pwm-meson: fix "BUG: scheduling while atomic"

[Martin Blumenstingl <martin.blumenstingl@googlemail.com>]

Hi Neil,

while working on some unrelated drivers I checked the kernel log on my
Meson8b EC-100 and found this BUG:
    BUG: scheduling while atomic: kworker/1:1/104/0x00000002
    Modules linked in:
    Preemption disabled at:
    [<00000000>]   (null)
    CPU: 1 PID: 104 Comm: kworker/1:1 Not tainted
4.20.0-10293-gfa81effbf874-dirty #3904
    Hardware name: Amlogic Meson platform
    Workqueue: events dbs_work_handler
    [<c0311c0c>] (unwind_backtrace) from [<c030cb88>] (show_stack+0x10/0x14)
    [<c030cb88>] (show_stack) from [<c0c77674>] (dump_stack+0x7c/0x90)
    [<c0c77674>] (dump_stack) from [<c034a348>] (__schedule_bug+0xb0/0xd0)
    [<c034a348>] (__schedule_bug) from [<c0c8ffb0>] (__schedule+0x4e0/0x7b4)
    [<c0c8ffb0>] (__schedule) from [<c0c902cc>] (schedule+0x48/0xa0)
    [<c0c902cc>] (schedule) from [<c0c90840>]
(schedule_preempt_disabled+0x14/0x20)
    [<c0c90840>] (schedule_preempt_disabled) from [<c0c91688>]
(__mutex_lock.constprop.5+0x220/0x5fc)
    [<c0c91688>] (__mutex_lock.constprop.5) from [<c0718398>]
(clk_prepare_lock+0x50/0xf8)
    [<c0718398>] (clk_prepare_lock) from [<c071a8c0>]
(clk_core_get_rate+0xc/0x60)
    [<c071a8c0>] (clk_core_get_rate) from [<c06d7a28>]
(meson_pwm_apply+0xe8/0x404)
    [<c06d7a28>] (meson_pwm_apply) from [<c06d6c94>]
(pwm_apply_state+0x6c/0x1b4)
    [<c06d6c94>] (pwm_apply_state) from [<c07435bc>]
(pwm_regulator_set_voltage+0x124/0x190)
    [<c07435bc>] (pwm_regulator_set_voltage) from [<c0734790>]
(_regulator_do_set_voltage+0x134/0x5a4)
    [<c0734790>] (_regulator_do_set_voltage) from [<c073502c>]
(regulator_balance_voltage+0x224/0x6b8)
    [<c073502c>] (regulator_balance_voltage) from [<c0734d90>]
(regulator_set_voltage_unlocked+0xbc/0x134)
    [<c0734d90>] (regulator_set_voltage_unlocked) from [<c0735504>]
(regulator_set_voltage+0x44/0x74)
    [<c0735504>] (regulator_set_voltage) from [<c0a1c618>]
(_set_opp_voltage.part.3+0x2c/0x94)
    [<c0a1c618>] (_set_opp_voltage.part.3) from [<c0a1cef4>]
(dev_pm_opp_set_rate+0x3bc/0x458)
    [<c0a1cef4>] (dev_pm_opp_set_rate) from [<c0a25fb4>] (set_target+0x2c/0x54)
    [<c0a25fb4>] (set_target) from [<c0a2147c>]
(__cpufreq_driver_target+0x194/0x520)
    [<c0a2147c>] (__cpufreq_driver_target) from [<c0a24868>]
(od_dbs_update+0xb4/0x160)
    [<c0a24868>] (od_dbs_update) from [<c0a253a8>] (dbs_work_handler+0x2c/0x54)
    [<c0a253a8>] (dbs_work_handler) from [<c033c63c>]
(process_one_work+0x204/0x568)
    [<c033c63c>] (process_one_work) from [<c033d4bc>] (worker_thread+0x44/0x580)
    [<c033d4bc>] (worker_thread) from [<c03422d8>] (kthread+0x14c/0x154)
    [<c03422d8>] (kthread) from [<c03010e8>] (ret_from_fork+0x14/0x2c)

The setup on EC-100 is:
- CPU frequency scaling will be enabled with v4.21
- the CPU voltage regulator (called VCCK) is a pwm-regulator

I have not seen this error in the past, even though I've been working on
CPU frequency scaling for a while. So it seems to be a rare issue in real-life.
However, it can happen again because clk_get_rate is using a mutex
(thus it's not supposed to be called from atomic context - but we're
still calling it with the spin lock held).

I tried to fix this myself but while doing so a few questions came up:
- PWM core does not hold any lock when applying the PWM settings
(unlike at "request" time where pwm_request_from_chip is holding the
"pwm_lock")
- first I tried to move the spin locks to meson_pwm_enable() and
meson_pwm_disable(), but then we're still modifying the data in struct
meson_pwm_channel without any locking
- should we use a mutex instead of a spinlock?
- should we get rid of the pre_div, lo, hi and state caching in struct
meson_pwm_channel so we don't have to modify it when applying new PWM
settings?

Please let me know what you (or anyone else - feedback is welcome) think.
I can also work on a patch - as soon as we discussed how to fix it -
if you don't have time for it.


Regards
Martin

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