[PATCH v2 0/2] PM / devfreq: Add dev_pm_qos support

[Leonard Crestez <leonard.crestez@nxp.com>]

Add dev_pm_qos notifiers to devfreq core in order to support frequency
limits via dev_pm_qos_add_request.

Unlike the rest of devfreq the dev_pm_qos frequency is measured in kHz,
this is consistent with current dev_pm_qos usage for cpufreq and
allows frequencies above 2Ghz (pm_qos expresses limits as s32).

Like with cpufreq the handling of min_freq/max_freq is moved to the
dev_pm_qos mechanism. Constraints from userspace are no longer clamped on
store, instead all values can be written and we only check against OPPs in a
new devfreq_get_freq_range function. This is consistent with the design of
dev_pm_qos.

Notifiers from pm_qos are executed under a single global dev_pm_qos_mtx and
need to take devfreq->lock, this means that calls into dev_pm_qos while holding
devfreq->lock are not allowed (lockdep warns about possible deadlocks).

Fix this by only adding the qos request and notifiers after devfreq->lock is
released inside devfreq_add_device. In theory this means sysfs writes
are possible before the min/max requests are initialized so we guard
against that explictly. The dev_pm_qos_update_request function would
otherwise print a big WARN splat.

This series depends on recently accepted series restoring
DEV_PM_QOS_MIN/MAX_FREQUENCY inside the pm core:

	https://patchwork.kernel.org/cover/11262633/

It would be great for this to get into 5.5-rc1

---
Changes since RFC v1:
* Trim cover letter
* Drop RFC since DEV_PM_QOS_MIN/MAX_FREQUENCY was restored
* Collect Acks.
* No code changes
Link to v1: https://patchwork.kernel.org/cover/11252415/

Changes since "big version" v10:
* Drop accepted cleanups
* Work with current locking approach (split cleanups into other series)
* Drop acks and deliberately relabel as a new series. It still incorporates
most previous discussion but takes a different approach to locking.
* Don't print errors if devfreq_dev_release is called on error cleanup from
devfreq_add_device, just accept that requests and notifiers might not be
registered yet. I wish dev_pm_qos cleanups behaved like standard "kfree" and
silently did nothing when there's nothing to be done.
Link to v10: https://patchwork.kernel.org/project/linux-arm-kernel/list/?series=196443

Leonard Crestez (2):
  PM / devfreq: Add PM QoS support
  PM / devfreq: Use PM QoS for sysfs min/max_freq

 drivers/devfreq/devfreq.c | 151 ++++++++++++++++++++++++++++++++++----
 include/linux/devfreq.h   |  14 +++-
 2 files changed, 145 insertions(+), 20 deletions(-)

-- 
2.17.1