Date:   Tue, 3 Oct 2017 20:04:09 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Stephen Warren <swarren@...dotorg.org>,
        Jon Hunter <jonathanh@...dia.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Laxman Dewangan <ldewangan@...dia.com>,
        Peter De Schrijver <pdeschrijver@...dia.com>,
        Prashant Gaikwad <pgaikwad@...dia.com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...eaurora.org>,
        Rob Herring <robh+dt@...nel.org>,
        Vinod Koul <vinod.koul@...el.com>
Cc:     linux-tegra@...r.kernel.org, devicetree@...r.kernel.org,
        dmaengine@...r.kernel.org, linux-clk@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 3/5] dt-bindings: Add DT bindings for NVIDIA Tegra AHB
 DMA controller

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@...il.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?
> 
> 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.

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