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. Cheers Jon -- nvpublic -- To unsubscribe from this list: send the line "unsubscribe linux-tegra" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
