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USB: add power/persist device attribute

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This patch (as920) adds an extra level of protection to the
USB-Persist facility.  Now it will apply by default only to hubs; for
all other devices the user must enable it explicitly by setting the
power/persist device attribute.

The disconnect_all_children() routine in hub.c has been removed and
its code placed inline.  This is the way it was originally as part of
hub_pre_reset(); the revised usage in hub_reset_resume() is
sufficiently different that the code can no longer be shared.
Likewise, mark_children_for_reset() is now inline as part of
hub_reset_resume().  The end result looks much cleaner than before.

The sysfs interface is updated to add the new attribute file, and
there are corresponding documentation updates.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
This commit is contained in:
Alan Stern
2007-05-30 15:39:33 -04:00
committed by Greg Kroah-Hartman
parent 54515fe528
commit b41a60eca8
6 changed files with 149 additions and 69 deletions
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38
Documentation/usb/persist.txt
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@ -2,7 +2,7 @@
Alan Stern <stern@rowland.harvard.edu>
September 2, 2006 (Updated March 27, 2007)
September 2, 2006 (Updated May 29, 2007)
What is the problem?
@ -52,9 +52,9 @@ you can convince the BIOS supplier to fix the problem (lots of luck!).
On many systems the USB host controllers will get reset after a
suspend-to-RAM. On almost all systems, no suspend current is
available during suspend-to-disk (also known as swsusp). You can
check the kernel log after resuming to see if either of these has
happened; look for lines saying "root hub lost power or was reset".
available during hibernation (also known as swsusp or suspend-to-disk).
You can check the kernel log after resuming to see if either of these
has happened; look for lines saying "root hub lost power or was reset".
In practice, people are forced to unmount any filesystems on a USB
device before suspending. If the root filesystem is on a USB device,
@ -71,15 +71,16 @@ structures are allowed to persist across a power-session disruption.
It works like this. If the kernel sees that a USB host controller is
not in the expected state during resume (i.e., if the controller was
reset or otherwise had lost power) then it applies a persistence check
to each of the USB devices below that controller. It doesn't try to
resume the device; that can't work once the power session is gone.
Instead it issues a USB port reset and then re-enumerates the device.
(This is exactly the same thing that happens whenever a USB device is
reset.) If the re-enumeration shows that the device now attached to
that port has the same descriptors as before, including the Vendor and
Product IDs, then the kernel continues to use the same device
structure. In effect, the kernel treats the device as though it had
merely been reset instead of unplugged.
to each of the USB devices below that controller for which the
"persist" attribute is set. It doesn't try to resume the device; that
can't work once the power session is gone. Instead it issues a USB
port reset and then re-enumerates the device. (This is exactly the
same thing that happens whenever a USB device is reset.) If the
re-enumeration shows that the device now attached to that port has the
same descriptors as before, including the Vendor and Product IDs, then
the kernel continues to use the same device structure. In effect, the
kernel treats the device as though it had merely been reset instead of
unplugged.
If no device is now attached to the port, or if the descriptors are
different from what the kernel remembers, then the treatment is what
@ -91,6 +92,17 @@ The end result is that the USB device remains available and usable.
Filesystem mounts and memory mappings are unaffected, and the world is
now a good and happy place.
Note that even when CONFIG_USB_PERSIST is set, the "persist" feature
will be applied only to those devices for which it is enabled. You
can enable the feature by doing (as root):
echo 1 >/sys/bus/usb/devices/.../power/persist
where the "..." should be filled in the with the device's ID. Disable
the feature by writing 0 instead of 1. For hubs the feature is
automatically and permanently enabled, so you only have to worry about
setting it for devices where it really matters.
Is this the best solution?