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

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Date:	Wed, 7 Oct 2015 17:38:11 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Jon Hunter <jonathanh@...dia.com>
Cc:	Stephen Warren <swarren@...dotorg.org>,
	Laxman Dewangan <ldewangan@...dia.com>,
	Vinod Koul <vinod.koul@...el.com>,
	Thierry Reding <thierry.reding@...il.com>,
	Alexandre Courbot <gnurou@...il.com>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Arnd Bergmann <arnd@...db.de>, dmaengine@...r.kernel.org,
	linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 1/2] Documentation: DT: Add binding documentation for
 NVIDIA ADMA

On Wed, Oct 07, 2015 at 09:43:45AM +0100, 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@...02d0000 {
>     ...
>     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?

The client shouldn't know anything about the format of the specifier. If
the client expects separate rx and tx DMA channels, it should describe
these separately with names as above, regardless of whether the DMA
controller requires TX and RX channels to be described differently.

I don't know if it make sense to also verify this from the DMA
controller's PoV.

Mark.
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