Re: [PATCH V2 1/2] Documentation: DT: Add binding documentation for NVIDIA ADMA

[Stephen Warren]

On 10/08/2015 03:58 AM, Jon Hunter wrote:
> On 07/10/15 20:36, Stephen Warren wrote:
> > On 10/07/2015 10:19 AM, Jon Hunter wrote:
> > > On 07/10/15 17:09, Stephen Warren wrote:
> > > > On 10/07/2015 02:43 AM, Jon Hunter wrote:
> > > > > On 07/10/15 00:04, Stephen Warren wrote:
> > > > > > On 10/05/2015 06:10 AM, Jon Hunter wrote:
> > > > > > > Add device-tree binding documentation for the Tegra210 Audio DMA
> > > > > > > controller.
> > > > > >
> > > > > > > diff --git a/Documentation/devicetree/bindings/dma/tegra210-adma.txt
> > > > > > > b/Documentation/devicetree/bindings/dma/tegra210-adma.txt
> > > > > >
> > > > > > > +- #dma-cells : Must be <2>. The first cell denotes the transmit or
> > > > > > > +  receive request number and should be between 1 and the maximum
> > > > > > > number
> > > > > > > +  of requests supported (see properties "dma-rx-requests" and
> > > > > > > +  "dma-tx-requests"). This value corresponds to the
> > > > > > > RX/TX_REQUEST_SELECT
> > > > > > > +  fields in the ADMA_CHn_CTRL register. The second cell denotes
> > > > > > > whether
> > > > > > > +  the channel is a receive or transmit channel and must be either 2
> > > > > > > for
> > > > > > > +  a receive channel and 4 for a transmit channel. These values
> > > > > > > correspond
> > > > > > > +  to the TRANSFER_DIRECTION field of the ADMA_CHn_CTRL register.
> > > > > >
> > > > > > Is it typical to encode the direction into the dma cells? I would have
> > > > > > thought the client would provide that information at run-time when
> > > > > > requesting a DMA channel.
> > > > >
> > > > > I have seen other dma bindings that do [0]. If we don't put the
> > > > > direction in the client binding, then it would appear as ...
> > > > >
> > > > > tegra_admaif: admaif@0x702d0000 {
> > > > >        ...
> > > > >        dmas = <&adma 1>, <&adma 1>, <&adma 2>, <&adma 2>,
> > > > >               <&adma 3>, <&adma 3>, <&adma 4>, <&adma 4>,
> > > > >               <&adma 5>, <&adma 5>, <&adma 6>, <&adma 6>,
> > > > >               <&adma 7>, <&adma 7>, <&adma 8>, <&adma 8>,
> > > > >               <&adma 9>, <&adma 9>, <&adma 10>, <&adma 10>;
> > > > >        dma-names = "rx1", "tx1", "rx2", "tx2", "rx3", "tx3",
> > > > >                    "rx4", "tx4", "rx5", "tx5", "rx6", "tx6",
> > > > >                    "rx7", "tx7", "rx8", "tx8", "rx9", "tx9",
> > > > >                    "rx10", "tx10";
> > > > >        ...
> > > > > };
> > > > >
> > > > > ... where "rxN" and "txN" appear to use the same request, but the
> > > > > reality is that "rxN" is using rx-request-N and "txN" is using
> > > > > tx-request-N. Arnd questioned this before. Obviously I can explain this
> > > > > in the binding document if the above is preferred. However, given that
> > > > > they are named "rx1", "rx2", etc, why not put the direction in the
> > > > > binding?
> > > >
> > > > Why would we need to duplicate the request IDs? I'd expect to have the
> > > > following property content:
> > > >
> > > > dmas = <&adma 1>, <&adma 2>, <&adma 3>, ...;
> > > > dma-names = "1", "2", "3"...;
> > > >
> > > > *and* not have a cell to represent the direction in DT. After all, the
> > > > direction of the channel is 100% implied by the use-case (and hence
> > > > defined by the DMA client's own DT binding); it is known by the client
> > > > driver and can be supplied at run-time.
> > >
> > > Right, but what does the 1, 2, 3, etc in the specifier represent?
> >
> > Aren't they the ADMAIF FIFO IDs?
>
> Yes.
>
> > We know the set/number of ADMAIF FIFOs, and each FIFO needs a request
> > selector ID. The list of those can be indexed by the identity of the
> > FIFO that is accessed via DMA.
>
> Right, but you have 10 RX FIFOs and 10 TX FIFOs. The FIFOs are
> unidirectional. This means that instead of having 20 FIFOs from 1-20
> (yes the FIFOs start from 1 and not 0), you have RX FIFOs from 1-10 and
> TX FIFOs from 1-10.
>
> > Thinking about this more, I think actually that the dmas/dma-names
> > property example that you posted above is exactly what is required here.
>
> Ok, good.
>
> > The counter-example I wrote makes this assumption and hence is invalid.
> > The ADMAIF binding should not assume that the RX and TX request selector
> > IDs are identical. As such, dmas/dma-names should have both an RX and TX
> > entry for each ADMAIF FIFO.
>
> Yes.
>
> > Still, there's no need to encode the DMA direction into the #dma-cells.
> > The client code will know that if it wants to configure DMA into (TX to)
> > FIFO ID 5, it must query dma-names entry "tx5", and simply use whatever
> > is in the DT. When it passes that DMA specifier to the DMA API, the
> > ADMAIF driver knows that it will be for TX, and can pass that
> > information to the DMA code.
>
> That's fine. From my perspective I don't have a strong objection either
> way, however, I can see that given that the name indicates rx or tx,
> then the direction in the binding could be seen as redundant.
>
> So to confirm you are happy with the client bindings being as follows?
>
> tegra_admaif: admaif@0x702d0000 {
>        ...
>       dmas = <&adma 1>, <&adma 1>, <&adma 2>, <&adma 2>,
>              <&adma 3>, <&adma 3>, <&adma 4>, <&adma 4>,
>              <&adma 5>, <&adma 5>, <&adma 6>, <&adma 6>,
>              <&adma 7>, <&adma 7>, <&adma 8>, <&adma 8>,
>              <&adma 9>, <&adma 9>, <&adma 10>, <&adma 10>;
>       dma-names = "rx1", "tx1", "rx2", "tx2", "rx3", "tx3",
>                   "rx4", "tx4", "rx5", "tx5", "rx6", "tx6",
>                   "rx7", "tx7", "rx8", "tx8", "rx9", "tx9",
>                   "rx10", "tx10";
>       ...
> };

Yes, that looks good for the client binding.

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