[PATCH v2 0/4] devfreq: check the get_cur_freq() return value and use it in ufshcd

[Bean Huo]

The devfreq core has three users of the optional ->get_cur_freq()
callback. Two of them check the return value, the third one does not and
passes an uninitialized frequency to the transition notifiers when the
callback fails. Patch 1 fixes that.

Patch 2 writes down what a driver is expected to return from the
callback. Today this has to be found by reading the devfreq core.

Patch 3 records the frequency the controller starts at.
ufshcd_init_clocks() puts the controller at its highest frequency, but
nothing writes that down, so clk_scaling.target_freq stays 0 and devfreq
starts with previous_freq at 0 as well. With use_pm_opp this makes
ufshcd_devfreq_get_dev_status() report 0 Hz, the ondemand governor then
asks for the maximum frequency, and ufshcd_devfreq_target() runs a full
ufshcd_devfreq_scale() that holds up the queue for up to a second only to
set the same OPP and the same gear again.

Patch 4 adds the ->get_cur_freq() callback to ufshcd. Without it the
cur_freq attribute shows the last frequency the governor selected, which
is wrong whenever the controller is scaled outside the governor, for
example after writing 0 to clkscale_enable.

The patches touch two subsystems. Patches 1 and 2 are for the devfreq
tree, patches 3 and 4 are for the SCSI tree. The two halves are
independent, at build time and at run time, and can be applied in either
order.

Patch was tested on a Radxa Dragon Q6A (1d84000.ufshc):

  before "echo 0 > clkscale_enable":  cur_freq 75000000, target_freq 75000000
  after  "echo 0 > clkscale_enable":  cur_freq 300000000, target_freq 75000000

Without it both files report 75000000 and keep doing so for as long as
clock scaling stays disabled. A 4 GiB direct read after enabling clock
scaling again counted the transitions in trans_stat and attributed time
to the 300000000 state, so the frequency the callback returns is one that
devfreq recognises.

One thing to be aware of: devfreq_monitor_resume() copies previous_freq
from the callback, but it does not call devfreq_update_status(). A
frequency change made while the governor was suspended therefore does not
show up as a transition. That is how devfreq behaves today and this
series does not change it.

Changes since v1:
- New patch 3, so that target_freq and devfreq's previous_freq are not 0
  at boot (suggested by Stanley Jhu).
- Patch 4: drop the !cur_freq check, it cannot happen any more.
- Drop the now stale comment in ufshcd_devfreq_get_dev_status().
- Patches 1 and 2 are unchanged.
- The devfreq and the ufshcd patches no longer depend on each other.
- Avri's Reviewed-by is on patches 1, 2 and 4. Patch 3 is new, so it does
  not carry it. Avri, please note that patch 4 changed since you reviewed
  it, the !cur_freq check is gone. Tell me if you want the tag dropped.


Bean Huo (4):
  PM / devfreq: Fall back to previous_freq when get_cur_freq() fails
  PM / devfreq: Add more details to the get_cur_freq() comment
  scsi: ufs: core: Record the frequency the controller starts at
  scsi: ufs: core: Report the current clock frequency to devfreq

 drivers/devfreq/devfreq.c |  5 ++---
 drivers/ufs/core/ufshcd.c | 37 +++++++++++++++++++++++++++++++------
 include/linux/devfreq.h   |  7 +++++--
 3 files changed, 38 insertions(+), 11 deletions(-)


base-commit: e30626823a406725ce29bc75cb8ec467d3e1e326
-- 
2.34.1