Re: [RFC PATCH 2/2] iommu: rockchip: Handle system-wide and runtime PM

[Tomasz Figa]

On Tue, Dec 16, 2014 at 4:53 AM, Laurent Pinchart
<laurent.pinchart@xxxxxxxxxxxxxxxx> wrote:
> Hi Tomasz,
>
> On Monday 15 December 2014 11:39:01 Tomasz Figa wrote:
>> On Sat, Dec 13, 2014 at 5:47 AM, Laurent Pinchart wrote:
>> > On Friday 12 December 2014 13:15:51 Tomasz Figa wrote:
>> >> On Fri, Dec 12, 2014 at 5:48 AM, Rafael J. Wysocki wrote:
>> >>> On Thursday, December 11, 2014 04:51:37 PM Ulf Hansson wrote:
>> >>>> On 11 December 2014 at 16:31, Kevin Hilman <khilman@xxxxxxxxxx> wrote:
>> >>>>> [+ Laurent Pinchart]
>> >>>>>
>> >>>>> Tomasz Figa <tfiga@xxxxxxxxxxxx> writes:
>> >>>>>> On Thu, Dec 11, 2014 at 8:58 PM, Ulf Hansson wrote:
>> >>>>> [...]
>> >>>>>
>> >>>>>>>> @@ -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.
>> >>
>> >> >> > 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.

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.

Best regards,
Tomasz
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