Re: [Intel-gfx] [PATCH v2 03/15] pwm: lpss: Add range limit check for the base_unit register value

From: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
To: Hans de Goede <hdegoede@redhat.com>
Cc: linux-pwm@vger.kernel.org,
	intel-gfx <intel-gfx@lists.freedesktop.org>,
	"Rafael J . Wysocki" <rjw@rjwysocki.net>,
	linux-acpi@vger.kernel.org, dri-devel@lists.freedesktop.org,
	"Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>,
	"Mika Westerberg" <mika.westerberg@linux.intel.com>,
	"Len Brown" <lenb@kernel.org>
Subject: Re: [Intel-gfx] [PATCH v2 03/15] pwm: lpss: Add range limit check for the base_unit register value
Date: Mon, 8 Jun 2020 06:50:23 +0300	[thread overview]
Message-ID: <20200608035023.GZ2428291@smile.fi.intel.com> (raw)
In-Reply-To: <20200607181840.13536-4-hdegoede@redhat.com>

On Sun, Jun 07, 2020 at 08:18:28PM +0200, Hans de Goede wrote:
> When the user requests a high enough period ns value, then the
> calculations in pwm_lpss_prepare() might result in a base_unit value of 0.
> 
> But according to the data-sheet the way the PWM controller works is that
> each input clock-cycle the base_unit gets added to a N bit counter and
> that counter overflowing determines the PWM output frequency. Adding 0
> to the counter is a no-op. The data-sheet even explicitly states that
> writing 0 to the base_unit bits will result in the PWM outputting a
> continuous 0 signal.

So, and why it's a problem?

> base_unit values > (base_unit_range / 256), or iow base_unit values using
> the 8 most significant bits, cause loss of resolution of the duty-cycle.
> E.g. assuming a base_unit_range of 65536 steps, then a base_unit value of
> 768 (256 * 3), limits the duty-cycle resolution to 65536 / 768 = 85 steps.
> Clamp the max base_unit value to base_unit_range / 32 to ensure a
> duty-cycle resolution of at least 32 steps. This limits the maximum
> output frequency to 600 KHz / 780 KHz depending on the base clock.

This part I don't understand. Why we limiting base unit? I seems like a
deliberate regression.

-- 
With Best Regards,
Andy Shevchenko

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