linux-kernel - Re: [PATCH v8 4/5] pwm: kona: Add debug info to config function

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Date:	Sat, 30 May 2015 09:42:38 -0700
From:	Tim Kryger <tim.kryger@...il.com>
To:	Jonathan Richardson <jonathar@...adcom.com>
Cc:	Dmitry Torokhov <dtor@...gle.com>,
	Anatol Pomazau <anatol@...gle.com>,
	Arun Ramamurthy <arun.ramamurthy@...adcom.com>,
	Thierry Reding <thierry.reding@...il.com>,
	Scott Branden <sbranden@...adcom.com>,
	bcm-kernel-feedback-list <bcm-kernel-feedback-list@...adcom.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux PWM <linux-pwm@...r.kernel.org>
Subject: Re: [PATCH v8 4/5] pwm: kona: Add debug info to config function

On Tue, May 26, 2015 at 1:08 PM, Jonathan Richardson
<jonathar@...adcom.com> wrote:
> Adds debugging info to config function where duty cycle and period
> are calculated and verified.
>
> Signed-off-by: Jonathan Richardson <jonathar@...adcom.com>
> ---
>  drivers/pwm/pwm-bcm-kona.c |   25 +++++++++++++++++++++++--
>  1 file changed, 23 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/pwm/pwm-bcm-kona.c b/drivers/pwm/pwm-bcm-kona.c
> index c87621f..0ddf19b 100644
> --- a/drivers/pwm/pwm-bcm-kona.c
> +++ b/drivers/pwm/pwm-bcm-kona.c
> @@ -138,18 +138,39 @@ static int kona_pwmc_config(struct pwm_chip *chip, struct pwm_device *pwm,
>                 dc = div64_u64(val, div);
>
>                 /* If duty_ns or period_ns are not achievable then return */
> -               if (pc < PERIOD_COUNT_MIN || dc < DUTY_CYCLE_HIGH_MIN)

The original code was based on the SPEAr PWM driver which has a non-zero
PWMDCR_MIN_DUTY such that the second condition here can evaluate to true.

This isn't the case for the Kona PWM where DUTY_CYCLE_HIGH_MIN is zero.

> +               if (pc < PERIOD_COUNT_MIN) {
> +                       dev_warn(chip->dev,
> +                               "%s: pwm[%d]: period=%d is not achievable, pc=%lu, prescale=%lu\n",
> +                               __func__, chan, period_ns, pc, prescale);
>                         return -EINVAL;
> +               }

Why not just print the minimum allowable period with the provided clock?

I don't think pc and prescale will be particularly helpful to users.

Also, do we really need to print __func__ here?

> +
> +               if (dc < DUTY_CYCLE_HIGH_MIN) {
> +                       if (0 != duty_ns) {
> +                               dev_warn(chip->dev,
> +                                       "%s: pwm[%d]: duty cycle=%d is not achievable, dc=%lu, prescale=%lu\n",
> +                                       __func__, chan, duty_ns, dc, prescale);
> +                       }
> +                       return -EINVAL;
> +               }

The above block is unreachable code.

>
>                 /* If pc and dc are in bounds, the calculation is done */
>                 if (pc <= PERIOD_COUNT_MAX && dc <= DUTY_CYCLE_HIGH_MAX)
>                         break;
>
>                 /* Otherwise, increase prescale and recalculate pc and dc */
> -               if (++prescale > PRESCALE_MAX)
> +               if (++prescale > PRESCALE_MAX) {
> +                       dev_warn(chip->dev,
> +                               "%s: pwm[%d]: Prescale (=%lu) within max (=%d) for period=%d and duty cycle=%d is not achievable\n",
> +                               __func__, chan, prescale, PRESCALE_MAX,
> +                               period_ns, duty_ns);
>                         return -EINVAL;
> +               }
>         }

The user got here because they specified a period larger than the maximum
supported so why not tell them largest value that can be supported instead
of confusing them with prescale and PRESCALE_MAX?

>
> +       dev_dbg(chip->dev, "pwm[%d]: period=%lu, duty_high=%lu, prescale=%lu\n",
> +               chan, pc, dc, prescale);
> +

This could be more clear.  It prints pc but calls it period.

>         /*
>          * Don't apply settings if disabled. The period and duty cycle are
>          * always calculated above to ensure the new values are
> --
> 1.7.9.5
>
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