Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
kernel
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linux
pwm: Formally describe the procedure used to pick a hardware waveform setting
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>
authored by
Uwe Kleine-König
and committed by
Uwe Kleine-König
d041b76a
164c4ac7
+23
-1
+23
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drivers/pwm/core.c
+23
-1
drivers/pwm/core.c
···
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*
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* Usually all values passed in @wf are rounded down to the nearest possible
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* value (in the order period_length_ns, duty_length_ns and then
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* duty_offset_ns). Only if this isn't possible, a value might grow.
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* duty_offset_ns). Only if this isn't possible, a value might grow. See the
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* documentation for pwm_set_waveform_might_sleep() for a more formal
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* description.
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*
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* Returns: 0 on success, 1 if at least one value had to be rounded up or a
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* negative errno.
···
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* Note that even with @exact = true, some rounding by less than 1 ns is
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* possible/needed. In the above example requesting .period_length_ns = 94 and
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* @exact = true, you get the hardware configured with period = 93.5 ns.
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*
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* Let C be the set of possible hardware configurations for a given PWM device,
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* consisting of tuples (p, d, o) where p is the period length, d is the duty
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* length and o the duty offset.
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*
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* The following algorithm is implemented to pick the hardware setting
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* (p, d, o) ∈ C for a given request (p', d', o') with @exact = false::
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*
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* p = max( { ṗ | ∃ ḋ, ȯ : (ṗ, ḋ, ȯ) ∈ C ∧ ṗ ≤ p' } ∪ { min({ ṗ | ∃ ḋ, ȯ : (ṗ, ḋ, ȯ) ∈ C }) })
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* d = max( { ḋ | ∃ ȯ : (p, ḋ, ȯ) ∈ C ∧ ḋ ≤ d' } ∪ { min({ ḋ | ∃ ȯ : (p, ḋ, ȯ) ∈ C }) })
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* o = max( { ȯ | (p, d, ȯ) ∈ C ∧ ȯ ≤ o' } ∪ { min({ ȯ | (p, d, ȯ) ∈ C }) })
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*
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* In words: The chosen period length is the maximal possible period length not
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* bigger than the requested period length and if that doesn't exist, the
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* minimal period length. The chosen duty length is the maximal possible duty
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* length that is compatible with the chosen period length and isn't bigger than
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* the requested duty length. Again if such a value doesn't exist, the minimal
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* duty length compatible with the chosen period is picked. After that the duty
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* offset compatible with the chosen period and duty length is chosen in the
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* same way.
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*
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* Returns: 0 on success, -EDOM if setting failed due to the exact waveform not
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* being possible (if @exact), or a different negative errno on failure.