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    gpio: sysfs interface · d8f388d8
    David Brownell authored
    
    
    This adds a simple sysfs interface for GPIOs.
    
        /sys/class/gpio
        	/export ... asks the kernel to export a GPIO to userspace
        	/unexport ... to return a GPIO to the kernel
            /gpioN ... for each exported GPIO #N
    	    /value ... always readable, writes fail for input GPIOs
    	    /direction ... r/w as: in, out (default low); write high, low
    	/gpiochipN ... for each gpiochip; #N is its first GPIO
    	    /base ... (r/o) same as N
    	    /label ... (r/o) descriptive, not necessarily unique
    	    /ngpio ... (r/o) number of GPIOs; numbered N .. N+(ngpio - 1)
    
    GPIOs claimed by kernel code may be exported by its owner using a new
    gpio_export() call, which should be most useful for driver debugging.
    Such exports may optionally be done without a "direction" attribute.
    
    Userspace may ask to take over a GPIO by writing to a sysfs control file,
    helping to cope with incomplete board support or other "one-off"
    requirements that don't merit full kernel support:
    
      echo 23 > /sys/class/gpio/export
    	... will gpio_request(23, "sysfs") and gpio_export(23);
    	use /sys/class/gpio/gpio-23/direction to (re)configure it,
    	when that GPIO can be used as both input and output.
      echo 23 > /sys/class/gpio/unexport
    	... will gpio_free(23), when it was exported as above
    
    The extra D-space footprint is a few hundred bytes, except for the sysfs
    resources associated with each exported GPIO.  The additional I-space
    footprint is about two thirds of the current size of gpiolib (!).  Since
    no /dev node creation is involved, no "udev" support is needed.
    
    Related changes:
    
      * This adds a device pointer to "struct gpio_chip".  When GPIO
        providers initialize that, sysfs gpio class devices become children of
        that device instead of being "virtual" devices.
    
      * The (few) gpio_chip providers which have such a device node have
        been updated.
    
      * Some gpio_chip drivers also needed to update their module "owner"
        field ...  for which missing kerneldoc was added.
    
      * Some gpio_chips don't support input GPIOs.  Those GPIOs are now
        flagged appropriately when the chip is registered.
    
    Based on previous patches, and discussion both on and off LKML.
    
    A Documentation/ABI/testing/sysfs-gpio update is ready to submit once this
    merges to mainline.
    
    [akpm@linux-foundation.org: a few maintenance build fixes]
    Signed-off-by: default avatarDavid Brownell <dbrownell@users.sourceforge.net>
    Cc: Guennadi Liakhovetski <g.liakhovetski@pengutronix.de>
    Cc: Greg KH <greg@kroah.com>
    Cc: Kay Sievers <kay.sievers@vrfy.org>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    d8f388d8