From: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
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,
Thierry Reding <thierry.reding@gmail.com>,
dri-devel@lists.freedesktop.org,
Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
Mika Westerberg <mika.westerberg@linux.intel.com>,
Len Brown <lenb@kernel.org>
Subject: Re: [PATCH v3 04/15] pwm: lpss: Add range limit check for the base_unit register value
Date: Mon, 22 Jun 2020 09:35:54 +0200 [thread overview]
Message-ID: <20200622073554.wf3smq3tvnr6t2xy@taurus.defre.kleine-koenig.org> (raw)
In-Reply-To: <20200620121758.14836-5-hdegoede@redhat.com>
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On Sat, Jun 20, 2020 at 02:17:47PM +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.
>
> When the user requestes a low enough period ns value, then the
> calculations in pwm_lpss_prepare() might result in a base_unit value
> which is bigger then base_unit_range - 1. Currently the codes for this
> deals with this by applying a mask:
>
> base_unit &= (base_unit_range - 1);
>
> But this means that we let the value overflow the range, we throw away the
> higher bits and store whatever value is left in the lower bits into the
> register leading to a random output frequency, rather then clamping the
> output frequency to the highest frequency which the hardware can do.
>
> This commit fixes both issues by clamping the base_unit value to be
> between 1 and (base_unit_range - 1).
>
> Fixes: 684309e5043e ("pwm: lpss: Avoid potential overflow of base_unit")
> Signed-off-by: Hans de Goede <hdegoede@redhat.com>
> ---
> Changes in v3:
> - Change upper limit of clamp to (base_unit_range - 1)
> - Add Fixes tag
> ---
> drivers/pwm/pwm-lpss.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/pwm/pwm-lpss.c b/drivers/pwm/pwm-lpss.c
> index 43b1fc634af1..80d0f9c64f9d 100644
> --- a/drivers/pwm/pwm-lpss.c
> +++ b/drivers/pwm/pwm-lpss.c
> @@ -97,6 +97,9 @@ static void pwm_lpss_prepare(struct pwm_lpss_chip *lpwm, struct pwm_device *pwm,
> freq *= base_unit_range;
>
> base_unit = DIV_ROUND_CLOSEST_ULL(freq, c);
DIV_ROUND_CLOSEST_ULL is most probably wrong, too. But I didn't spend
the time to actually confirm that.
> + /* base_unit must not be 0 and we also want to avoid overflowing it */
> + base_unit = clamp_t(unsigned long long, base_unit, 1,
> + base_unit_range - 1);
.get_state seems to handle base_unit == 0 just fine?! Though this
doesn't look right either ...
Best regards
Uwe
--
Pengutronix e.K. | Uwe Kleine-König |
Industrial Linux Solutions | https://www.pengutronix.de/ |
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next prev parent reply other threads:[~2020-06-22 7:36 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-20 12:17 [PATCH v3 00/15] acpi/pwm/i915: Convert pwm-crc and i915 driver's PWM code to use the atomic PWM API Hans de Goede
2020-06-20 12:17 ` [PATCH v3 01/15] ACPI / LPSS: Resume Cherry Trail PWM controller in no-irq phase Hans de Goede
2020-06-22 16:03 ` Rafael J. Wysocki
2020-06-20 12:17 ` [PATCH v3 02/15] ACPI / LPSS: Save Cherry Trail PWM ctx registers only once (at activation) Hans de Goede
2020-06-22 16:04 ` Rafael J. Wysocki
2020-06-20 12:17 ` [PATCH v3 03/15] pwm: lpss: Fix off by one error in base_unit math in pwm_lpss_prepare() Hans de Goede
2020-06-22 7:25 ` Uwe Kleine-König
2020-06-20 12:17 ` [PATCH v3 04/15] pwm: lpss: Add range limit check for the base_unit register value Hans de Goede
2020-06-22 7:35 ` Uwe Kleine-König [this message]
2020-07-06 20:53 ` Hans de Goede
2020-07-07 7:34 ` Uwe Kleine-König
2020-07-07 8:04 ` Hans de Goede
2020-07-07 17:31 ` Hans de Goede
2020-07-07 19:09 ` Uwe Kleine-König
2020-07-07 19:41 ` Hans de Goede
2020-06-20 12:17 ` [PATCH v3 05/15] pwm: lpss: Use pwm_lpss_apply() when restoring state on resume Hans de Goede
2020-06-20 12:17 ` [PATCH v3 06/15] pwm: crc: Fix period / duty_cycle times being off by a factor of 256 Hans de Goede
2020-06-20 12:17 ` [PATCH v3 07/15] pwm: crc: Fix off-by-one error in the clock-divider calculations Hans de Goede
2020-06-20 12:17 ` [PATCH v3 08/15] pwm: crc: Fix period changes not having any effect Hans de Goede
2020-06-20 12:17 ` [PATCH v3 09/15] pwm: crc: Enable/disable PWM output on enable/disable Hans de Goede
2020-06-22 7:55 ` Uwe Kleine-König
2020-07-06 21:03 ` Hans de Goede
2020-07-07 7:26 ` Uwe Kleine-König
2020-06-20 12:17 ` [PATCH v3 10/15] pwm: crc: Implement apply() method to support the new atomic PWM API Hans de Goede
2020-06-20 12:17 ` [PATCH v3 11/15] pwm: crc: Implement get_state() method Hans de Goede
2020-06-22 7:57 ` Uwe Kleine-König
2020-07-06 21:05 ` Hans de Goede
2020-07-07 7:24 ` Uwe Kleine-König
2020-06-20 12:17 ` [PATCH v3 12/15] drm/i915: panel: Add get_vbt_pwm_freq() helper Hans de Goede
2020-06-20 12:17 ` [PATCH v3 13/15] drm/i915: panel: Honor the VBT PWM frequency for devs with an external PWM controller Hans de Goede
2020-06-20 12:17 ` [PATCH v3 14/15] drm/i915: panel: Honor the VBT PWM min setting " Hans de Goede
2020-06-20 12:17 ` [PATCH v3 15/15] drm/i915: panel: Use atomic PWM API " Hans de Goede
2020-07-07 7:50 ` Uwe Kleine-König
2020-07-07 19:21 ` Hans de Goede
2020-06-30 13:51 ` [PATCH v3 00/15] acpi/pwm/i915: Convert pwm-crc and i915 driver's PWM code to use the atomic PWM API Jani Nikula
2020-07-06 20:53 ` Hans de Goede
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