libata: update documentation for sysfs interfaces

Dcoumentation has been added by parsing through git commit history and
reading code. This might be useful for scripting and tracking changes in
the ABI.

I do not have complete descriptions for the following 3 attributes; they
have been annotated with the comment [to be documented] -

/sys/class/scsi_host/hostX/ahci_port_cmd
/sys/class/scsi_host/hostX/ahci_host_caps
/sys/class/scsi_host/hostX/ahci_host_cap2

Signed-off-by: Aishwarya Pant <[e-mail]>
Signed-off-by: Tejun Heo <[e-mail]>

Documentation/ABI/testing/sysfs-block-device

···
+What:		/sys/block/*/device/sw_activity
+Date:		Jun, 2008
+KernelVersion:	v2.6.27
+Contact:	[e-mail]
+Description:
+		(RW) Used by drivers which support software controlled activity
+		LEDs.
+
+		It has the following valid values:
+
+		0	OFF - the LED is not activated on activity
+		1	BLINK_ON - the LED blinks on every 10ms when activity is
+			detected.
+		2	BLINK_OFF - the LED is on when idle, and blinks off
+			every 10ms when activity is detected.
+
+		Note that the user must turn sw_activity OFF it they wish to
+		control the activity LED via the em_message file.
+
+
+What:		/sys/block/*/device/unload_heads
+Date:		Sep, 2008
+KernelVersion:	v2.6.28
+Contact:	[e-mail]
+Description:
+		(RW) Hard disk shock protection
+
+		Writing an integer value to this file will take the heads of the
+		respective drive off the platter and block all I/O operations
+		for the specified number of milliseconds.
+
+		- If the device does not support the unload heads feature,
+		  access is denied with -EOPNOTSUPP.
+		- The maximal value accepted for a timeout is 30000
+		  milliseconds.
+		- A previously set timeout can be cancelled and disk can resume
+		  normal operation immediately by specifying a timeout of 0.
+		- Some hard drives only comply with an earlier version of the
+		  ATA standard, but support the unload feature nonetheless.
+		  There is no safe way Linux can detect these devices, so this
+		  is not enabled by default. If it is known that your device
+		  does support the unload feature, then you can tell the kernel
+		  to enable it by writing -1. It can be disabled again by
+		  writing -2.
+		- Values below -2 are rejected with -EINVAL
+
+		For more information, see
+		Documentation/laptops/disk-shock-protection.txt
+
+
+What:		/sys/block/*/device/ncq_prio_enable
+Date:		Oct, 2016
+KernelVersion:	v4.10
+Contact:	[e-mail]
+Description:
+		(RW) Write to the file to turn on or off the SATA ncq (native
+		command queueing) support. By default this feature is turned
+		off.

Documentation/ABI/testing/sysfs-class-scsi_host

···
 		the direct i/o path to physical devices.  This setting is
 		controller wide, affecting all configured logical drives on the
 		controller.  This file is readable and writable.
+
+What:		/sys/class/scsi_host/hostX/link_power_management_policy
+Date:		Oct, 2007
+KernelVersion:	v2.6.24
+Contact:	[e-mail]
+Description:
+		(RW) This parameter allows the user to read and set the link
+		(interface) power management.
+
+		There are four possible options:
+
+		min_power: Tell the controller to try to make the link use the
+		least possible power when possible. This may sacrifice some
+		performance due to increased latency when coming out of lower
+		power states.
+
+		max_performance: Generally, this means no power management.
+		Tell the controller to have performance be a priority over power
+		management.
+
+		medium_power: Tell the controller to enter a lower power state
+		when possible, but do not enter the lowest power state, thus
+		improving latency over min_power setting.
+
+		med_power_with_dipm: Identical to the existing medium_power
+		setting except that it enables dipm (device initiated power
+		management) on top, which makes it match the Windows IRST (Intel
+		Rapid Storage Technology) driver settings. This setting is also
+		close to min_power, except that:
+		a) It does not use host-initiated slumber mode, but it does
+		allow device-initiated slumber
+		b) It does not enable low power device sleep mode (DevSlp).
+
+What:		/sys/class/scsi_host/hostX/em_message
+What:		/sys/class/scsi_host/hostX/em_message_type
+Date:		Jun, 2008
+KernelVersion:	v2.6.27
+Contact:	[e-mail]
+Description:
+		em_message: (RW) Enclosure management support. For the LED
+		protocol, writes and reads correspond to the LED message format
+		as defined in the AHCI spec.
+
+		The user must turn sw_activity (under /sys/block/*/device/) OFF
+		it they wish to control the activity LED via the em_message
+		file.
+
+		em_message_type: (RO) Displays the current enclosure management
+		protocol that is being used by the driver (for eg. LED, SAF-TE,
+		SES-2, SGPIO etc).
+
+What:		/sys/class/scsi_host/hostX/ahci_port_cmd
+What:		/sys/class/scsi_host/hostX/ahci_host_caps
+What:		/sys/class/scsi_host/hostX/ahci_host_cap2
+Date:		Mar, 2010
+KernelVersion:	v2.6.35
+Contact:	[e-mail]
+Description:
+		[to be documented]
+
+What:		/sys/class/scsi_host/hostX/ahci_host_version
+Date:		Mar, 2010
+KernelVersion:	v2.6.35
+Contact:	[e-mail]
+Description:
+		(RO) Display the version of the AHCI spec implemented by the
+		host.
+
+What:		/sys/class/scsi_host/hostX/em_buffer
+Date:		Apr, 2010
+KernelVersion:	v2.6.35
+Contact:	[e-mail]
+Description:
+		(RW) Allows access to AHCI EM (enclosure management) buffer
+		directly if the host supports EM.
+
+		For eg. the AHCI driver supports SGPIO EM messages but the
+		SATA/AHCI specs do not define the SGPIO message format of the EM
+		buffer. Different hardware(HW) vendors may have different
+		definitions. With the em_buffer attribute, this issue can be
+		solved by allowing HW vendors to provide userland drivers and
+		tools for their SGPIO initiators.
+
+What:		/sys/class/scsi_host/hostX/em_message_supported
+Date:		Oct, 2009
+KernelVersion:	v2.6.39
+Contact:	[e-mail]
+Description:
+		(RO) Displays supported enclosure management message types.