ata: Add documentation for hard disk shock protection interface (v3)

Put some information (and pointers to more) into the kernel's doc tree,
describing briefly the interface to the kernel's disk head unloading
facility. Information about how to set up a complete shock protection
system under GNU/Linux can be found on the web and is referenced
accordingly.

v3:
Here is some final polish including various spelling corrections
pointed out by Grant Grundler and Peter Moulder. Also, I have added some
information about the timing constraints related to disk head parking.
The patch looks more impressive than it really is and I think it would
be alright just to incorporate it into the original patch so as not to
clutter up the git log.
Signed-off-by: NElias Oltmanns <eo@nebensachen.de>
Cc: Jeff Garzik <jeff@garzik.org>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Tejun Heo <htejun@gmail.com>
Signed-off-by: NBartlomiej Zolnierkiewicz <bzolnier@gmail.com>

Documentation/laptops/disk-shock-protection.txt

+Hard disk shock protection
+==========================
+
+Author: Elias Oltmanns <eo@nebensachen.de>
+Last modified: 2008-10-03
+
+
+0. Contents
+-----------
+
+1. Intro
+2. The interface
+3. References
+4. CREDITS
+
+
+1. Intro
+--------
+
+ATA/ATAPI-7 specifies the IDLE IMMEDIATE command with unload feature.
+Issuing this command should cause the drive to switch to idle mode and
+unload disk heads. This feature is being used in modern laptops in
+conjunction with accelerometers and appropriate software to implement
+a shock protection facility. The idea is to stop all I/O operations on
+the internal hard drive and park its heads on the ramp when critical
+situations are anticipated. The desire to have such a feature
+available on GNU/Linux systems has been the original motivation to
+implement a generic disk head parking interface in the Linux kernel.
+Please note, however, that other components have to be set up on your
+system in order to get disk shock protection working (see
+section 3. References below for pointers to more information about
+that).
+
+
+2. The interface
+----------------
+
+For each ATA device, the kernel exports the file
+block/*/device/unload_heads in sysfs (here assumed to be mounted under
+/sys). Access to /sys/block/*/device/unload_heads is denied with
+-EOPNOTSUPP if the device does not support the unload feature.
+Otherwise, 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. When the timeout expires and
+no further disk head park request has been issued in the meantime,
+normal operation will be resumed. The maximal value accepted for a
+timeout is 30000 milliseconds. Exceeding this limit will return
+-EOVERFLOW, but heads will be parked anyway and the timeout will be
+set to 30 seconds. However, you can always change a timeout to any
+value between 0 and 30000 by issuing a subsequent head park request
+before the timeout of the previous one has expired. In particular, the
+total timeout can exceed 30 seconds and, more importantly, you can
+cancel a previously set timeout and resume normal operation
+immediately by specifying a timeout of 0. Values below -2 are rejected
+with -EINVAL (see below for the special meaning of -1 and -2). If the
+timeout specified for a recent head park request has not yet expired,
+reading from /sys/block/*/device/unload_heads will report the number
+of milliseconds remaining until normal operation will be resumed;
+otherwise, reading the unload_heads attribute will return 0.
+
+For example, do the following in order to park the heads of drive
+/dev/sda and stop all I/O operations for five seconds:
+
+# echo 5000 > /sys/block/sda/device/unload_heads
+
+A simple
+
+# cat /sys/block/sda/device/unload_heads
+
+will show you how many milliseconds are left before normal operation
+will be resumed.
+
+A word of caution: The fact that the interface operates on a basis of
+milliseconds may raise expectations that cannot be satisfied in
+reality. In fact, the ATA specs clearly state that the time for an
+unload operation to complete is vendor specific. The hint in ATA-7
+that this will typically be within 500 milliseconds apparently has
+been dropped in ATA-8.
+
+There is a technical detail of this implementation that may cause some
+confusion and should be discussed here. When a head park request has
+been issued to a device successfully, all I/O operations on the
+controller port this device is attached to will be deferred. That is
+to say, any other device that may be connected to the same port will
+be affected too. The only exception is that a subsequent head unload
+request to that other device will be executed immediately. Further
+operations on that port will be deferred until the timeout specified
+for either device on the port has expired. As far as PATA (old style
+IDE) configurations are concerned, there can only be two devices
+attached to any single port. In SATA world we have port multipliers
+which means that a user-issued head parking request to one device may
+actually result in stopping I/O to a whole bunch of devices. However,
+since this feature is supposed to be used on laptops and does not seem
+to be very useful in any other environment, there will be mostly one
+device per port. Even if the CD/DVD writer happens to be connected to
+the same port as the hard drive, it generally *should* recover just
+fine from the occasional buffer under-run incurred by a head park
+request to the HD. Actually, when you are using an ide driver rather
+than its libata counterpart (i.e. your disk is called /dev/hda
+instead of /dev/sda), then parking the heads of one drive (drive X)
+will generally not affect the mode of operation of another drive
+(drive Y) on the same port as described above. It is only when a port
+reset is required to recover from an exception on drive Y that further
+I/O operations on that drive (and the reset itself) will be delayed
+until drive X is no longer in the parked state.
+
+Finally, there are some hard drives that only comply with an earlier
+version of the ATA standard than ATA-7, but do support the unload
+feature nonetheless. Unfortunately, there is no safe way Linux can
+detect these devices, so you won't be able to write to the
+unload_heads attribute. If you know that your device really does
+support the unload feature (for instance, because the vendor of your
+laptop or the hard drive itself told you so), then you can tell the
+kernel to enable the usage of this feature for that drive by writing
+the special value -1 to the unload_heads attribute:
+
+# echo -1 > /sys/block/sda/device/unload_heads
+
+will enable the feature for /dev/sda, and giving -2 instead of -1 will
+disable it again.
+
+
+3. References
+-------------
+
+There are several laptops from different vendors featuring shock
+protection capabilities. As manufacturers have refused to support open
+source development of the required software components so far, Linux
+support for shock protection varies considerably between different
+hardware implementations. Ideally, this section should contain a list
+of pointers at different projects aiming at an implementation of shock
+protection on different systems. Unfortunately, I only know of a
+single project which, although still considered experimental, is fit
+for use. Please feel free to add projects that have been the victims
+of my ignorance.
+
+- http://www.thinkwiki.org/wiki/HDAPS
+  See this page for information about Linux support of the hard disk
+  active protection system as implemented in IBM/Lenovo Thinkpads.
+
+
+4. CREDITS
+----------
+
+This implementation of disk head parking has been inspired by a patch
+originally published by Jon Escombe <lists@dresco.co.uk>. My efforts
+to develop an implementation of this feature that is fit to be merged
+into mainline have been aided by various kernel developers, in
+particular by Tejun Heo and Bartlomiej Zolnierkiewicz.