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

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Date:	Fri, 9 Oct 2015 09:26:01 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Jon Hunter <jonathanh@...dia.com>
Cc:	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>,
	Mark Rutland <mark.rutland@....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 10/09/2015 04:20 AM, Jon Hunter wrote:
>
> On 08/10/15 15:27, Stephen Warren wrote:
>> On 10/08/2015 03:58 AM, Jon Hunter wrote:
>
> [snip]
>
>>> 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@...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";
>>>        ...
>>> };
>>
>> Yes, that looks good for the client binding.
>
> One more clarifying question ... should the xlate verify that no other
> dma channel is using the same hardware request signal?
>
> I understand that typically the xlate decodes the binding to get the
> channel info, but because this is invoked by dmaengine while allocating
> a channel, I was wondering if we should prevent dmaengine allocating
> more than one channel to be used with the same hardware request? If so,
> then passing the direction to the xlate would be necessary (so I can
> determine in the xlate that no one else is currently using this, which
> is what I currently do).
>
> Alternatively, I could check that no one else is using the request
> signal at a later when the transfer is being prepared.

I think that handling this at prepare/usage time is probably most 
appropriate. That is the time when the resource conflict /actually/ occurs.

The only time when two clients would be given the same DMA request 
signal is if there are multiple different drivers that can DMA into the 
same FIFO in a time-multiplexed fashion. That seems pretty unlikely off 
the top of my head, but I don't think we want to actively ban that, in 
case we come up with a cunning use-case for it.

> If you are wondering why I am worried about this, I my mind I think that
> the driver should be robust enough to check for conflicts in the request
> signals used by the various channels.

Sure, makes sense.
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