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

[VokÃÄ Michal]

On 12.10.2018 10:57, Uwe Kleine-KÃnig wrote:
> Hello,
> 
> On Wed, Oct 10, 2018 at 09:33:26AM +0000, VokÃÄ Michal wrote:
>> Normally the PWM output is held LOW when PWM is disabled. This can cause
>> problems when inverted PWM signal polarity is needed. With this behavior
>> the connected circuit is fed by 100% duty cycle instead of being shut-off.
>>
>> Allow users to define a "gpio" and a "pwm" pinctrl states. The pwm pinctrl
>> state is then selected when PWM is enabled and the gpio pinctrl state is
>> selected when PWM is disabled. Also add a new pwm-gpios GPIO that is used
>> to drive the output in the gpio state.
>>
>> If all the pinctrl states and the pwm-gpios are not correctly specified
>> in DT the logic will work as before.
>>
>> As an example, with this patch a PWM controlled backlight with inversed
>> signal polarity can be used in full brightness range. Without this patch
>> the backlight can not be turned off as brightness = 0 disables the PWM
>> and that in turn set PWM output LOW, that is full brightness.
>>
>> Output of the PWM with "default" pinctrl and with "pwm"+"gpio" pinctrl:
>>
>> +--------------+------------+---------------+---------------------------+
>> | After reset  | Bootloader | Linux pinctrl | User (sysfs, backlight..) |
>> | 100k pull-up | (not used) |               |   enable    |   disable   |
>> +--------------+------------+---------------+---------------------------+
>>   ___________________________    default        _   _   _
>>                              |_________________| |_| |_| |_|_____________
>>
>>                                 pwm + gpio
>>   ___________________________________________   _   _   _   _____________
>>                                              |_| |_| |_| |_|
> 
> I was made aware of this patch by Thierry while discussion about a patch
> opportunity. I already pointed out some stuff I don't like about this
> patch in the repective thread, but I repeat it here to have it at the
> right place.

Thank you for taking time to comment on this one as well Uwe.

>> Signed-off-by: Michal VokÃÄ <michal.vokac@xxxxxxxxx>
>> ---
>> Changes in v2:
>>   - Utilize the "pwm" and "gpio" pinctrl states.
>>   - Use the pwm-gpios signal to drive the output in "gpio" pinctrl state.
>>   - Select the right pinctrl state in probe.
>>
>>   drivers/pwm/pwm-imx.c | 86 +++++++++++++++++++++++++++++++++++++++++++++++++++
>>   1 file changed, 86 insertions(+)
>>
>> diff --git a/drivers/pwm/pwm-imx.c b/drivers/pwm/pwm-imx.c
>> index 6cd3b72..3502123 100644
>> --- a/drivers/pwm/pwm-imx.c
>> +++ b/drivers/pwm/pwm-imx.c
>> @@ -10,11 +10,13 @@
>>   #include <linux/clk.h>
>>   #include <linux/delay.h>
>>   #include <linux/err.h>
>> +#include <linux/gpio/consumer.h>
>>   #include <linux/io.h>
>>   #include <linux/kernel.h>
>>   #include <linux/module.h>
>>   #include <linux/of.h>
>>   #include <linux/of_device.h>
>> +#include <linux/pinctrl/consumer.h>
>>   #include <linux/platform_device.h>
>>   #include <linux/pwm.h>
>>   #include <linux/slab.h>
>> @@ -92,10 +94,45 @@ struct imx_chip {
>>   	void __iomem	*mmio_base;
>>   
>>   	struct pwm_chip	chip;
>> +
>> +	struct pinctrl *pinctrl;
>> +	struct pinctrl_state *pinctrl_pins_gpio;
>> +	struct pinctrl_state *pinctrl_pins_pwm;
>> +	struct gpio_desc *pwm_gpiod;
> 
> The pinctrl framework already knows about "init" and "default". These
> should be enough. i.e. "init" configures as gpio and "default" als pwm.

That is totally new information for me. I've never seen any usage of
"init" pinctrl state before. The total number of users is 6 (rockchip)
so that explains a lot.

I think that it would not work though. The basic idea behind my
solution is that the pinctrl driver does not know what is the correct
pin configuration at probe whereas the pwm-imx driver does.

With the "init" and "default" states the pinctrl driver will always
select the "default" if it find the pins in "init" state.

	https://elixir.bootlin.com/linux/latest/source/drivers/pinctrl/core.c#L1469

But we want the pins to stay in the "init" GPIO state unless a client
enables the PWM.

>>   };
>>   
>> +
>>   #define to_imx_chip(chip)	container_of(chip, struct imx_chip, chip)
>>   
>> +static int imx_pwm_init_pinctrl_info(struct imx_chip *imx_chip,
>> +		struct platform_device *pdev)
>> +{
>> +	imx_chip->pinctrl = devm_pinctrl_get(&pdev->dev);
>> +	if (!imx_chip->pinctrl || IS_ERR(imx_chip->pinctrl)) {
>> +		dev_info(&pdev->dev, "can not get pinctrl\n");
>> +		return PTR_ERR(imx_chip->pinctrl);
>> +	}
>> +
>> +	imx_chip->pinctrl_pins_pwm = pinctrl_lookup_state(imx_chip->pinctrl,
>> +			"pwm");
>> +	imx_chip->pinctrl_pins_gpio = pinctrl_lookup_state(imx_chip->pinctrl,
>> +			"gpio");
>> +	imx_chip->pwm_gpiod = devm_gpiod_get_optional(&pdev->dev, "pwm",
>> +			GPIOD_IN);
>> +
>> +	if (PTR_ERR(imx_chip->pwm_gpiod) == -EPROBE_DEFER) {
> 
> You must not use PTR_ERR on a value that might not contain an error
> pointer.

OK, thank you for valuable info.
So it seems like the I2C folks are in troubles as well:

	https://elixir.bootlin.com/linux/latest/source/drivers/i2c/busses/i2c-imx.c#L996

>> +		return -EPROBE_DEFER;
>> +	} else if (IS_ERR(imx_chip->pwm_gpiod) ||
>> +		   IS_ERR(imx_chip->pinctrl_pins_pwm) ||
>> +		   IS_ERR(imx_chip->pinctrl_pins_gpio)) {
> 
> Would it be more correct to handle imx_chip->pinctrl_pins_pwm ==
> ERR_PTR(-EPROBE_DEFER) similar to imx_chip->pwm_gpiod ==
> ERR_PTR(-EPROBE_DEFER)?

Sorry, I can't answer that now. I need to look deeper into that.

>> +		dev_dbg(&pdev->dev, "PWM pinctrl information incomplete\n");
> 
> I wouldn't call that "incomplete". It's incomplete for the gpio
> switching trick, but enough in general.

True. It is a copy-paste and I could not really come up with something
more sensible for our case. I am happy to change that.
  
>> +		devm_pinctrl_put(imx_chip->pinctrl);
>> +		imx_chip->pinctrl = NULL;
>> +	}
>> +
>> +	return 0;
>> +}
>> +
>>   static void imx_pwm_get_state(struct pwm_chip *chip,
>>   		struct pwm_device *pwm, struct pwm_state *state)
>>   {
>> @@ -306,7 +343,31 @@ static int imx_pwm_apply_v2(struct pwm_chip *chip, struct pwm_device *pwm,
>>   					MX3_PWMCR_POUTC_INVERTED);
>>   
>>   		writel(cr, imx->mmio_base + MX3_PWMCR);
>> +
>> +		/*
>> +		 * If we are in charge of pinctrl then switch output to
>> +		 * the PWM signal.
>> +		 */
>> +		if (imx->pinctrl)
>> +			pinctrl_select_state(imx->pinctrl,
>> +					imx->pinctrl_pins_pwm);
>>   	} else if (cstate.enabled) {
>> +		/*
>> +		 * PWM block will be disabled. Normally its output will be set
>> +		 * low no matter what output polarity is configured. Lets use
> 
> s/Lets/Let's/

Ack.
  
>> +		 * pinctrl to switch the output pin to GPIO functon and keep
>> +		 * the output at the same level as for duty-cycle = 0.
> 
> Is it obvious that using a GPIO is more efficient/better/worth the
> complexity than just enabling the PWM with duty-cycle 0 and the right
> polarity?

To me, it does more than just setting duty-cycle = 0. I think that the
user expect to find the PWM chip in disabled state after this. So that
when you read the HW registers you see the chip is disabled.

>> +		 * First set the GPIO to the desired level, then switch the
>> +		 * muxing and at last disable PWM. In that order we do not get
>> +		 * unwanted logic level changes on the output.
>> +		 */
>> +		if (imx->pinctrl) {
>> +			gpiod_set_value_cansleep(imx->pwm_gpiod, 0);
> 
> You must call gpiod_direction_output for this to have any effect.

Obviously not. It is working (not only) for me like that a long time.
But I agree, a comment in gpiolib I found recently is clear about that
you must call that function beforehand.

> There might be mechanisms in pincontrol that automatically mux the pin
> if it's configured as gpio, I didn't follow the details though.
> 
> Also it should be possible to configure the GPIO as output immediatly.
> If the pinmuxing is set to the PWM function this doesn't have a visible
> side effect.

This could be the first time that we want to disable the PWM as you
might start with PWM enabled from probe. And hence the GPIO is still
configured as input.

>> +			pinctrl_select_state(imx->pinctrl,
>> +					imx->pinctrl_pins_gpio);
> 
> Usually align function arguments to the opening (.

Oops, I will fix those. There is more than this one..
  
>> +		}
>> +
>>   		writel(0, imx->mmio_base + MX3_PWMCR);
>>   
>>   		clk_disable_unprepare(imx->clk_per);
>> @@ -354,6 +415,7 @@ static int imx_pwm_probe(struct platform_device *pdev)
>>   	const struct of_device_id *of_id =
>>   			of_match_device(imx_pwm_dt_ids, &pdev->dev);
>>   	const struct imx_pwm_data *data;
>> +	struct pwm_state cstate;
>>   	struct imx_chip *imx;
>>   	struct resource *r;
>>   	int ret = 0;
>> @@ -385,6 +447,10 @@ static int imx_pwm_probe(struct platform_device *pdev)
>>   		imx->chip.of_pwm_n_cells = 3;
>>   	}
>>   
>> +	ret = imx_pwm_init_pinctrl_info(imx, pdev);
>> +	if (ret)
>> +		return ret;
>> +
>>   	r = platform_get_resource(pdev, IORESOURCE_MEM, 0);
>>   	imx->mmio_base = devm_ioremap_resource(&pdev->dev, r);
>>   	if (IS_ERR(imx->mmio_base))
>> @@ -394,6 +460,26 @@ static int imx_pwm_probe(struct platform_device *pdev)
>>   	if (ret < 0)
>>   		return ret;
>>   
>> +	if (imx->pinctrl) {
>> +		/*
>> +		 * Update cstate after pwmchip_add() call as the core might
>> +		 * call the get_state() function to read the PWM registers
>> +		 * to get the actual HW state.
>> +		 */
>> +		pwm_get_state(imx->chip.pwms, &cstate);
>> +		if (cstate.enabled) {
>> +			dev_dbg(&pdev->dev,
>> +				"PWM entered probe in enabled state\n");
>> +			pinctrl_select_state(imx->pinctrl,
>> +					imx->pinctrl_pins_pwm);
>> +		} else {
>> +			gpiod_set_value_cansleep(imx->pwm_gpiod, 0);
>> +			pinctrl_select_state(imx->pinctrl,
>> +					imx->pinctrl_pins_gpio);
>> +
>> +		}
>> +	}
>> +
>>   	platform_set_drvdata(pdev, imx);
>>   	return 0;
>>   }
> 
> There is nothing in this patch that would prevent this code to live in a
> place where other drivers could reuse this. (But attention, there are
> dragons: Thierry already replied on my topic that his view is different
> in this aspect compared to other maintainers though. His POV is that as
> long as there is only a single driver known that has a problem this
> should be handled in driver specific code.)

When I tripped over that issue I also thought it is i.MX specific.
It never came to my mind that something like that should be done in
higher layer.

But I have to admit that my current understanding of the overall
architecture is at such level that I will just do it how the maintainer
wants me to do it. Given that the solution improves the situation and
solves my original problem. And the pinctrl solution was already
suggested by Fabio Estevam a year ago [1] so I went that way.

[1] https://patchwork.ozlabs.org/patch/839834/#1819865

Michal