On Wednesday, December 17, 2014 02:15:31 AM Laurent Pinchart wrote:

Hi Tomasz,

On Tuesday 16 December 2014 11:18:33 Tomasz Figa wrote:

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On Tue, Dec 16, 2014 at 4:53 AM, Laurent Pinchart wrote:

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On Monday 15 December 2014 11:39:01 Tomasz Figa wrote:

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On Sat, Dec 13, 2014 at 5:47 AM, Laurent Pinchart wrote:

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On Friday 12 December 2014 13:15:51 Tomasz Figa wrote:

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On Fri, Dec 12, 2014 at 5:48 AM, Rafael J. Wysocki wrote:

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On Thursday, December 11, 2014 04:51:37 PM Ulf Hansson wrote:

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On 11 December 2014 at 16:31, Kevin Hilman wrote:

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[+ Laurent Pinchart]

Tomasz Figa [off-list ref] writes:

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On Thu, Dec 11, 2014 at 8:58 PM, Ulf Hansson wrote:

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@@ -988,11 +1107,28 @@ static int rk_iommu_probe(struct

platform_device *pdev)
                return -ENXIO;
        }

+       pm_runtime_no_callbacks(dev);
+       pm_runtime_enable(dev);
+
+       /* Synchronize state of the domain with driver data. */
+       pm_runtime_get_sync(dev);
+       iommu->is_powered = true;

Doesn't the runtime PM status reflect the value of "is_powered",
thus why do you need to have a copy of it? Could it perpahps be
that you try to cope with the case when CONFIG_PM is unset?

It's worth noting that this driver fully relies on status of other
devices in the power domain the IOMMU is in and does not enforce
the status on its own. So in general, as far as my understanding
of PM runtime subsystem, the status of the IOMMU device will be
always suspended, because nobody will call pm_runtime_get() on it
(except the get and put pair in probe). So is_powered is here to
track status of the domain, not the device. Feel free to suggest a
better way, though.

I still don't like these notifiers.  I think they add ways to
bypass having proper runtime PM implemented for devices/subsystems.

I do agree, but I haven't found another good solution to the
problem.

For the record, I'm not liking this mostly because it "fixes" a
generic problem in a way that's hidden in the genpd code and very
indirect.

Well, that's true. This is indeed a generic problem of PM dependencies
between devices (other than those represented by parent-child
relation), which in fact doesn't have much to do with genpd, but
rather with those devices directly. It is just that genpd is the most
convenient location to solve this in current code and in a simple way.
In other words, I see this solution as a reasonable way to get the
problem solved quickly for now, so that we can start thinking about a
more elegant solution.

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From a high-level, the IOMMU is just another device inside the PM
domain, so ideally it should be doing it's own _get() and _put()
calls so the PM domain code would just do the right thing without
the need for notifiers.

As I understand it, the IOMMU (or for other similar cases)
shouldn't be doing any get() and put() at all because there are no
IO API to serve request from.

Speaking purely from an IOMMU point of view that's not entirely tree.
IOMMU drivers expose map and unmap operations, so they can track
whether any memory is mapped. From a bus master point of view the IOMMU
map and unmap operations are hidden by the DMA mapping API. The IOMMU
can thus track the existence of mappings without any IOMMU awareness in
the bus master driver.

If no mapping exist the IOMMU shouldn't receive any translation
request. An IOMMU driver could thus call pm_runtime_get_sync() in the
map handler when creating the first mapping, and pm_runtime_put() in
the unmap handler when tearing the last mapping down.

One could argue that the IOMMU would end up being powered more often
than strictly needed, as bus masters drivers, even when written
properly, could keep mappings around at times they don't perform bus
access. This is true, and that's an argument I've raised during the
last kernel summit. The general response (including Linus Torvald's)
was that micro-optimizing power management might not be worth it, and
that measurements proving that the gain is worth it are required before
introducing new APIs to solve the problem. I can't disagree with that
argument.

This would be a micro optimization if the IOMMU was located in its own
power domain. Unfortunately in most cases it is not, so keeping all
the devices in the domain powered on, because one of them have a
mapping created doesn't sound like a good idea.

Moreover, most of the drivers will keep the mapping for much longer
than one run cycle. Please take a look at V4L2's videobuf2 subsystem
(which I guess you are more familiar with than me;)), which will keep
MMAP buffers mapped in IOMMU address space for their whole lifetime. I
believe similar is the case for DRM drivers.

Yes, but that doesn't mean it gets out of control. Buffers shouldn't be
allocated if they won't be used. Granted, they could be preallocated (or
rather premapped) slightly before being used, but in sane use cases that
shouldn't be long before the hardware needs to be turned on.

Assuming that we don't have a third party, called "user", involved.

Who needs that ? :-D

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A simple use case is video playback pause. Usually all the software state
(including output buffers that can be used as reference for decoding next
frames) needs to be preserved to continue decoding after resume, but it
would be nice to power off the decoder, if it is unused for some period. In
addition, we would like the pause/resume operation to be fast, so
unmapping/freeing buffers and then exactly the opposite on resume doesn't
sound like a good idea.

OK, then we have one possible use case. I expect people to still want to see 
power consumption numbers though.

Well, we have them, kind of.

In the ACPI world there's something called _DEP which gives us a list of devices
depended on by the given one.  Those may be devices whose drivers provide so
called "operation region" handling which means that an ACPI method executed for
the dependent device may access a device it depends on indirectly.  Because of
that indirection we basically need the devices listed by _DEP to be "on" whenever
the dependent device is "on" or things may break in nasty ways otherwise.

Now, on (some) Intel SoCs some devices listed by _DEP cannot be "on" all the
time, because the lowest-power states of the whole SoC cannot be used then,
which makes hours of battery life of a difference.

This isn't exactly the same problem, but it maps to the IOMMU one quite well IMO.

You can call me annoying, but I'm not sure whether a generic PM dependency 
implementation, while it could be a good idea in general, is the best solution 
here, especially if the bus master and the IOMMU are in a different power 
domain. The bus master could provide functions that don't require DMA access. 
For instance a camera controller could feed its output to the display 
directly, without going through memory. In that case we probably don't want to 
power the IOMMU and its complete power domain on when using the camera 
controller in that mode.

That's a fair point, but it really boils down to energy usage numbers again.

One alternative solution would be to extend the DMA mapping API with two 
functions to signal that DMA is about to be started and that DMA has now 
finished. It might be considered too ad-hoc though.

It would be better to be able to reference count the DMA engine from the
bus master IMO and arguably you can use the runtime PM framework for that.
Namely, give bus masters someting like

	pm_runtime_get_my_DMA_engine(bus_master_device)
	pm_runtime_put_my_DMA_engine(bus_master_device)

and let them call these as they see fit.


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.