Re: [PATCH v3 04/15] pwm: lpss: Add range limit check for the base_unit register value - Uwe Kleine-König

From: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
To: Hans de Goede <hdegoede@redhat.com>
Cc: "Thierry Reding" <thierry.reding@gmail.com>,
	"Jani Nikula" <jani.nikula@linux.intel.com>,
	"Joonas Lahtinen" <joonas.lahtinen@linux.intel.com>,
	"Ville Syrjälä" <ville.syrjala@linux.intel.com>,
	"Rafael J . Wysocki" <rjw@rjwysocki.net>,
	"Len Brown" <lenb@kernel.org>,
	linux-pwm@vger.kernel.org,
	intel-gfx <intel-gfx@lists.freedesktop.org>,
	dri-devel@lists.freedesktop.org,
	"Andy Shevchenko" <andriy.shevchenko@linux.intel.com>,
	"Mika Westerberg" <mika.westerberg@linux.intel.com>,
	linux-acpi@vger.kernel.org, kernel@pengutronix.de
Subject: Re: [PATCH v3 04/15] pwm: lpss: Add range limit check for the base_unit register value
Date: Tue, 7 Jul 2020 21:09:55 +0200	[thread overview]
Message-ID: <20200707190955.e7wzmphdgi5reeyg@pengutronix.de> (raw)
In-Reply-To: <1496178b-ce39-9285-ff75-cd39bc0e9aa7@redhat.com>

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Hello Hans,

On Tue, Jul 07, 2020 at 07:31:29PM +0200, Hans de Goede wrote:
> On 7/7/20 9:34 AM, Uwe Kleine-König wrote:
> > On Mon, Jul 06, 2020 at 10:53:08PM +0200, Hans de Goede wrote:
> > > But if we do then I think closest to the truth would be:
> > > 
> > > state->period     = UINT_MAX;
> > > state->duty_cycle = 0;
> > 
> > I'd say state->period = 1 & state->duty_cycle = 0 is a better
> > representation.
> 
> But that would suggest the output is configured for an
> infinitely high output frequency, but the frequency is
> actually 0, the reason why get_state needs to treat a
> base_unit val of 0 special at all is to avoid a division
> by 0, and in math dividing by 0 gives infinite, isn't
> UINT_MAX a better way to represent infinity ?

Given that duty_cycle is 0, how can to tell anything about the period
when only seeing the signal (= a constant low)?

Given that (ideally) a period is completed when pwm_apply_state() is
called, a short period is much more sensible.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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