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

[Jon Hunter]

On 03/10/17 13:07, Dmitry Osipenko wrote:
> On 03.10.2017 13:32, 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@xxxxxxxxx>
>>>>>>>>>> ---
>>>>>>>>>> ÂÂ .../bindings/dma/nvidia,tegra20-ahbdma.txtÂÂÂÂÂÂÂÂ | 23
>>>>>>>>>> ++++++++++++++++++++++
>>>>>>>>>> ÂÂ 1 file changed, 23 insertions(+)
>>>>>>>>>> ÂÂ 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:ÂÂÂ Must be "nvidia,tegra20-ahbdma"
>>>>>>>>>> +- reg:ÂÂÂÂÂÂÂ Should contain registers base address and length.
>>>>>>>>>> +- interrupts:ÂÂÂ Should contain one entry, DMA controller interrupt.
>>>>>>>>>> +- clocks:ÂÂÂ Should contain one entry, DMA controller clock.
>>>>>>>>>> +- resets :ÂÂÂ Should contain one entry, DMA controller reset.
>>>>>>>>>> +- #dma-cells:ÂÂÂ Should be <1>. The cell represents DMA request select
>>>>>>>>>> value
>>>>>>>>>> +ÂÂÂÂÂÂÂ for the peripheral. For more details consult the Tegra TRM's
>>>>>>>>>> +ÂÂÂÂÂÂÂ documentation, in particular AHB DMA channel control register
>>>>>>>>>> +ÂÂÂÂÂÂÂ 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?
>>>
>>> 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?
>>
>>> #dma-cells=<2>: req_sel, trig_sel
>>>
>>> The only difference between request and trigger is that trigger issues the whole
>>> transfer, while request only a single burst. Isn't it possible to have a case in
>>> HW for the "trigger-only" option? If not or it's a rareness, then I agree that
>>> REQ_SEL must be mandatory.
>>
>> I think that what Stephen is proposing is that for now we go with
>> '#dma-cells=<1>' and if we ever need to support the trigger cell we
>> could add support for '#dma-cells=<2>'. So with this proposal the
>> 'req_sel' would always be required for both '#dma-cells=<1>' and
>> '#dma-cells=<2>'. Even if the req_sel is not actually used but the
>> 'trig_sel' is, the user would have to set 'req_sel' to some pre-defined
>> value (eg. -1) where we know to ignore it.
>>
> 
> Okay, I see now. Thank you for the clarification, but then we should have that
> pre-defined value declared in the binding?

I would have thought it should be in the dt-binding header.

Cheers
Jon

-- 
nvpublic