On Wed, Feb 26, 2025 at 03:58:54PM +0200, Abel Vesa wrote:
> Ideally, the requested duty cycle should never translate to a PWM
> value higher than the selected resolution (PWM size), but currently the
> best matched period is never reported back to the PWM consumer, so the
> consumer will still be using the requested period which is higher than
> the best matched one. This will result in PWM consumer requesting
> duty cycle values higher than the allowed PWM value.
>
> Currently, the consumer driver known to fail this way is the PWM backlight
> (pwm_bl) and should be reworked in such a way that the best matched period
> is used instead.
>
As you're rebasing the patch (per Anjelique's addition of 6-bit pwm) I
think you should omit this paragraph. Whether we like it or not, this
seems to be the way the PWM api is designed, and there's no need to
promise a redesign of the API in this bug fix.
> As for the current implementation of the duty cycle calculation, it is
> capping the max value, fix that by using the resolution to figure out the
> maximum allowed PWM value.
>
> Cc: stable@xxxxxxxxxxxxxxx # 6.4
> Fixes: b00d2ed37617 ("leds: rgb: leds-qcom-lpg: Add support for high resolution PWM")
> Signed-off-by: Abel Vesa <abel.vesa@xxxxxxxxxx>
Reviewed-by: Bjorn Andersson <andersson@xxxxxxxxxx>
Regards,
Bjorn
> ---
> drivers/leds/rgb/leds-qcom-lpg.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/leds/rgb/leds-qcom-lpg.c b/drivers/leds/rgb/leds-qcom-lpg.c
> index f3c9ef2bfa572f9ee86c8b8aa37deb8231965490..146cd9b447787bf170310321e939022dfb176e9f 100644
> --- a/drivers/leds/rgb/leds-qcom-lpg.c
> +++ b/drivers/leds/rgb/leds-qcom-lpg.c
> @@ -529,7 +529,7 @@ static void lpg_calc_duty(struct lpg_channel *chan, uint64_t duty)
> unsigned int clk_rate;
>
> if (chan->subtype == LPG_SUBTYPE_HI_RES_PWM) {
> - max = LPG_RESOLUTION_15BIT - 1;
> + max = BIT(lpg_pwm_resolution_hi_res[chan->pwm_resolution_sel]) - 1;
> clk_rate = lpg_clk_rates_hi_res[chan->clk_sel];
> } else {
> max = LPG_RESOLUTION_9BIT - 1;
>
> --
> 2.34.1
>
