Re: [PATCH V3 1/2] of: Add generic device tree DMA helpers

[Arnd Bergmann <arnd@arndb.de>]

On Wednesday 13 June 2012, Jon Hunter wrote:

> > As I said previously, I think just encoding the direction but not
> > the client specific ID (meaning we would have to disambiguate
> > the more complex cases that Stephen mentioned using a dma-names
> > property) would be the best because it keeps the common case simple,
> > but I could live with other things going in there too.
> 
> Ok, so you are saying that there are some DMA controllers where there is
> no channel/request ID and only a direction is needed? So an even simpler
> case than what I had imagined.

No, that was not the case I was thinking about.

> So in that case, I don't see why the first cell after the phandle could
> not be an index which could be either a direction or request-id and then
> the next cell after that be a secondary match variable.
> 
> So simple case with just a index (either req-id or direction) ...
> 
> dmas = <&dmac0 index>
> 
> More complex case ...
> 
> dmas = <&dmac0 index match>
> 
> For example, for OMAP index = req-id and match = direction ...
> 
> dmas = <&dmac0 req-id direction>
> 
> Or am I way off the page?

The intention was instead to remove the need for the /index/ in those
cases, because having a client-specific index in here makes it inconsistent
with other similar bindings (reg, interrupts, gpios, ...) that people
are familiar with. They use an implicit index by counting the
fields in the respective properties.

The existing method we have for avoiding index numbers is to use
named fields, like

	dmas = <&dmac0 matchA>, <dmac1 matchB>, <dmac2 matchC>;
	dma-names = "rx", "rx", "tx";

This is similar to how we use named interrupt and mmio resources, but
it requires that we always request the dma lines by name, which is
slightly more complex than we might want it to be.

Because the vast majority of cases just use a single channel, or one
channel per direction, my idea was to encode the direction in the
dmas property, which lets us request a dma channel by direction from
the driver side, without explicitly encoding the name.

This would let us handle the following cases very easily:

1. one read-write channel

	dmas = <&dmac 0x3 match>;

2. a choice of two read-write channels:

	dmas = <&dmacA 0x3 matchA>, <&dmacB 0x3 matchB>;

3. one read-channel, one write channel:

	dmas = <&dmac 0x1 match-read>, <&dmac 0x2 match-write>;

4. a choice of two read channels and one write channel:

	dmas = <&dmacA 0x1 match-readA>, <&dmacA 0x2 match-write> 
			<&dmacB match-readB>;

And only the cases where we have more multiple channels that differ
in more aspects would require named properties:

5. two different channels

	dmas = <&dmac 0x3 match-rwdata>, <&dmac 0x1 match-status>;
	dma-names = "rwdata", "status";

6. same as 5, but with a choice of channels:

	dmas = <&dmacA 0x3 match-rwdataA>, <&dmacA 0x1 match-status>;
		<dmacB 0x3 match-rwdataB>;
	dma-names = "rwdata", "status", "rwdata";


With a definition like that, we can implement a very simple device
driver interface for the common cases, and a slightly more complex
one for the more complex cases:

1. chan = of_dma_request_channel(dev->of_node, 0);
2. chan = of_dma_request_channel(dev->of_node, 0);
3. rxchan = of_dma_request_channel(dev->of_node, DMA_MEM_TO_DEV);
   txchan = of_dma_request_channel(dev->of_node, DMA_DEV_TO_MEM);
4. rxchan = of_dma_request_channel(dev->of_node, DMA_MEM_TO_DEV);
   txchan = of_dma_request_channel(dev->of_node, DMA_DEV_TO_MEM);
5. chan = of_dma_request_named_channel(dev->of_node, "rwdata", 0);
   auxchan = of_dma_request_named_channel(dev->of_node, "status", DMA_DEV_TO_MEM);
6. chan = of_dma_request_named_channel(dev->of_node, "rwdata", 0);
   auxchan = of_dma_request_named_channel(dev->of_node, "status", DMA_DEV_TO_MEM);

	Arnd