On 03.10.2017 18:38, Stephen Warren wrote:

On 10/03/2017 04:32 AM, Jon Hunter wrote:

quoted

On 03/10/17 00:02, Dmitry Osipenko wrote:

quoted

On 02.10.2017 20:05, Stephen Warren wrote:

quoted

On 09/29/2017 09:11 PM, Dmitry Osipenko wrote:

quoted

On 29.09.2017 22:30, Stephen Warren wrote:

quoted

On 09/27/2017 02:34 AM, Jon Hunter wrote:

quoted

On 27/09/17 02:57, Dmitry Osipenko wrote:

quoted

On 26.09.2017 17:50, Jon Hunter wrote:

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On 26/09/17 00:22, Dmitry Osipenko wrote:

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Document DT bindings for NVIDIA Tegra AHB DMA controller that presents
on Tegra20/30 SoC's.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
---
    .../bindings/dma/nvidia,tegra20-ahbdma.txt         | 23
++++++++++++++++++++++
    1 file changed, 23 insertions(+)
    create mode 100644
Documentation/devicetree/bindings/dma/nvidia,tegra20-ahbdma.txt

diff --git
a/Documentation/devicetree/bindings/dma/nvidia,tegra20-ahbdma.txt
b/Documentation/devicetree/bindings/dma/nvidia,tegra20-ahbdma.txt
new file mode 100644
index 000000000000..2af9aa76ae11

--- /dev/null
+++ b/Documentation/devicetree/bindings/dma/nvidia,tegra20-ahbdma.txt

@@ -0,0 +1,23 @@
+* NVIDIA Tegra AHB DMA controller
+
+Required properties:
+- compatible:    Must be "nvidia,tegra20-ahbdma"
+- reg:        Should contain registers base address and length.
+- interrupts:    Should contain one entry, DMA controller interrupt.
+- clocks:    Should contain one entry, DMA controller clock.
+- resets :    Should contain one entry, DMA controller reset.
+- #dma-cells:    Should be <1>. The cell represents DMA request select

value
+        for the peripheral. For more details consult the Tegra TRM's
+        documentation, in particular AHB DMA channel control register
+        REQ_SEL field.

What about the TRIG_SEL field? Do we need to handle this here as well?

Actually, DMA transfer trigger isn't related a hardware description. It's
up to
software to decide what trigger to select. So it shouldn't be in the
binding.

I think it could be, if say a board wanted a GPIO to trigger a transfer.

quoted

And I think the same applies to requester... any objections?

Well, the REQ_SEL should definitely be in the binding.

Laxman, Stephen, what are your thoughts on the TRIG_SEL field? Looks
like we never bothered with it for the APB DMA and so maybe no ones uses
this.

I don't think TRIG_SEL should be in the binding, at least at present. While
TRIG_SEL certainly is something used to configure the transfer, I believe the
semantics of the current DMA binding only cover DMA transfers that are
initiated
when SW desires, rather than being a combination of after SW programs the
transfer plus some other HW event. So, we always use a default/hard-coded
TRIG_SEL value. As such, there's no need for a TRIG_SEL value in DT. There's
certainly no known use-case that requires a non-default TRIG_SEL value at
present. We could add an extra #dma-cells value later if we find a use for
it,
and the semantics of that use-case make sense to add it to the DMA specifier,
rather than some other separate higher-level property/driver/...

Thank you for the comment. If we'd want to extend the binding further with the
trigger, how to differentiate trigger from the requester in a case of a single
#data-cell?

Of course realistically a chance that the further extension would be needed is
very-very low, so we may defer the efforts to solve that question and for now
make driver aware of the potential #dma-cells extension.

The request selector cell isn't optional, so is always present. If we later add
an optional trig_sel cell, we'll either have:

#dma-cells=<1>: req_sel

or:

#dma-cells=<2>: req_sel, trig_sel

Why request sel. couldn't be optional? Could you please elaborate a bit more?

The documentation currently says it's mandatory, and DT bindings must be evolved
in a backwards-compatible fashion.

quoted

quoted

I think possible options are:

#dma-cells=<1>: req_sel
#dma-cells=<1>: trig_sel

With the above, how would you know that it is the req_sel or trig_sel
that is specified?

Also, if req_sel were optional, then it'd be impossible to distinguish between
those cases, so we can't design a binding like that. In general, when adding
extra optional cells to an #xxx-cells style binding, then whenever cell N is
present, all cells before cell N must be present even if optional.

I've checked how extending of #dma-cells looks like and it is indeed a good
variant since it preserves backward compatibility. Thank you and Jon for the
comments and suggestions, I'll send out v2 soon.