The Power Management Quality of Service (PM QoS) allows to
aggregate constraints from multiple entities. It is currently
used to manage the min/max frequency of a given policy.

Frequency constraints can come from:
- Thermal framework: acpi_thermal_cpufreq_init()
- Firmware: _PPC objects: acpi_processor_ppc_init()
- User: by setting policyX/scaling_[min|max]_freq
The minimum of the max frequency constraints is used to compute
the resulting maximum allowed frequency.

When enabling boost frequencies, the same frequency request object
(policy->max_freq_req) as to handle requests from users is used.
As a result, when setting:
- scaling_max_freq
- boost
The last sysfs file used overwrites the request from the other
sysfs file.

To avoid this:
1. Create a per-policy boost_freq_req to save the boost
constraints instead of overwriting the last scaling_max_freq
constraint.

2. policy_set_boost() calls the cpufreq set_boost callback.
Update the newly added boost_freq_req request from there:
- whenever boost is toggled
- to cover all possible paths

3. In the existing set_boost() callbacks:
- Don't update policy->max as this is done through the qos notifier
  cpufreq_notifier_max() which calls cpufreq_set_policy().
- Remove freq_qos_update_request() calls as the qos request is now
  done in policy_set_boost() and updates the new boost_freq_req

---

E.g.:
On a Juno with available frequencies: 600.000, 1.000.000
Boost frequencies: 1.200.000
Using the cppc-cpufreq driver.

---
Without the patches:
# ## Init state
scaling_max_freq:1000000
cpuinfo_max_freq:1000000

# echo 700000 > scaling_max_freq
scaling_max_freq:700000
cpuinfo_max_freq:1000000

# echo 1 > ../boost
scaling_max_freq:1200000
cpuinfo_max_freq:1200000

# echo 800000 > scaling_max_freq
scaling_max_freq:800000
cpuinfo_max_freq:1200000

# echo 0 > ../boost
scaling_max_freq:1000000
cpuinfo_max_freq:1000000

---
With the patches:
# ## Init
scaling_max_freq:1000000
cpuinfo_max_freq:1000000

# echo 700000 > scaling_max_freq
scaling_max_freq:700000
cpuinfo_max_freq:1000000

# echo 1 > ../boost
scaling_max_freq:700000
cpuinfo_max_freq:1200000

# echo 800000 > scaling_max_freq
scaling_max_freq:800000
cpuinfo_max_freq:1200000

# echo 0 > ../boost
scaling_max_freq:800000
cpuinfo_max_freq:1000000

With the patches, the maximum scaling frequency requested is
conserved even though boosting is enabled/disabled.

---

Note:
It seems that there is a confusion in the cpufreq framework between:
- the min/max frequency requested by the user
- the min/max frequency constraint applied when selecting a frequency.

E.g:
A.
$ echo XXX > scaling_max_freq
updates the max_freq_req QoS request.

B.
$ cat scaling_max_freq
shows the content of policy->max, which is the not representing
the value of the max_freq_req QoS request.

C.
Whenever policy->max is accessed in the cpufreq framework,
the aggregation of all the requests on the maximum frequency should
be used instead.

cpufreq_set_policy() aggregates min/max constraints and
writes the resulting value in policy->min/max. These values
are then used in the cpufreq drivers.

Creating a clear distinction would be doable but quite invasive.
This patchset focuses on handling the boost frequency QoS request
first and should not change the current behaviour of policy->min
and max.

---

v1: https://lore.kernel.org/all/20251204101344.192678-1-pierre.gondois@arm.com/#t
v2: https://lore.kernel.org/all/20251208105933.1369125-1-pierre.gondois@arm.com/#t
Changes:
- Fixed error path
- Integrated [PATCH 1/4] Revert "cpufreq: Fix re-boost issue after hotplugging a CPU"
  to another patch
v3:
Changes:
- Fixed error path
- Extracted the revert of:
  "cpufreq: Fix re-boost issue after hotplugging a CPU"
  for clarity purpose
- Set cpuinfo.max_freq as a max_freq_req QoS constraint by default
New patches:
- "cpufreq: Allow decreasing cpuinfo.max_freq"
- "cpufreq: Set policy->min and max as QoS constraints"
v4:
- Correct reported issues
v5:
- Corrections
v6:
- Folded patches:
  - cpufreq: Centralize boost freq QoS requests
  - cpufreq: Update .set_boost() callbacks to rely on boost_freq_req
  inside:
  - cpufreq: Add boost_freq_req QoS request
- Simplified allocation handling of boost_freq_req
- Removed unnecessary bits
v7:
- Removed the following patches to submit them separately
  - cpufreq: Set policy->min and max as real QoS constraints
  - cpufreq/freq_table: Allow decreasing cpuinfo.max_freq
- Fixed blocking_notifier_call_chain() call order when removing
  a policy.
- Updated the commit message of:
  - cpufreq: Remove per-CPU QoS constraint

Pierre Gondois (2):
  cpufreq: Remove per-CPU QoS constraint
  cpufreq: Add boost_freq_req QoS request

 drivers/cpufreq/amd-pstate.c   |  2 --
 drivers/cpufreq/cppc_cpufreq.c | 10 ++-----
 drivers/cpufreq/cpufreq.c      | 55 ++++++++++++++++++++++++----------
 include/linux/cpufreq.h        |  1 +
 4 files changed, 43 insertions(+), 25 deletions(-)

--
2.43.0