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

[Stephen Warren <swarren@wwwdotorg.org>]

On 10/12/2015 07:55 AM, Jon Hunter wrote:
>
> On 09/10/15 16:26, Stephen Warren wrote:
>> 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@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.
>>>
>>> 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.
>
> Although that makes sense, the more I look at this, the more I think it
> should be handled during the channel allocate/free phases as it makes
> sense to allocate the required resources then. It is probably simpler
> and safer too.

I fail to see how it's simpler or safer. Everything is still 100% safe 
if the checks are handled when the channel is actually used. I think 
it's simpler too, since there's less to worry about in DT, and less 
state to carry around in the code.

>> 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.
>
> I know this is purely an example, but if such a time-multiplexed scheme
> was a real use-case, then it would seem more likely to have a shim layer
> between the clients that talked to the dmaengine and hence, it would
> still only be necessary for one client to interface to a given channel.

I don't agree at all. There's no reason why clients shouldn't simply go 
to the dmaengine code and request to use channels when they need them. 
Why would a shim layer be needed for that?

> What I don't like about the above binding is that someone can request
> the dma channel "tx5" and then call dmaengine_prep_dma_cyclic() and say
> you know what, I am gonna receive data instead.

It's always possible to write bugs. The DMA binding can't fix that.

> That seems odd and I
> think that such a scenario should be greeted with an error code of
> -EINVAL. It seems to me that if channels are uni-directional (in the
> sense you either use it for tx or rx), you should request the
> appropriate channel for the direction you want and then set the
> direction in dmaengine_prep_dma_cyclic() so that it matches and if it
> does not then we return an error.

Channels (in HW) are uni-directional for a particular transfer, but can 
operate in any arbitrary combination of directions for different transfers.

Do note that the name "tx5" is something 100% isolated to the client of 
the DMA channel and meaningless for the DMA controller itself. This is 
simply a name that the client uses to look up data that it must pass to 
the DMA controller.

> So I still like the idea of the direction of the request being in the
> binding so we know what the client intends (sorry to keep changing my
> mind). Do you completely deplore the idea?

I still believe it's wrong yes.