[PATCH 1/5] dmaengine: dw_dmac: move to generic DMA binding

On Tue, Jan 29, 2013 at 02:55:49PM +0000, Arnd Bergmann wrote:

On Tuesday 29 January 2013, Russell King - ARM Linux wrote:

quoted

No, that's not what I mean.  I mean the situation we find on Versatile
platforms:

            8     3                   >3
PL080 DMA --/--+--/------> FPGA Mux --/--> {bunch of off-CPU peripherals}
               |  5
               `--/------> {On-CPU peripherals}

This is something that I've been raising _every time_ I've been involved
with DMA engine discussions when it comes to the matching stuff, so this
is nothing new, and it's not unknown about.

Ok, I see. I have not actually been involved with the DMA engine API
much, so for me it's the first time /I/ see this being explained.

From the DT binding perspective, doing this should be no problem. I guess
you would model the MUX as a trivial dma engine device that forwards to
another one, just like we do for nested interrupt controllers:

	pl080: dma-engine at 10000000 {
		compatible ="arm,pl080", "arm,primecell";
		reg = <0x10000000 0x1000>;
		dma-requests = <8>;
		dma-channels = <4>;
		#dma-cells = <2>;
	};

	fpga {
		mux: dma-mux at f0008000 {
			reg = <0xf0008000 100>;
			compatible = "arm,versatile-fpga-dmamux";
			dma-requests = <7>;
			dma-channels = <3>;
			#dma-cells = <1>;
			dmas = <&pl080 5 0>, <&pl080 6 0> <&pl080 7 0>;
			dma-names = "mux5", "mux6", "mux7";
		};
		...

		some-device at f000a000 {
			compatible = "foo,using-muxed-dma";
			dmas = <&mux 3>, <&mux 4>;
			dma-names = "rx", "tx";
		};
	};

The driver for foo,using-muxed-dma just requests a slave channel for its
lines, which ends up calling the xlate function of the driver for the mux.
That driver still needs to get written, of course, but it should be able
to recursively call dma_request_slave_channel on the pl080 device.
The arm,versatile-fpga-dmamux driver would not be registered to the dmaengine
layer, but it would register as an of_dma_controller.

That's a good way to represent it but it fails in a very big way:
You're stuffing N peripherals down to 3 request lines to the DMA
engine, and you may want more than 3 of those peripherals to be
making use of the DMA engine at any one time.

Before anyone says "you shouldn't be doing this" consider this:
your typical DMA slave engine already has this structure:

N DMA channels <---> M DMA requests <---> M peripherals

where N < M.  In other words, there is _already_ a MUX between the
peripherals and the DMA engine channels themselves (what do you think
the "request index" which you have to program into DMA channel control
registers is doing...

We support this external mux today in the PL080 driver - and we do that
by having the PL080 driver do the scheduling of virtual DMA channels on
the actual hardware itself.  The PL080 driver has knowledge that there
may be some sort of additional muxing layer between it and the
peripheral.

As the APIs stand today, you just can't do this without having the
DMA engine driver itself intimately involved because a layer above
doesn't really have much clue as to what's going on, and the DMA
engine stuff itself doesn't layer particularly well.