linux-kernel - Re: [PATCH v1 3/5] dt-bindings: Add DT bindings for NVIDIA Tegra AHB DMA controller

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Message-ID: <5a5ce745-750f-9c8f-5129-c9b0f7aee614@gmail.com>
Date:   Sat, 30 Sep 2017 06:11:07 +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 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.