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authorAlan Stern <stern@rowland.harvard.edu>2008-03-03 15:15:59 -0500
committerGreg Kroah-Hartman <gregkh@suse.de>2008-04-25 00:16:32 -0400
commitfeccc30d90155bcbc937f87643182a43d25873eb (patch)
tree96394e24075a885f1a8bb3e53203f8397e78ea46 /Documentation/usb/persist.txt
parent5e6effaed6da94e727cd45f945ad2489af8570b3 (diff)
USB: remove CONFIG_USB_PERSIST setting
This patch (as1047) removes the USB_PERSIST Kconfig option, enabling it permanently. It also prevents the power/persist attribute from being created for hub devices; there's no point in having it since USB-PERSIST is always turned on for hubs. Signed-off-by: Alan Stern <stern@rowland.harvard.edu> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Diffstat (limited to 'Documentation/usb/persist.txt')
-rw-r--r--Documentation/usb/persist.txt35
1 files changed, 19 insertions, 16 deletions
diff --git a/Documentation/usb/persist.txt b/Documentation/usb/persist.txt
index df54d645cbb5..bea58dbd30fe 100644
--- a/Documentation/usb/persist.txt
+++ b/Documentation/usb/persist.txt
@@ -2,7 +2,7 @@
2 2
3 Alan Stern <stern@rowland.harvard.edu> 3 Alan Stern <stern@rowland.harvard.edu>
4 4
5 September 2, 2006 (Updated May 29, 2007) 5 September 2, 2006 (Updated February 25, 2008)
6 6
7 7
8 What is the problem? 8 What is the problem?
@@ -65,9 +65,10 @@ much better.)
65 65
66 What is the solution? 66 What is the solution?
67 67
68Setting CONFIG_USB_PERSIST will cause the kernel to work around these 68The kernel includes a feature called USB-persist. It tries to work
69issues. It enables a mode in which the core USB device data 69around these issues by allowing the core USB device data structures to
70structures are allowed to persist across a power-session disruption. 70persist across a power-session disruption.
71
71It works like this. If the kernel sees that a USB host controller is 72It works like this. If the kernel sees that a USB host controller is
72not in the expected state during resume (i.e., if the controller was 73not in the expected state during resume (i.e., if the controller was
73reset or otherwise had lost power) then it applies a persistence check 74reset or otherwise had lost power) then it applies a persistence check
@@ -80,28 +81,30 @@ re-enumeration shows that the device now attached to that port has the
80same descriptors as before, including the Vendor and Product IDs, then 81same descriptors as before, including the Vendor and Product IDs, then
81the kernel continues to use the same device structure. In effect, the 82the kernel continues to use the same device structure. In effect, the
82kernel treats the device as though it had merely been reset instead of 83kernel treats the device as though it had merely been reset instead of
83unplugged. 84unplugged. The same thing happens if the host controller is in the
85expected state but a USB device was unplugged and then replugged.
84 86
85If no device is now attached to the port, or if the descriptors are 87If no device is now attached to the port, or if the descriptors are
86different from what the kernel remembers, then the treatment is what 88different from what the kernel remembers, then the treatment is what
87you would expect. The kernel destroys the old device structure and 89you would expect. The kernel destroys the old device structure and
88behaves as though the old device had been unplugged and a new device 90behaves as though the old device had been unplugged and a new device
89plugged in, just as it would without the CONFIG_USB_PERSIST option. 91plugged in.
90 92
91The end result is that the USB device remains available and usable. 93The end result is that the USB device remains available and usable.
92Filesystem mounts and memory mappings are unaffected, and the world is 94Filesystem mounts and memory mappings are unaffected, and the world is
93now a good and happy place. 95now a good and happy place.
94 96
95Note that even when CONFIG_USB_PERSIST is set, the "persist" feature 97Note that the "USB-persist" feature will be applied only to those
96will be applied only to those devices for which it is enabled. You 98devices for which it is enabled. You can enable the feature by doing
97can enable the feature by doing (as root): 99(as root):
98 100
99 echo 1 >/sys/bus/usb/devices/.../power/persist 101 echo 1 >/sys/bus/usb/devices/.../power/persist
100 102
101where the "..." should be filled in the with the device's ID. Disable 103where the "..." should be filled in the with the device's ID. Disable
102the feature by writing 0 instead of 1. For hubs the feature is 104the feature by writing 0 instead of 1. For hubs the feature is
103automatically and permanently enabled, so you only have to worry about 105automatically and permanently enabled and the power/persist file
104setting it for devices where it really matters. 106doesn't even exist, so you only have to worry about setting it for
107devices where it really matters.
105 108
106 109
107 Is this the best solution? 110 Is this the best solution?
@@ -112,19 +115,19 @@ centralized Logical Volume Manager. Such a solution would allow you
112to plug in a USB flash device, create a persistent volume associated 115to plug in a USB flash device, create a persistent volume associated
113with it, unplug the flash device, plug it back in later, and still 116with it, unplug the flash device, plug it back in later, and still
114have the same persistent volume associated with the device. As such 117have the same persistent volume associated with the device. As such
115it would be more far-reaching than CONFIG_USB_PERSIST. 118it would be more far-reaching than USB-persist.
116 119
117On the other hand, writing a persistent volume manager would be a big 120On the other hand, writing a persistent volume manager would be a big
118job and using it would require significant input from the user. This 121job and using it would require significant input from the user. This
119solution is much quicker and easier -- and it exists now, a giant 122solution is much quicker and easier -- and it exists now, a giant
120point in its favor! 123point in its favor!
121 124
122Furthermore, the USB_PERSIST option applies to _all_ USB devices, not 125Furthermore, the USB-persist feature applies to _all_ USB devices, not
123just mass-storage devices. It might turn out to be equally useful for 126just mass-storage devices. It might turn out to be equally useful for
124other device types, such as network interfaces. 127other device types, such as network interfaces.
125 128
126 129
127 WARNING: Using CONFIG_USB_PERSIST can be dangerous!! 130 WARNING: USB-persist can be dangerous!!
128 131
129When recovering an interrupted power session the kernel does its best 132When recovering an interrupted power session the kernel does its best
130to make sure the USB device hasn't been changed; that is, the same 133to make sure the USB device hasn't been changed; that is, the same
@@ -152,5 +155,5 @@ but yourself.
152YOU HAVE BEEN WARNED! USE AT YOUR OWN RISK! 155YOU HAVE BEEN WARNED! USE AT YOUR OWN RISK!
153 156
154That having been said, most of the time there shouldn't be any trouble 157That having been said, most of the time there shouldn't be any trouble
155at all. The "persist" feature can be extremely useful. Make the most 158at all. The USB-persist feature can be extremely useful. Make the
156of it. 159most of it.