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

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Date:   Mon, 2 Oct 2017 11:05:11 -0600
From:   Stephen Warren <swarren@...dotorg.org>
To:     Dmitry Osipenko <digetx@...il.com>,
        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 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