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    SubjectRe: [PATCH 4/4 v2] Add documentation for hard disk shock protection interface
    I found one typo here...can be fixed up by hand I think.

    On Wed, Sep 17, 2008 at 9:40 AM, Elias Oltmanns <> wrote:
    > 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.
    > Signed-off-by: Elias Oltmanns <>
    > ---
    > Documentation/laptops/disk-shock-protection.txt | 144 +++++++++++++++++++++++
    > 1 files changed, 144 insertions(+), 0 deletions(-)
    > create mode 100644 Documentation/laptops/disk-shock-protection.txt
    > diff --git a/Documentation/laptops/disk-shock-protection.txt b/Documentation/laptops/disk-shock-protection.txt
    > new file mode 100644
    > index 0000000..1f93462
    > --- /dev/null
    > +++ b/Documentation/laptops/disk-shock-protection.txt
    > @@ -0,0 +1,144 @@
    > +Hard disk shock protection
    > +==========================
    > +
    > +Author: Elias Oltmanns <>
    > +Last modified: 2008-09-16
    > +
    > +
    > +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).

    Unfortunate place for a line break: "section 3" should be on one line if
    possible. Otherwise the "3. References" looks like section header with
    mangled white space (at first glance).

    > +
    > +
    > +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 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.
    > +
    > +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 devvice 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. Hwoever,

    Typo: 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 it's libata counterpart (i.e. your disk is called /dev/hda
    > +instead of /dev/sda), then parking the heads of drive A will generally
    > +not affect the mode of operation of drive B on the same port as
    > +described above. It is only when a port reset is required to recover
    > +from an exception on drive B that further I/O operations on that drive
    > +(and the reset itself) will be delayed until drive A 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 systeems. 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.
    > +
    > +-
    > + See this page for information about Linux support of the hard disk
    > + active protection system as implemented in IBM/Lenovo Thinkpads.
    > + (FIXME: The information there will have to be updated once this
    > + patch has been approved or the user interface has been agreed upon
    > + at least.)

    Does this still belong here?

    > +
    > +
    > +4. CREDITS
    > +----------
    > +
    > +This implementation of disk head parking has been inspired by a patch
    > +originally published by Jon Escombe <>. 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.

    otherwise looks pretty good - thanks!

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