[PATCH] USB: correct the USB info in Documentation/power/swsusp.txt

 *

 * (*) suspend/resume support is needed to make it safe.

 *

 * If you have any filesystems on USB devices mounted before suspend,

 * If you have any filesystems on USB devices mounted before software suspend,

 * they won't be accessible after resume and you may lose data, as though

 * you have unplugged the USB devices with mounted filesystems on them

 * (see the FAQ below for details).

 * you have unplugged the USB devices with mounted filesystems on them;

 * see the FAQ below for details.  (This is not true for more traditional

 * power states like "standby", which normally don't turn USB off.)

You need to append resume=/dev/your_swap_partition to kernel command

line. Then you suspend by

distinctions between SUSPEND and FREEZE.

A: Doing SUSPEND when you are asked to do FREEZE is always correct,

but it may be unneccessarily slow. If you want USB to stay simple,

but it may be unneccessarily slow. If you want your driver to stay simple,

slowness may not matter to you. It can always be fixed later.

For devices like disk it does matter, you do not want to spindown for

I suspend to disk, I can lose data unless the filesystem has been mounted

with "sync"?

A: That's right.  It depends on your hardware, and it could be true even for

suspend-to-RAM.  In fact, even with "-o sync" you can lose data if your

programs have information in buffers they haven't written out to disk.

A: That's right ... if you disconnect that device, you may lose data.

In fact, even with "-o sync" you can lose data if your programs have

information in buffers they haven't written out to a disk you disconnect,

or if you disconnect before the device finished saving data you wrote.

If you're lucky, your hardware will support low-power modes for USB

controllers while the system is asleep.  Lots of hardware doesn't,

however.  Shutting off the power to a USB controller is equivalent to

unplugging all the attached devices.

Software suspend normally powers down USB controllers, which is equivalent

to disconnecting all USB devices attached to your system.

Your system might well support low-power modes for its USB controllers

while the system is asleep, maintaining the connection, using true sleep

modes like "suspend-to-RAM" or "standby".  (Don't write "disk" to the

/sys/power/state file; write "standby" or "mem".)  We've not seen any

hardware that can use these modes through software suspend, although in

theory some systems might support "platform" or "firmware" modes that

won't break the USB connections.

Remember that it's always a bad idea to unplug a disk drive containing a

mounted filesystem.  With USB that's true even when your system is asleep!

The safest thing is to unmount all USB-based filesystems before suspending

and remount them after resuming.

mounted filesystem.  That's true even when your system is asleep!  The

safest thing is to unmount all filesystems on removable media (such USB,

Firewire, CompactFlash, MMC, external SATA, or even IDE hotplug bays)

before suspending; then remount them after resuming.