Query about Open drain flag with active LOW

Hi Linus,
I need help to understand the open drain flag passing from the DT. The DT binding header says: In the file include/dt-bindings/gpio/gpio.h

/*
* Open Drain/Collector is the combination of single-ended active low,
* Open Source/Emitter is the combination of single-ended active high. */
#define GPIO_OPEN_DRAIN (GPIO_SINGLE_ENDED | GPIO_ACTIVE_LOW) #define GPIO_OPEN_SOURCE (GPIO_SINGLE_ENDED | GPIO_ACTIVE_HIGH)

I have the schematics, where I need to set it as
Pin state HIGH (by setting gpio-input mode) to enable the circuit.
Pin state LOW (by setting gpio-output mode with value to 0) to disable the circuit.

With this, I think the GPIO is active HIGH and open drain type.
If I set the active LOW from DT then I can not set pin to logical 1 using the API gpiod_set_value() with “1”. As this invert before calling _gpiod_set_raw_value() and so the value = 0 and it set the pin in the gpio mode output-Low.

Shouldn'’t open drain pin are active HIGH?

Thanks,
Laxman

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(...)

Sorry I do not understand, have you solved it?

Else can you try to break down the problem, do you mean there is a bug in inverted handling for open drain lines in gpiolib?

Yours,
Linus Walleij

I wanted to say that open drain flag should be with ACTIVE_HIGH | SINGLE_ENDED which is in inverted as of now.

Yes, I am suspecting the bug here with combination of ACTIVE state and SINGLE_ENDED flag. The logic to get OPEN DRAIN flag should be in inverted form. Else we need to decouple the ACTIVE state with SINGLE_ENDED flag to get Open drain or open source.

If I want to say logical "1" for the open drain, it is setting pin in drive-0 mode. Actually, it should be in input mode so that pin state become HIGH.

In function of_find_gpio(), it is as follows:

        if (of_flags & OF_GPIO_SINGLE_ENDED) {
                if (of_flags & OF_GPIO_ACTIVE_LOW)
                        *flags |= GPIO_OPEN_DRAIN;
                else
                        *flags |= GPIO_OPEN_SOURCE;
        }

I think it should be

        if (of_flags & OF_GPIO_SINGLE_ENDED) {
                if (of_flags & OF_GPIO_ACTIVE_LOW)
                        *flags |= GPIO_OPEN_SOURCE;
                else
                        *flags |= GPIO_OPEN_DRAIN;
        }

Similar change is needed in the dtbinding gpio header also to invert the ACTIVE state with open drain/source.

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Oh that seems correct... can you send a patch for this?

OK I think there are no real users of this feature yet, so it would be great if you could send a patch, especially if you also have hardware so you can test it.

Yours,
Linus Walleij

Thinking more, active low/high for line is independent of the open drain/source behavior.
Per my understanding, Active low/high depends on the how external device provides signal or glue logic designed. Open drain/source is kind of setting pin state between GND or VDD or high impedance.

Let me send patch on decoupling the active HIGH/LOW with open drain/source and discuss more in the patch.

Thanks,
Laxman

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