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pwm: Formally describe the procedure used to pick a hardware waveform setting

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This serves as specification for both, PWM consumers and the respective
callback for lowlevel drivers.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/d2916bfa70274961ded26b07ab6998c36b90e69a.1746010245.git.u.kleine-koenig@baylibre.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>
This commit is contained in:
Uwe Kleine-König 2025-04-30 13:56:00 +02:00 committed by Uwe Kleine-König
parent 164c4ac754
commit d041b76ac9
1 changed files with 23 additions and 1 deletions
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24
drivers/pwm/core.c
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@ -231,7 +231,9 @@ static int __pwm_write_waveform(struct pwm_chip *chip, struct pwm_device *pwm, c
*
* Usually all values passed in @wf are rounded down to the nearest possible
* value (in the order period_length_ns, duty_length_ns and then
* duty_offset_ns). Only if this isn't possible, a value might grow.
* duty_offset_ns). Only if this isn't possible, a value might grow. See the
* documentation for pwm_set_waveform_might_sleep() for a more formal
* description.
*
* Returns: 0 on success, 1 if at least one value had to be rounded up or a
* negative errno.
@ -411,6 +413,26 @@ static int __pwm_set_waveform(struct pwm_device *pwm,
* possible/needed. In the above example requesting .period_length_ns = 94 and
* @exact = true, you get the hardware configured with period = 93.5 ns.
*
* Let C be the set of possible hardware configurations for a given PWM device,
* consisting of tuples (p, d, o) where p is the period length, d is the duty
* length and o the duty offset.
*
* The following algorithm is implemented to pick the hardware setting
* (p, d, o) ∈ C for a given request (p', d', o') with @exact = false::
*
* p = max( { ṗ | ∃ ḋ, ȯ : (ṗ, ḋ, ȯ) ∈ C ∧ ṗ ≤ p' } ∪ { min({ ṗ | ∃ ḋ, ȯ : (ṗ, ḋ, ȯ) ∈ C }) })
* d = max( { ḋ | ∃ ȯ : (p, ḋ, ȯ) ∈ C ∧ ḋ ≤ d' } ∪ { min({ ḋ | ∃ ȯ : (p, ḋ, ȯ) ∈ C }) })
* o = max( { ȯ | (p, d, ȯ) ∈ C ∧ ȯ ≤ o' } ∪ { min({ ȯ | (p, d, ȯ) ∈ C }) })
*
* In words: The chosen period length is the maximal possible period length not
* bigger than the requested period length and if that doesn't exist, the
* minimal period length. The chosen duty length is the maximal possible duty
* length that is compatible with the chosen period length and isn't bigger than
* the requested duty length. Again if such a value doesn't exist, the minimal
* duty length compatible with the chosen period is picked. After that the duty
* offset compatible with the chosen period and duty length is chosen in the
* same way.
*
* Returns: 0 on success, -EDOM if setting failed due to the exact waveform not
* being possible (if @exact), or a different negative errno on failure.
* Context: May sleep.