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If so, adding a > link here would be awesome. Unfortunately, there is currently no publicly available reference manual for this hardware. > >> +/* >> + * Limitations: >> + * - Only normal polarity is supported. >> + * - The hardware has no shadow registers, so configuration changes take >> + * effect immediately without waiting for the current period to complete. >> + * - The output is driven high while the hardware is disabled. >> + * - Supported period range: 2 through 0xfffffffe PWM input clock cycles; >> + * 0xffffffff is reserved by the hardware for a constant low output. >> + * Longer periods are clamped to the maximum. >> + * - A 0% duty cycle is emitted using the constant low encoding because the >> + * hardware interprets a zero high time as a constant high output. The >> + * requested period is lost in that case and is reported as a single clock >> + * cycle. >> + * - A 100% duty cycle is supported and emits a constant high output. >> + */ >> [...] >> +static int >> +axiado_pwm_round_waveform_tohw(struct pwm_chip *chip, >> + struct pwm_device *pwm, >> + const struct pwm_waveform *wf, >> + void *_wfhw) >> +{ >> + struct axiado_pwm_chip *axpwm = pwmchip_get_drvdata(chip); >> + struct axiado_pwm_waveform *wfhw = _wfhw; >> + u64 period; >> + u64 duty; >> + int ret = 0; >> + >> + /* Encode a disabled request as a zeroed hardware waveform. */ >> + if (!wf->period_length_ns) { >> + *wfhw = (struct axiado_pwm_waveform) { }; >> + >> + return 0; >> + } >> + >> + period = mul_u64_u64_div_u64(wf->period_length_ns, axpwm->rate, >> + NSEC_PER_SEC); >> + duty = mul_u64_u64_div_u64(wf->duty_length_ns, axpwm->rate, >> + NSEC_PER_SEC); >> + >> + if (!duty) { >> + /* >> + * A zero high time makes the hardware emit a constant high >> + * output, so use the constant low period encoding for a 0% >> + * duty cycle. The high time must stay non-zero because it >> + * takes precedence over that encoding. The requested period >> + * is lost, .round_waveform_fromhw() reports a single clock >> + * cycle instead. >> + */ >> + *wfhw = (struct axiado_pwm_waveform) { >> + .period = AXIADO_PWM_PERIOD_CONST_LOW, >> + .duty = AXIADO_PWM_DUTY_MIN, >> + .enabled = true, >> + }; >> + >> + /* That single cycle is longer than a sub-cycle request. */ >> + return period ? 0 : 1; >> + } >> + >> + if (period < AXIADO_PWM_PERIOD_MIN) { >> + period = AXIADO_PWM_PERIOD_MIN; >> + ret = 1; >> + } else if (period > AXIADO_PWM_PERIOD_MAX) { >> + period = AXIADO_PWM_PERIOD_MAX; >> + } >> + >> + /* >> + * Period clamping can leave the converted duty greater than the >> + * final hardware period. In that case, clamp it to 100% duty. >> + */ >> + if (duty > period) >> + duty = period; > So with period in [AXIADO_PWM_PERIOD_MIN, AXIADO_PWM_PERIOD_MAX], duty = > period yields a constant high output? If so, why is there a special > encoding for const high? Yes. With a normal period value, |duty == period| produces a constant-high output. There isn't a separate constant-high encoding used by the driver. The unusual hardware behavior is that a zero high time also leaves the output high. To generate a constant-low output, the hardware provides a special encoding: the period register is set to |0xffffffff| while the high-time register remains non-zero. The driver uses that encoding for a 0% duty cycle because |duty == 0| would otherwise leave the output high. > >> + *wfhw = (struct axiado_pwm_waveform) { >> + .period = period, >> + .duty = duty, >> + .enabled = true, >> + }; >> + >> + return ret; >> +} >> + >> +static int >> +axiado_pwm_round_waveform_fromhw(struct pwm_chip *chip, >> + struct pwm_device *pwm, >> + const void *_wfhw, >> + struct pwm_waveform *wf) >> +{ >> + struct axiado_pwm_chip *axpwm = pwmchip_get_drvdata(chip); >> + const struct axiado_pwm_waveform *wfhw = _wfhw; >> + u32 period = wfhw->period; >> + u32 duty = wfhw->duty; >> + >> + if (!wfhw->enabled) { >> + *wf = (struct pwm_waveform) { >> + .period_length_ns = 0, >> + }; >> + >> + return 0; >> + } >> + >> + /* >> + * The constant low encoding doesn't hold a period. Report the >> + * shortest one, the output is low for all of it either way. >> + */ >> + if (period == AXIADO_PWM_PERIOD_CONST_LOW) { >> + period = 1; >> + duty = 0; >> + } else if (duty > period) { >> + duty = period; >> + } > You don't handle wfhw->duty == 0 here. I think this is what Sashiko > pointed out. While .round_waveform_tohw() doens't produce this setting, > it would still be good to handle that in case the driver is loaded with > this setting applied in hardware. Yes, you're right. A zero high-time value results in a constant-high output, so |.round_waveform_fromhw()|should handle that case as well in case the hardware was configured before the driver was loaded. I'll update it to interpret |duty == 0|as a 100% duty cycle. I'll also check this before the constant-low period encoding, since a zero high time takes precedence over that encoding. >> + *wf = (struct pwm_waveform) { >> + .period_length_ns = >> + mul_u64_u64_div_u64_roundup(period, NSEC_PER_SEC, >> + axpwm->rate), >> + .duty_length_ns = >> + mul_u64_u64_div_u64_roundup(duty, NSEC_PER_SEC, >> + axpwm->rate), >> + .duty_offset_ns = 0, >> + }; >> + >> + return 0; >> +} Best regards, Petar