On 03.10.2017 13:32, 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? >> >> 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? > >> #dma-cells=<2>: req_sel, trig_sel >> >> The only difference between request and trigger is that trigger issues the whole >> transfer, while request only a single burst. Isn't it possible to have a case in >> HW for the "trigger-only" option? If not or it's a rareness, then I agree that >> REQ_SEL must be mandatory. > > I think that what Stephen is proposing is that for now we go with > '#dma-cells=<1>' and if we ever need to support the trigger cell we > could add support for '#dma-cells=<2>'. So with this proposal the > 'req_sel' would always be required for both '#dma-cells=<1>' and > '#dma-cells=<2>'. Even if the req_sel is not actually used but the > 'trig_sel' is, the user would have to set 'req_sel' to some pre-defined > value (eg. -1) where we know to ignore it. > Okay, I see now. Thank you for the clarification, but then we should have that pre-defined value declared in the binding? -- 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
