Re: [PATCH V2 1/2] Documentation: DT: Add binding documentation for NVIDIA ADMA
From: Stephen Warren
Date: Mon Oct 12 2015 - 13:51:53 EST
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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