linux-kernel - Re: [PATCH v2 2/2] iio: adc: ad7606: finish pwm_get_state

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Message-ID: <iajs2rk7odutqwoih4h6besd4b4nnksap6om5r7i2cw5arqcip@rvztnliokuk3>
Date: Wed, 30 Oct 2024 09:28:01 +0100
From: Uwe Kleine-König <u.kleine-koenig@...libre.com>
To: David Lechner <dlechner@...libre.com>
Cc: Jonathan Cameron <jic23@...nel.org>, 
	Guillaume Stols <gstols@...libre.com>, Michael Hennerich <Michael.Hennerich@...log.com>, 
	linux-pwm@...r.kernel.org, linux-kernel@...r.kernel.org, linux-iio@...r.kernel.org
Subject: Re: [PATCH v2 2/2] iio: adc: ad7606: finish pwm_get_state_hw() TODO

On Tue, Oct 29, 2024 at 04:18:50PM -0500, David Lechner wrote:
> Replace the call to pwm_get_state() with a call to pwm_get_state_hw() in
> the ad7606 driver. This allows reading the sampling_frequency attribute
> to return the rate the hardware is actually running at rather than the
> rate that was requested. These may differ when the hardware isn't
> capable of running at exactly the requested frequency.
> 
> Signed-off-by: David Lechner <dlechner@...libre.com>
> ---
> 
> I went ahead and made this patch since it is trivial, but it would be
> nice to get a Tested-by from Guillaume to make sure it actually works
> as expected.
> ---
>  drivers/iio/adc/ad7606.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/iio/adc/ad7606.c b/drivers/iio/adc/ad7606.c
> index 8b2046baaa3e..1581eb31b8f9 100644
> --- a/drivers/iio/adc/ad7606.c
> +++ b/drivers/iio/adc/ad7606.c
> @@ -762,11 +762,9 @@ static int ad7606_read_raw(struct iio_dev *indio_dev,
>  		*val = st->oversampling;
>  		return IIO_VAL_INT;
>  	case IIO_CHAN_INFO_SAMP_FREQ:
> -		/*
> -		 * TODO: return the real frequency intead of the requested one once
> -		 * pwm_get_state_hw comes upstream.
> -		 */
> -		pwm_get_state(st->cnvst_pwm, &cnvst_pwm_state);
> +		ret = pwm_get_state_hw(st->cnvst_pwm, &cnvst_pwm_state);
> +		if (ret < 0)
> +			return ret;
>  		*val = DIV_ROUND_CLOSEST_ULL(NSEC_PER_SEC, cnvst_pwm_state.period);
>  		return IIO_VAL_INT;
>  	}

Acked-by: Uwe Kleine-König <u.kleine-koenig@...libre.com>

There is a slight inconsistency compared to ad7606_set_sampling_freq():

ad7606_set_sampling_freq uses

	cnvst_pwm_state.period = DIV_ROUND_UP_ULL(NSEC_PER_SEC, freq);

. So if cnvst_pwm_state.period happens to be 3 ns then reading
the freq value yields 333333333, but if you feed freq=333333333 into
ad7606_set_sampling_freq() it sets period = 4.

To fix that you'd better use a plain / here in ad7606_read_raw().
(Note that with using round-closest for both there are still corner
cases, e.g. period = 31796 ns yields freq = 31450.496917851302 but
setting freq = 31450 yields 31796.50238473768 and so 31797.)

Best regards
Uwe

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