stm32: pwm capture with four channel support skips captures

[gumulka]

Describe the bug

When trying to implement tests for pwm pattern generation, I found that, when four-channel-capture support is enabled on a pwm channel, then every second capture is only passed on to the system, and every other capture is silently dropped.

If I disable four-channel-capture-support everything works fine.

See the code in https://github.com/gumulka/zephyr/tree/pwm_pattern_generation. To be precise: main...gumulka:zephyr:pwm_pattern_generation

To Reproduce

• Check out my zephyr branch and execute the twister test for the nucleo board.
• run the test west twister -T tests/drivers/pwm/pwm_loopback -v --inline-logs -p nucleo_h743zi --device-testing --device-serial /dev/serial/by-id/usb-STMicroelectronics_STLINK-V3_004C00243133510537363734-if02 -X pwm_loopback
• remove the line about four-channel-capture-support in the overlay.
• run again

Expected behavior

The description read, as if the results for four-channel-capture-support are less accurate, but not, that they are missing samples. I expected every sample to be there and captured by the system.

Impact

Annoyance

Logs and console output

I added the following patch to the code:

diff --git a/tests/drivers/pwm/pwm_loopback/src/test_pwm_loopback.c b/tests/drivers/pwm/pwm_loopback/src/test_pwm_loopback.c
index 588796c98a4..170b912954b 100644
--- a/tests/drivers/pwm/pwm_loopback/src/test_pwm_loopback.c
+++ b/tests/drivers/pwm/pwm_loopback/src/test_pwm_loopback.c
@@ -459,6 +459,15 @@ ZTEST(pwm_loopback, test_set_pattern)
        err = pwm_disable_capture(in.dev, in.pwm);
        zassert_equal(err, 0, "failed to disable pwm capture (err %d)", err);
 
+       printf("captured samples: %d\n", data_in.count);
+
+       for (i = 0; i < data_in.count; i++) {
+               uint64_t period, pulse;
+               pwm_cycles_to_usec(in.dev, in.pwm, periods_in[i], &period);
+               pwm_cycles_to_usec(in.dev, in.pwm, pulses_in[i], &pulse);
+               printf("%d: %lld/%lld\n", i, pulse, period);
+       }
+
        zassert_between_inclusive(data_in.count, NUM_SAMPLES / 2, NUM_SAMPLES,
                                  "Did not capture enough samples!");
 

The resulting log:

START - test_set_pattern
captured samples: 3
0: 1196/3596
1: 1996/5996
2: 2796/8396

    Assertion failed at WEST_TOPDIR/zephyr/tests/drivers/pwm/pwm_loopback/src/test_pwm_loopback.c:471: pwm_loopback_test_set_pattern: (data_in.count not between NUM_SAMPLES / 2 and NUM_SAMPLES inclusive)
Did not capture enough samples!

When the samples should have been something along the lines of:
0: 800/2400
1: 1200/3600
2: 1600/4800
3: 2000/6000
4: 2400/7200
5: 2800/8400
6: 3200/9600

Environment (please complete the following information):

• OS: Linux
• on my Zephyr fork the commit 2618a25, but the problem is also in zephyr main, just not visible because it is never tested.

I'm currently only testing on a nucleo_h753zi, but compiling it as a nucleo_h743zi, because for PWM it is equal enough. If I remove the line

https://github.com/gumulka/zephyr/blob/2618a257aaf8a1bb825507bbc445c567aa2a9bf4/tests/drivers/pwm/pwm_loopback/boards/nucleo_h743zi.overlay#L38

then everything works fine. I the line is there, then every second signal is not captured.