Question about gpio sysfs interface

[Michael Heimpold <mhei@xxxxxxxxxxx>]

Hi,

I have a question regarding user-space gpio support with sysfs.

Documentation/gpio/sysfs.txt states
> GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42)
> and have the following read/write attributes:
>
>    /sys/class/gpio/gpioN/
>
>	"direction" ... reads as either "in" or "out". This value may
>		normally be written. ...
>
>		Note that this attribute *will not exist* if the kernel
>		doesn't support changing the direction of a GPIO, or
>		it was exported by kernel code that didn't explicitly
>		allow userspace to reconfigure this GPIO's direction.

Imagine for example a generic user-space tool which enumerates
all available GPIOs to the user and sorts by input and output GPIOs.
How would such a tool classify an exported GPIO when kernel
doesn't export the 'direction' file via sysfs? Or in other words: how
can userspace detect the state (in vs. out) of a GPIO when 'direction'
is not exported?

I propose to change the API that direction is always exported, but
is only readable when hardware doesn't support changing the
direction and/or kernel code wants to prohibit this.

Best regards,
Michael

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