Hi Jon,

On Tue, Sep 20, 2016 at 12:28 PM, Jon Hunter [off-list ref] wrote:

Some devices may require more than one PM domain to operate and this is
not currently by the PM domain framework. Furthermore, the current Linux
'device' structure only allows devices to be associated with a single PM
domain and so cannot easily be associated with more than one. To allow
devices to be associated with more than one PM domain, if multiple
domains are defined for a given device (eg. via device-tree), then:
1. Create a new PM domain for this device. The name of the new PM domain
   created matches the device name for which it was created for.
2. Register the new PM domain as a sub-domain for all PM domains
   required by the device.
3. Attach the device to the new PM domain.

This looks a suboptimal to me: if you have n devices sharing the same PM
domains, you would add n new subdomains?

Having a clean way to specify multiple PM domains is very useful, though.

E.g. on Renesas ARM SoCs, devices are usually part of two PM domains:
  1. A power area (can be "always-on",
  2. The clock domain.

As power areas and clock domains are fairly orthogonal (the former use the
.power_{off,on}() callbacks, the latter set GENPD_FLAG_PM_CLK and use the
{at,de}tach_dev() callbacks), we currently setup both in the same driver
(SYSC, for controlling power areas), which forwards the clock domain operations
to the clock driver (CPG/MSTP or CPG/MSSR).
Hence we have only single references in the power-domains properties, but
having two would allow to drop the hardcoded links between the two drivers.

(Oh no, more DT backwards compatibility issues if this is accepted ;-)

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds