Re: delayed dma trigger

[Takashi Iwai <tiwai@suse.de>]

[Removed Paul Collins from Cc now]

At Mon, 23 Oct 2006 15:46:52 +0200,
Johannes Berg wrote:
> 
> > > For AOA, there are some DMA engine programming pitfalls I wasn't aware
> > > of previously (actually, I was aware of the bug it triggered but not
> > > quite why it happened). But now we have two questions about the trigger
> > > callback.
> > > 
> > > (1)
> > > When correctly using the i2s bus (aoa isn't at the moment), it is not
> > > possible to immediately stop doing DMA, the DMA engine has to finish the
> > > current period first, then go into a stop condition, and only after that
> > > we can stop the engine completely. Hence, this can take quite a while
> > > (up to the period length). Is it ok to set the stop condition and return
> > > from the trigger function without having DMA stopped? It'll be
> > > completely stopped asynchronously on the next interrupt, and we'd have
> > > to make the hw_free callback wait for that, I guess.
> > 
> > Yes, but hw_free isn't enough.  You'd need a DMA sync in prepare
> > callback since a typical repeated play flow is like:
> > 	open -> hw_params -> prepare -> trigger(START) ->
> > 		running -> trigger(STOP) ->
> > 		 prepare -> trigger(START) -> ...
> 
> Right, but prepare can sleep as well so it's easy to do. Though if the
> prepare doesn't actually change anything, couldn't we not stop the DMA
> and just continue?

It's up to you.  My point is the prepare callback is called anyway,
and if you want to resolve the DMA call conflict, prepare callback is
the right place.  But, if trigger(START) checks whether the DMA is
already is running and just continues, then yes, you don't have to
wait in prepare.

> > > (2)
> > > Relating to that, if a start trigger comes in before the DMA engine is
> > > stopped, it would be nice if we could just clear the stop condition and
> > > let it continue to run. Can recording start at any arbitrary period in
> > > the buffer relying only on the pointer callback?
> > > [Actually, come to think of it, isn't this required for proper 'pause'
> > > function during playback or recording? I guess aoa is wrong now then?]
> > 
> > Well, you can of course trick in the lowlevel code as if DMA is newly
> > started.  For PCM middle layer, it doesn't matter.
> > 
> > But, the value returned from pointer callback must be reset to zero
> > once after prepare callback is called.  So, the lowlevel code should
> > handle an offset from the actual DMA pointer value in such a case.
> 
> Let me put it in my words to see if I understand things correctly.
> Let me always use a picture like this
>   |...|...|...|...|
> for the sample buffer, where the | indicates a new period.
> 
> First, pause/resume. If we pause playing in the middle of some period
>   |...|...|...|...|
>         ^ pause request here
> what we'll have to do is make it play until the period ends:
>   |...|...|...|...|
>           ^ pauses here
> Then, if we want to resume playing, we should probably continue at
> exactly that same spot, right? Currently, I think I restart at the first
> period.

Ah now I understood what you meant.

> Now, with stop/play instead of pause/resume, does the picture change?
> There's a prepare between the trigger calls, and that could give us a
> whole new buffer, right? And the period size might even be different.
> Hence, we'll need to create a whole new set of DBDMA commands after
> waiting for the stop to complete, I guess.

Yes, almost.  The prepare callback won't assign any new buffers but
just reset the positions to zero in most cases.  But, the app could
resize the buffer, so yes, it can happen after the following sequence:
	stop -> hw_free -> hw_params -> prepare -> start

> But if the prepare can give us a whole new buffer, what guarantees that
> the old buffer is still there to do DMA on after triggering a stop? Or
> does one need to close/re-open the device to change
> buffers/periodsize/...?

It's hw_free/hw_params call as mentioned in the above.

> Anyway, except for that I'd think that in the play/resume trigger, we
> can see if things are stopped and if it's still running we just clear
> the stop condition. If in prepare we wait for it to stop completely,
> that's fine since we'll have created a whole new dbdma command set.

Sounds feasible.  You can add a check whether you can reuse the
running DBDMA (i.e. period/buffer address and size are unchanged), for
example.

But, still I think a DMA sync in prepare would be an easier solution
since the same check would be needed for hw_free anyway.
Another merit is that the trigger(START) callback does start the
stream at the very same moment it's issued.  With the continous DBDMA,
the start of stream might be delayed than the triggered time.


Takashi

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