What:		/sys/firmware/acpi/bgrt/
Date:		January 2012
Contact:	Matthew Garrett <mjg@redhat.com>
Description:
		The BGRT is an ACPI 5.0 feature that allows the OS
		to obtain a copy of the firmware boot splash and
		some associated metadata. This is intended to be used
		by boot splash applications in order to interact with
		the firmware boot splash in order to avoid jarring
		transitions.

		image: The image bitmap. Currently a 32-bit BMP.
		status: 1 if the image is valid, 0 if firmware invalidated it.
		type: 0 indicates image is in BMP format.

		======== ===================================================
		version: The version of the BGRT. Currently 1.
		xoffset: The number of pixels between the left of the screen
			 and the left edge of the image.
		yoffset: The number of pixels between the top of the screen
			 and the top edge of the image.
		======== ===================================================

What:		/sys/firmware/acpi/hotplug/
Date:		February 2013
Contact:	Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Description:
		There are separate hotplug profiles for different classes of
		devices supported by ACPI, such as containers, memory modules,
		processors, PCI root bridges etc.  A hotplug profile for a given
		class of devices is a collection of settings defining the way
		that class of devices will be handled by the ACPI core hotplug
		code.  Those profiles are represented in sysfs as subdirectories
		of /sys/firmware/acpi/hotplug/.

		The following setting is available to user space for each
		hotplug profile:

		======== =======================================================
		enabled: If set, the ACPI core will handle notifications of
			 hotplug events associated with the given class of
			 devices and will allow those devices to be ejected with
			 the help of the _EJ0 control method.  Unsetting it
			 effectively disables hotplug for the correspoinding
			 class of devices.
		======== =======================================================

		The value of the above attribute is an integer number: 1 (set)
		or 0 (unset).  Attempts to write any other values to it will
		cause -EINVAL to be returned.

What:		/sys/firmware/acpi/interrupts/
Date:		February 2008
Contact:	Len Brown <lenb@kernel.org>
Description:
		All ACPI interrupts are handled via a single IRQ,
		the System Control Interrupt (SCI), which appears
		as "acpi" in /proc/interrupts.

		However, one of the main functions of ACPI is to make
		the platform understand random hardware without
		special driver support.  So while the SCI handles a few
		well known (fixed feature) interrupts sources, such
		as the power button, it can also handle a variable
		number of a "General Purpose Events" (GPE).

		A GPE vectors to a specified handler in AML, which
		can do a anything the BIOS writer wants from
		OS context.  GPE 0x12, for example, would vector
		to a level or edge handler called _L12 or _E12.
		The handler may do its business and return.
		Or the handler may send send a Notify event
		to a Linux device driver registered on an ACPI device,
		such as a battery, or a processor.

		To figure out where all the SCI's are coming from,
		/sys/firmware/acpi/interrupts contains a file listing
		every possible source, and the count of how many
		times it has triggered::

		  $ cd /sys/firmware/acpi/interrupts
		  $ grep . *
		  error:	     0
		  ff_gbl_lock:	     0   enable
		  ff_pmtimer:	     0  invalid
		  ff_pwr_btn:	     0   enable
		  ff_rt_clk:	     2  disable
		  ff_slp_btn:	     0  invalid
		  gpe00:	     0	invalid
		  gpe01:	     0	 enable
		  gpe02:	   108	 enable
		  gpe03:	     0	invalid
		  gpe04:	     0	invalid
		  gpe05:	     0	invalid
		  gpe06:	     0	 enable
		  gpe07:	     0	 enable
		  gpe08:	     0	invalid
		  gpe09:	     0	invalid
		  gpe0A:	     0	invalid
		  gpe0B:	     0	invalid
		  gpe0C:	     0	invalid
		  gpe0D:	     0	invalid
		  gpe0E:	     0	invalid
		  gpe0F:	     0	invalid
		  gpe10:	     0	invalid
		  gpe11:	     0	invalid
		  gpe12:	     0	invalid
		  gpe13:	     0	invalid
		  gpe14:	     0	invalid
		  gpe15:	     0	invalid
		  gpe16:	     0	invalid
		  gpe17:	  1084	 enable
		  gpe18:	     0	 enable
		  gpe19:	     0	invalid
		  gpe1A:	     0	invalid
		  gpe1B:	     0	invalid
		  gpe1C:	     0	invalid
		  gpe1D:	     0	invalid
		  gpe1E:	     0	invalid
		  gpe1F:	     0	invalid
		  gpe_all:	  1192
		  sci:		  1194
		  sci_not:	     0

		===========  ==================================================
		sci	     The number of times the ACPI SCI
			     has been called and claimed an interrupt.

		sci_not	     The number of times the ACPI SCI
			     has been called and NOT claimed an interrupt.

		gpe_all	     count of SCI caused by GPEs.

		gpeXX	     count for individual GPE source

		ff_gbl_lock  Global Lock

		ff_pmtimer   PM Timer

		ff_pwr_btn   Power Button

		ff_rt_clk    Real Time Clock

		ff_slp_btn   Sleep Button

		error	     an interrupt that can't be accounted for above.

		invalid      it's either a GPE or a Fixed Event that
			     doesn't have an event handler.

		disable	     the GPE/Fixed Event is valid but disabled.

		enable       the GPE/Fixed Event is valid and enabled.
		===========  ==================================================

		Root has permission to clear any of these counters.  Eg.::

		  # echo 0 > gpe11

		All counters can be cleared by clearing the total "sci"::

		  # echo 0 > sci

		None of these counters has an effect on the function
		of the system, they are simply statistics.

		Besides this, user can also write specific strings to these files
		to enable/disable/clear ACPI interrupts in user space, which can be
		used to debug some ACPI interrupt storm issues.

		Note that only writing to VALID GPE/Fixed Event is allowed,
		i.e. user can only change the status of runtime GPE and
		Fixed Event with event handler installed.

		Let's take power button fixed event for example, please kill acpid
		and other user space applications so that the machine won't shutdown
		when pressing the power button::

		  # cat ff_pwr_btn
		  0	enabled
		  # press the power button for 3 times;
		  # cat ff_pwr_btn
		  3	enabled
		  # echo disable > ff_pwr_btn
		  # cat ff_pwr_btn
		  3	disabled
		  # press the power button for 3 times;
		  # cat ff_pwr_btn
		  3	disabled
		  # echo enable > ff_pwr_btn
		  # cat ff_pwr_btn
		  4	enabled
		  /*
		   * this is because the status bit is set even if the enable
		   * bit is cleared, and it triggers an ACPI fixed event when
		   * the enable bit is set again
		   */
		  # press the power button for 3 times;
		  # cat ff_pwr_btn
		  7	enabled
		  # echo disable > ff_pwr_btn
		  # press the power button for 3 times;
		  # echo clear > ff_pwr_btn	/* clear the status bit */
		  # echo disable > ff_pwr_btn
		  # cat ff_pwr_btn
		  7	enabled