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

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Date:   Mon, 26 Nov 2018 15:50:49 +0000
From:   Vokáč Michal <Michal.Vokac@...ft.com>
To:     Thierry Reding <thierry.reding@...il.com>
CC:     Lothar Waßmann <LW@...o-electronics.de>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Mark Rutland <mark.rutland@....com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-pwm@...r.kernel.org" <linux-pwm@...r.kernel.org>,
        Lukasz Majewski <l.majewski@...ess.pl>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>,
        Fabio Estevam <fabio.estevam@....com>,
        Linus Walleij <linus.walleij@...aro.org>,
        viresh kumar <viresh.kumar@...aro.org>
Subject: Re: [RCF PATCH,v2,2/2] pwm: imx: Configure output to GPIO in disabled
 state

On 26.11.2018 14:34, Thierry Reding wrote:
> On Mon, Nov 26, 2018 at 01:23:16PM +0100, Lothar Waßmann wrote:
>> Thierry Reding <thierry.reding@...il.com> wrote:
>>
>>> On Fri, Nov 23, 2018 at 03:15:11PM +0000, Vokáč Michal wrote:
>>>> 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
>>>
>>> Does it help if you additionally set the ODE bit (bit 11) here?
>>>
>> That only helps to NOT actively pulling the pin HIGH, but the opposite
>> is what is needed here.
> 
>  From the description in the reference manual it sounded like the ODE
> would avoid the pin from actively being driven anywhere if configured as
> output. So I was hoping that in conjunction with the pull-up it would
> actually do the right thing.

Not exactly. I tested/measured this and the ODE setting works like
Lothar said.

If ODE=1 and GPIO is in output mode:

- The internal pull resistors are enabled even if PKE=0 (pull/keeper
   is disabled). That's news to me but it actually matches the GPIO pad
   functional diagram. See Figure 29-3 in RM.

- If the GPIO output is set LOW, the output is actively pulled down by
   the low side transistor.

- If the GPIO output is set HIGH the output is passively pulled up by
   the internal pull-up resistor. The value of the resistor can be
   selected by PUS and I am able to measure it.

That does not solve the issue that the pin may me configured as output
and set LOW from bootloader though.

Michal