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

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Message-ID: <ca7ce856-208b-2753-a8a2-411b29288306@gmail.com>
Date:   Wed, 27 Sep 2017 17:29:19 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     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 27.09.2017 16:46, Jon Hunter wrote:
> 
> On 27/09/17 14:44, Jon Hunter wrote:
>>
>> On 27/09/17 13:12, Dmitry Osipenko wrote:
>>> On 27.09.2017 11:34, 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.
>>>>
>>>
>>> GPIO isn't a very good example, there is no "GPIO" trigger. To me all triggers
>>> are software-defined, so that software could create transfer chains.
>>
>> TRM shows the following in the APBDMA_TRIG_REG_0 ...
>>
>> "XRQ_A: XRQ.A (GPIOA) (Hardware initiated DMA request)"
> 
> Furthermore there are timer and hw-semaphore triggers as well.
> 

Aha, I wasn't sure about what XRQ is. AHB DMA doesn't have XRQ.A as a trigger,
but XRQ.C/D which I suppose corresponds to GPIO C/D.

Timer and hw-semaphore are more questionable, aren't semaphores software-only
triggerable?

-- 
Dmitry