On Mon, Feb 13, 2017 at 04:32:32PM +0100, Ulf Hansson wrote:

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Although, I don't know of other examples, besides the runtime PM use
case, where non-atomic channel prepare/unprepare would make sense. Do
you?

The primary ask for that has been to enable runtime_pm for drivers. It's not
a new ask, but we somehow haven't gotten around to do it.

Okay, I see.

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As I said earlier, if we want to solve that problem a better idea is to
actually split the prepare as we discussed in [1]

This way we can get a non atomic descriptor allocate/prepare and release.
Yes we need to redesign the APIs to solve this, but if you guys are up for
it, I think we can do it and avoid any further round abouts :)

Adding/re-designing dma APIs is a viable option to solve the runtime PM case.

Changes would be needed for all related dma client drivers as well,
although if that's what we need to do - let's do it.

Yes, but do bear in mind that some cases do need atomic prepare. The primary
cases for DMA had that in mind and also submitting next transaction from the
callback (tasklet) context, so that won't go away.

It would help in other cases where clients know that they will not be in
atomic context so we provide additional non-atomic "allocation" followed by
prepare, so that drivers can split the work among these and people can do
runtime_pm and other things..

That for sharing the details.

It seems like some dma expert really need to be heavily involved if we
ever are going to complete this work. :-)

Sure, I will help out :)

If anyone of you are in Portland next week, then we can discuss these f2f. I
will try taking a stab at the new API design next week.

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1) Dependencies between dma drivers and dma client drivers during system
PM. For example, a dma client driver needs the dma controller to be
operational (remain system resumed), until the dma client driver itself
becomes system suspended.

The *only* currently available solution for this, is to try to system
suspend the dma controller later than the dma client, via using the *late
or the *noirq system PM callbacks. This works for most cases, but it
becomes a problem when the dma client also needs to be system suspended at
the *late or the *noirq phase. Clearly this solution that doesn't scale.

Using device links explicitly solves this problem as it allows to specify
this dependency between devices.

Yes this is an interesting point. Yes till now people have been doing above
to workaround this problem, but hey this is not a unique to dmaengine. Any
subsystem which provides services to others has this issue, so the solution
much be driver or pm framework and not unique to dmaengine.

I definitely agree, these problems aren't unique to the dmaengine
subsystem. Exactly how/where to manage them, that I guess, is the key
question.

However, I can't resist from finding the device links useful, as those
really do address and solve our issues from a runtime/system PM point
of view.

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2) We won't avoid dma clients from getting -EPROBE_DEFER when requesting
their dma channels in their ->probe() routines. This would be possible, if
we can set up the device links at device initialization.

Well setting those links is not practical at initialization time. Most
modern dma controllers feature a SW mux, with multiple clients connecting
and requesting, would we link all of them? Most of times dmaengine driver
wont know about those..

Okay, I see!

Kind regards
Uffe

-- 
~Vinod