[PATCH v8 01/34] opp: Add dev_pm_opp_sync() helper

[Dmitry Osipenko]

18.08.2021 07:53, Viresh Kumar пишет:
> On 18-08-21, 07:37, Dmitry Osipenko wrote:
>> This will set voltage level without having an actively used hardware.
>> Take a 3d driver for example, if you set the rate on probe and
>> rpm-resume will never be called, then the voltage will be set high,
>> while hardware is kept suspended if userspace will never wake it up by
>> executing a 3d job.
> 
> What exactly are we looking to achieve with this stuff ? Cache the
> current performance state with genpd (based on the state bootloader's
> has set) ?

Yes, GENPD will cache the perf state across suspend/resume and initially
cached value is out of sync with h/w.

> Or anything else as well ?

Nothing else. But let me clarify it all again.

Initially the performance state of all GENPDs is 0 for all devices.

The clock rate is preinitialized for all devices to a some default rate
by clk driver, or by bootloader or by assigned-clocks in DT.

When device is rpm-resumed, the resume callback of a device driver
enables the clock.

Before clock is enabled, the voltage needs to be configured in
accordance to the clk rate.

So now we have a GENPD with pstate=0 on a first rpm-resume, which
doesn't match the h/w configuration. Calling dev_pm_opp_sync() sets the
pstate in accordance to the h/w config.

In a previous v7 I proposed to preset the rpm_pstate of GENPD (perf
level that is restored before device is rpm-resumed) from PD's
attach_dev callback, but Ulf didn't like that because it requires to use
and modify GENPD 'private' variables from a PD driver. We decided that
will be better to make device drivers to explicitly sync the perf state,
which I implemented in this v8.