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Tue, 21 Aug 2018 14:41:03 +0000 From: =?UTF-8?q?Michal=20Vok=C3=A1=C4=8D?= To: Thierry Reding , Rob Herring Cc: Mark Rutland , devicetree@vger.kernel.org, linux-pwm@vger.kernel.org, linux-kernel@vger.kernel.org, Lukasz Majewski , Fabio Estevam , =?UTF-8?q?Lothar=20Wa=C3=9Fmann?= , =?UTF-8?q?Michal=20Vok=C3=A1=C4=8D?= Subject: [RFC PATCH 1/2] dt-bindings: pwm: imx: Allow switching PWM output between PWM and GPIO Date: Tue, 21 Aug 2018 16:38:52 +0200 Message-Id: <1534862333-27950-2-git-send-email-michal.vokac@ysoft.com> X-Mailer: git-send-email 2.1.4 In-Reply-To: <1534862333-27950-1-git-send-email-michal.vokac@ysoft.com> References: <1534862333-27950-1-git-send-email-michal.vokac@ysoft.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Originating-IP: [89.24.100.190] X-ClientProxiedBy: HE1PR0301CA0012.eurprd03.prod.outlook.com (2603:10a6:3:76::22) To AM6PR04MB4661.eurprd04.prod.outlook.com (2603:10a6:20b:1b::20) X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: 6c446380-e5fe-41b9-6fbf-08d6077420e7 X-Microsoft-Antispam: BCL:0;PCL:0;RULEID:(7020095)(4652040)(8989137)(4534165)(4627221)(201703031133081)(201702281549075)(8990107)(5600074)(711020)(2017052603328)(7153060)(7193020);SRVR:AM6PR04MB4661; 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This can caue issues when inverted PWM polarity is needed. With inverted polarity a duty cycle = 0% corresponds to solid high level on the output. If the PWM is dissabled its output instantly goes to solid zero which corresponds to duty cycle = 100%. To have a trully inverted PWM output configure the PWM pad as a GPIO with pull-up. Then switch the pad to PWM output whenever non-zero duty cycle is needed. Signed-off-by: Michal Vokáč --- Documentation/devicetree/bindings/pwm/imx-pwm.txt | 44 +++++++++++++++++++++++ 1 file changed, 44 insertions(+) diff --git a/Documentation/devicetree/bindings/pwm/imx-pwm.txt b/Documentation/devicetree/bindings/pwm/imx-pwm.txt index c61bdf8..3b1bc4c 100644 --- a/Documentation/devicetree/bindings/pwm/imx-pwm.txt +++ b/Documentation/devicetree/bindings/pwm/imx-pwm.txt @@ -14,6 +14,12 @@ See the clock consumer binding, Documentation/devicetree/bindings/clock/clock-bindings.txt - interrupts: The interrupt for the pwm controller +Optional properties: +- pinctrl: For i.MX27 and newer SoCs. Add extra pinctrl to configure the PWM + pin to gpio function. It allows control over the pin output level when the + PWM block is disabled. This is meant to be used if inverted polarity of the + PWM signal is required. See "Inverted PWM output" section bellow. + Example: pwm1: pwm@53fb4000 { @@ -25,3 +31,41 @@ pwm1: pwm@53fb4000 { clock-names = "ipg", "per"; interrupts = <61>; }; + +Inverted PWM output +------------------- + +The i.MX SoC has such limitation that whenever a pad is configured as a PWM +output, the output level is always zero volts when the PWM block is disabled. +The zero output level is actively driven by the output stage of the PWM block +and can not be overridden by pull-up. It also does not matter what PWM polarity +a PWM client (e.g. backlight) requested. + +To gain control of the PWM output level in disabled state two pinctrl states +can be used. The "default" state and the "pwm" state. In the default state the +PWM output is configured as a GPIO with pull-up. In the "pwm" state the output +is configured as a PWM output. This setup assures that the PWM output is at +the required level that corresponds to duty cycle = 0 when PWM is disabled. +E.g. at boot. + +Example: + +&pwm1 { + pinctrl-names = "default", "pwm"; + pinctrl-0 = <&pinctrl_backlight_gpio>; + pinctrl-1 = <&pinctrl_backlight_pwm>; +} + +pinctrl_backlight_gpio: pwm1grp-gpio { + fsl,pins = < + /* GPIO with 22kOhm pull-up */ + MX6QDL_PAD_GPIO_9__GPIO1_IO09 0xF008 + >; +}; + +pinctrl_backlight_pwm: pwm1grp-pwm { + fsl,pins = < + /* PWM output */ + MX6QDL_PAD_GPIO_9__PWM1_OUT 0x8 + >; +}; -- 2.1.4