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    SubjectRe: [PATCH v1 3/5] dt-bindings: Add DT bindings for NVIDIA Tegra AHB DMA controller
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    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@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.
    >>>>>>
    >>>>>>> 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?

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