lkml.org 
[lkml]   [2023]   [Mar]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] pwm: mediatek: support inverted polarity

On Fri, Mar 3 2023 at 22:17:25 +01:00:00, Uwe Kleine-König
<u.kleine-koenig@pengutronix.de> wrote:
> On Fri, Mar 03, 2023 at 09:58:21PM +0100, Lorenz Brun wrote:
>> According to the MT7986 Reference Manual the Mediatek PWM
>> controller
>> doesn't appear to have support for inverted polarity.
>>
>> This implements the same solution as in pwm-meson and just inverts
>> the
>> duty cycle instead, which results in the same outcome.
>
> This idea is broken. This was recently discussed on the linux-pwm list
> and I hope will be fixed soon. See
> https://lore.kernel.org/linux-pwm/20230228093911.bh2sbp4tyfir2z5g@pengutronix.de/T/#meda75ffbd4ef2048991ea2cd091c0c14b1bb09c2
>
Is the issue here emulating PWM_POLARITY_INVERSED by inverting the
period or the overflow issues?
This driver currently rejects PWM_POLARITY_INVERSED, but the problem is
that I have a board which inverts the output of the PWM peripheral
(low-side MOSFET for higher-voltage open-drain output), thus I need to
set the PWM node to output an inverted signal so that the final
open-drain output behaves correctly as the signal has been inverted
twice now.

In my specific case this logic could also be added to pwm-fan, but this
would lead to more complexity there as this type of circuit is
generally handled by the PWM driver.

> So this patch won't be accepted, still pointing out a style problem
> below.
>
>> Signed-off-by: Lorenz Brun <lorenz@brun.one>
>> ---
>> drivers/pwm/pwm-mediatek.c | 13 +++++++++----
>> 1 file changed, 9 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/pwm/pwm-mediatek.c b/drivers/pwm/pwm-mediatek.c
>> index 5b5eeaff35da..6f4a54c8299f 100644
>> --- a/drivers/pwm/pwm-mediatek.c
>> +++ b/drivers/pwm/pwm-mediatek.c
>> @@ -202,9 +202,7 @@ static int pwm_mediatek_apply(struct pwm_chip
>> *chip, struct pwm_device *pwm,
>> const struct pwm_state *state)
>> {
>> int err;
>> -
>> - if (state->polarity != PWM_POLARITY_NORMAL)
>> - return -EINVAL;
>> + u64 duty_cycle;
>>
>> if (!state->enabled) {
>> if (pwm->state.enabled)
>> @@ -213,7 +211,14 @@ static int pwm_mediatek_apply(struct pwm_chip
>> *chip, struct pwm_device *pwm,
>> return 0;
>> }
>>
>> - err = pwm_mediatek_config(pwm->chip, pwm, state->duty_cycle,
>> state->period);
>> + // According to the MT7986 Reference Manual the peripheral does
>> not
>> + // appear to have the capability to invert the output. Instead
>> just
>> + // invert the duty cycle.
>
> Wrong commenting style, please stick to C-style comments (/* ... */)
I can fix that if I end up submitting a V2 of this patch, but this
didn't get picked up by checkpatch.

>
>> + duty_cycle = state->duty_cycle;
>> + if (state->polarity == PWM_POLARITY_INVERSED)
>> + duty_cycle = state->period - state->duty_cycle;
>> +
>> + err = pwm_mediatek_config(pwm->chip, pwm, duty_cycle,
>> state->period);
>> if (err)
>> return err;
>
> Best regards
> Uwe
>
> --
> Pengutronix e.K. | Uwe Kleine-König
> |
> Industrial Linux Solutions |
> https://www.pengutronix.de/ |

Regards,
Lorenz


\
 
 \ /
  Last update: 2023-03-27 00:43    [W:0.216 / U:0.188 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site