Re: [PATCH v1 3/5] dt-bindings: Add DT bindings for NVIDIA Tegra AHB DMA controller

From: Stephen Warren
Date: Mon Oct 02 2017 - 13:05:26 EST

On 09/29/2017 09:11 PM, Dmitry Osipenko wrote:
On 29.09.2017 22:30, Stephen Warren wrote:
On 09/27/2017 02:34 AM, Jon Hunter wrote:

On 27/09/17 02:57, Dmitry Osipenko wrote:
On 26.09.2017 17:50, Jon Hunter wrote:

On 26/09/17 00:22, Dmitry Osipenko wrote:
Document DT bindings for NVIDIA Tegra AHB DMA controller that presents
on Tegra20/30 SoC's.

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

diff --git
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:ÂÂÂ Must be "nvidia,tegra20-ahbdma"
+- reg:ÂÂÂÂÂÂÂ Should contain registers base address and length.
+- interrupts:ÂÂÂ Should contain one entry, DMA controller interrupt.
+- clocks:ÂÂÂ Should contain one entry, DMA controller clock.
+- resets :ÂÂÂ Should contain one entry, DMA controller reset.
+- #dma-cells:ÂÂÂ Should be <1>. The cell represents DMA request select value
+ÂÂÂÂÂÂÂ for the peripheral. For more details consult the Tegra TRM's
+ÂÂÂÂÂÂÂ documentation, in particular AHB DMA channel control register

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.

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

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

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


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