I would just add a phandle to the dma controller here.

(although perhaps dma-request-select would be a better name, since each
of the DMA controller's 16 channels can be used for any of the DMA
selects)

You'll find h/w with all sorts of relationships between requests and
channels on DMA controller h/w...

For reference, the code turns out as below; it might be nicer if the
DT parsing code could create this resource for us, although there was
some previous discussion about IORESOURCE_DMA not being the correct
representation for this anyway.

	dmareq = platform_get_resource(pdev, IORESOURCE_DMA, 0);
	if (!dmareq) {
		if (of_property_read_u32(pdev->dev.of_node,
					 "nvidia,dma-channel",
					 &dma_ch) < 0) {

It would, but I think IORESOURCE_DMA is supposed to be for ISA bus DMA.
Also, I think often you need more information than just a request line
like a mode or type of transfer or burst length parameters.

Rob