Messages in this thread | | | Subject | Re: [PATCH V2 1/2] Documentation: DT: Add binding documentation for NVIDIA ADMA | From | Stephen Warren <> | Date | Mon, 12 Oct 2015 11:51:59 -0600 |
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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.
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