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

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Date:   Fri, 29 Sep 2017 13:30:51 -0600
From:   Stephen Warren <swarren@...dotorg.org>
To:     Jon Hunter <jonathanh@...dia.com>,
        Dmitry Osipenko <digetx@...il.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 09/27/2017 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/...