On 03.10.2017 18:38, Stephen Warren wrote: > On 10/03/2017 04:32 AM, Jon Hunter wrote: >> >> >> On 03/10/17 00:02, Dmitry Osipenko wrote: >>> On 02.10.2017 20:05, Stephen Warren wrote: >>>> 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 >>>>>>>>>> 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. >>>>>>> >>>>>>>> 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. > >>> 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. -- To unsubscribe from this list: send the line "unsubscribe dmaengine" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
