[PATCH v4 4/4] pwm: meson: make full use of common clock framework

[Martin Blumenstingl]

Hello Heiner,

On Thu, Apr 13, 2023 at 7:55 AM Heiner Kallweit <hkallweit1 at gmail.com> wrote:
[...]
> Tested-by: Martin Blumenstingl <martin.blumenstingl at googlemail.com>
Unfortunately I have some bad news and I need to take back my Tested-by :-(
Previously my test was: cycle through all available CPU frequencies
while stressing the CPU.
My assumption was: if the system doesn't lock up everything's fine
because we have a high enough voltage.

This evening however I got a memory corruption error while trying to
log in via UART - which I thought was strange.
So I connected my logic analyzer to my Odroid-C1 and did some experiments:

period = 30518, duty cycle = 15259 (typically used for the 32kHz
output to the SDIO wifi chip)
before your patches / after applying your patches:
PWM: duty cycle: 50.000000% / 50.000000%
PWM: period: 30.6 µs / 30.5 µs
Timing: Time: 15.292 µs (65.395 kHz) / 15.250 µs (65.574 kHz)
Timing: Average: 15.296 µs (65.377 kHz) / 15.264 µs (65.513 kHz)
driver debug messages with your patches applied:
fin_freq: 850000000 Hz
period=30518 cnt=25940
duty=15259 duty_cnt=12970

Then I tried period = 12218, duty cycle = 0 (typically used for the
highest CPU voltage):
before your patches / after applying your patches:
PWM: duty cycle: 0.338983% / n/a (constant low output)
PWM: period: 12.3 µs / n/a
Timing: Time: 12.250 µs (81.633 kHz) / n/a
Timing: Average: 6.148 µs (162.668 kHz) / n/a
driver debug messages with your patches applied:
fin_freq: 850000000 Hz
period=12218 cnt=10385

Finally I tried period = 12218, duty cycle = 12218 (typically used for
the lowest CPU voltage):
before your patches / after applying your patches:
PWM: duty cycle: 99.661017% / n/a (constant low output)
PWM: period: 12.3 µs / n/a
Timing: Time: 12.250 µs (81.633 kHz) / n/a
Timing: Average: 6.148 µs (162.668 kHz) / n/a
driver debug messages with your patches applied:
fin_freq: 850000000 Hz
period=12218 cnt=10385

After seeing the constant low output with period 12218 I realized that
my previous test was no good: the CPU was fed the highest possible
voltage all the time.
It's not clear to me why period 12218 would give no PWM output at all
while period 30518 works fine.
I did an experiment by removing CLK_SET_RATE_PARENT from the divider's
init.flags -> now XTAL (24MHz) is the only possible clock (it's the
hardware default). It does indeed bring back the exact same results as
before (where the XTAL clock was also used; with the changes from this
series FCLK_DIV3 is now chosen, which runs at 850MHz).

Do you have any idea what could cause this?
The FCLK_DIV3 input seems to work as otherwise period 30518 would also not work.
The calculated values also look sane, so it's not that we have some
32-bit overflow (as I'm testing on a 32-bit Meson8b SoC).


Best regards,
Martin