Re: [RCF PATCH,v2,2/2] pwm: imx: Configure output to GPIO in disabled state

From: "Vokáč Michal" <Michal.Vokac@ysoft.com>
To: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
Cc: "Lothar Waßmann" <LW@KARO-electronics.de>,
	"Thierry Reding" <thierry.reding@gmail.com>,
	"Mark Rutland" <mark.rutland@arm.com>,
	"devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
	"linux-pwm@vger.kernel.org" <linux-pwm@vger.kernel.org>,
	"Lukasz Majewski" <l.majewski@majess.pl>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"Rob Herring" <robh+dt@kernel.org>,
	"kernel@pengutronix.de" <kernel@pengutronix.de>,
	"Fabio Estevam" <fabio.estevam@nxp.com>,
	"Linus Walleij" <linus.walleij@linaro.org>,
	"viresh kumar" <viresh.kumar@linaro.org>
Subject: Re: [RCF PATCH,v2,2/2] pwm: imx: Configure output to GPIO in disabled state
Date: Fri, 23 Nov 2018 15:15:11 +0000	[thread overview]
Message-ID: <9da8c6d9-d97c-200c-d8e4-2eb9f73eedc5@ysoft.com> (raw)
In-Reply-To: <20181122190321.ktqegs7kpvhcemvi@pengutronix.de>

On 22.11.2018 20:03, Uwe Kleine-König wrote:
> On Thu, Nov 22, 2018 at 04:46:39PM +0000, Vokáč Michal wrote:
>> On 22.11.2018 17:23, Uwe Kleine-König wrote:
>>> On Thu, Nov 22, 2018 at 03:42:14PM +0000, Vokáč Michal wrote:
>>>> On 16.11.2018 09:25, Uwe Kleine-König wrote:
>>>>> On Fri, Nov 16, 2018 at 08:34:30AM +0100, Lothar Waßmann wrote:
>>>>>> No. You can disable the output driver via pinctrl, so that only the
>>>>>> selected pull-up/down is relevant. The pin function and GPIO register
>>>>>> settings don't matter at all in this case.
>>>>
>>>> Lothar, please can you be more specific how would you do that? IFAIK the
>>>> pull-up/down internal resistors have effect only if the pin is configured
>>>> as GPIO *input* (on i.MX6 at least). See the TRM, 29.4.2.2 Output driver:
>>>>
>>>>     "Internal pull-up, pull-down resistors, and pad keeper are disabled in
>>>>      output mode."
> 
> This would mean you'd have to rely on an external pull up for your use
> case. I wouldn't be surprised however if DSE=0 wouldn't count as "output
> mode". Given the reliability of NXP documentation I wouldn't bet neither
> on one nor the other possibility.

Yeah, the NXP documentation sometimes does not really match reality.
My use case is based on the fact that I configure the pin as input in
the driver. Then it works just fine.

>>> So I'd expect this to really work on i.MX6 but not the earlier SoCs
>>> without a gpio specifier.
>>
>> Maybe you would expect it to work but I already tested and measured
>> that weeks ago ;) It did not work.
> 
> Which pin/gpio do we talk about? Which i.MX6 variant did you test this
> on? (Assuming i.MX6D or i.MX6Q and PAD_DISP0_DATA09, did you try setting
> 
> 	IOMUXC_SW_MUX_CTL_PAD_DISP0_DATA09 (0x020E0194) = 0x00000005
> 	IOMUXC_SW_PAD_CTL_PAD_DISP0_DATA09 (0x020E04A8) = 0x0000b080
> 
> and then play with GPIO 4.30 direction and output value?)

My test setup is as follows:
- SoC is i.MX6DL or i.MX6S - I have three board variants in total.
- Pin used for PWM/GPIO is PAD_GPIO9.
- The pin is not connected to any circuit. Just a test point.
- pinctrl setup in DT:
   - for "pwm":
     - fsl,pins = <MX6QDL_PAD_GPIO_9__PWM1_OUT 0x8>
     - IOMUXC_SW_MUX_CTL_PAD_GPIO09 = 0x00000004
     - IOMUXC_SW_PAD_CTL_PAD_GPIO09 = 0x00000008

   - for "gpio":
    - fsl,pins = <MX6QDL_PAD_GPIO_9__GPIO1_IO09 0xb000>
    - IOMUXC_SW_MUX_CTL_PAD_GPIO09 = 0x00000005
    - IOMUXC_SW_PAD_CTL_PAD_GPIO09 = 0x0000b000

Test scenario:

- In bootloader configure the pin as GPIO output, set it LOW (0V).
   I set it LOW to start in the oposite state than I want to end up in.
- Boot Linux
- Use sysfs to configure the PWM to produce some signal.
- In the PWM driver select the "pwm" pinctrl state.
- Measure the output - expected waveform on the scope.
- Use sysfs to stop the PWM "echo 0 > /sys/class/pwm/pwmchip0/pwm0/enable°
- In the PWM driver select the "gpio" pinctrl state.
- Then in the PWM driver:

a) non-working variant
- Do not request the GPIO - so do not touch its pad control reg.
   It is set as output from the bootloader, data register = 0.
- Measure voltage on the output
   - against GND = 0V
   - against VCC = 0V
- Measure resistance against VCC: Hi-Z

The pin is in Hi-Z mode. That is fine but that does not satisfy any
logic level.

b) working variant
- Request the GPIO, configure it as input.
- Measure voltage on the output
   - against GND = VCC (3.3V)
   - against VCC = 0V
- Measure resistance against VCC = 91.1kOhm

The pull-up is active.

I can repeat the results on both i.MX6DL and i.MX6S on pwm1 and pwm4.
All pull-up and pull-down combinations gave me the expected results
only if the pin is configured as input. So if I repeat the test and
do not touch the pin in bootloader at all, it works as well even if
I do not request/configure the GPIO in the PWM driver.

But we can not rely on the fact that the pin is correctly configured
from bootloader or that it is in the after-reset state. It should
just work regardless of what state it was left in.

Best regards,
Michal