Re: [PATCH v5 7/7] iommu/exynos: Use device dependency links to control runtime pm

[Lukas Wunner <lukas@wunner.de>]

On Tue, Nov 08, 2016 at 08:27:12AM +0100, Marek Szyprowski wrote:
> On 2016-11-07 22:47, Luis R. Rodriguez wrote:
> > Has there been any review of the existing similar solutions out there
> > such as the DRM / audio component framework? Would that help ?
> 
> Nope, none of that solution deals with runtime pm.

Well, they do.  Hybrid graphics laptops often have an HDA controller
on the discrete GPU and I assume that's what Luis meant. There's code
in drivers/gpu/vga/vga_switcheroo.c to make this (only sort of) work:

* When the GPU is powered up/down, the HDA controller's driver is
  instructed to pm_runtime_get/put the HDA device (see call to
  set_audio_state() in vga_switcheroo_set_dynamic_switch()).

* When a runtime PM ref is acquired on the HDA device, the
  GPU is powered up (see vga_switcheroo_runtime_resume_hdmi_audio()).


Unfortunately this is all fairly broken, e.g.:

* If a runtime PM ref on the HDA device is held for more than 5 sec
  (autosuspend delay of the GPU), the GPU will be powered down and
  the HDA device will become inaccessible, regardless of the runtime
  PM ref still being held (because vga_switcheroo_set_dynamic_switch()
  doesn't check the refcount of the HDA device).

* The DRM device is afforded direct-complete but the HDA device is not.
  If the GPU is handled earlier by dpm_suspend(), then runtime PM will
  have been disabled on the GPU and thus the HDA device will fail to
  runtime resume before system sleep.

Rafael's series allows representation of such inter-device dependencies
in the PM core and can thus replace kludgy and broken "solutions" like
the one above.

There's one thing that I haven't understood myself though:  In an e-mail
exchange in September Rafael has argued that the above-mentioned hybrid
graphics use case "isn't a good [example] IMO.  That clearly is a case
when two (or more) devices share power resources controlled by a single
on/off switch.  Which is a clear use case for a PM domain."

The same seems to apply to Marek's SYSMMU use case.  When applying device
links to SYSMMU or hybrid graphics, we select one of the devices in the
PM domain as master and have the other one depend on it as slave, i.e.
a synthetic hierarchical relationship is established.

I've responded to Rafael on September 18 that this can't be solved with
a struct dev_pm_domain, but haven't received a reply since:
https://lkml.org/lkml/2016/9/18/103

Thanks,

Lukas