Commit 32a3bebc authored by Mauro Carvalho Chehab's avatar Mauro Carvalho Chehab Committed by Jonathan Corbet
Browse files

usb/persist.txt: convert to ReST and add to driver-api book 



This document describe some USB core features. Add it to the
driver-api book.

Signed-off-by: default avatarMauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: default avatarJonathan Corbet <corbet@lwn.net>
parent 76f650f0

Documentation/driver-api/usb/index.rst

+1 −0
Original line number Diff line number Diff line
@@ -12,6 +12,7 @@ Linux USB API 
   dma

   power-management

   hotplug

   persist

   error-codes

   writing_usb_driver

   writing_musb_glue_layer

Documentation/usb/persist.txt→Documentation/driver-api/usb/persist.rst

+13 −9
Original line number Diff line number Diff line 
USB device persistence during system suspend

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~



		   Alan Stern <stern@rowland.harvard.edu>



		September 2, 2006 (Updated February 25, 2008)

:Author: Alan Stern <stern@rowland.harvard.edu>

:Date: September 2, 2006 (Updated February 25, 2008)





What is the problem?

====================



According to the USB specification, when a USB bus is suspended the

bus must continue to supply suspend current (around 1-5 mA).  This
@@ -64,6 +65,7 @@ much better.) 




What is the solution?

=====================



The kernel includes a feature called USB-persist.  It tries to work

around these issues by allowing the core USB device data structures to
@@ -99,7 +101,7 @@ now a good and happy place. 


Note that the "USB-persist" feature will be applied only to those

devices for which it is enabled.  You can enable the feature by doing

(as root):

(as root)::



	echo 1 >/sys/bus/usb/devices/.../power/persist
@@ -111,6 +113,7 @@ devices where it really matters. 




Is this the best solution?

==========================



Perhaps not.  Arguably, keeping track of mounted filesystems and

memory mappings across device disconnects should be handled by a
@@ -131,6 +134,7 @@ other device types, such as network interfaces. 




WARNING: USB-persist can be dangerous!!

=======================================



When recovering an interrupted power session the kernel does its best

to make sure the USB device hasn't been changed; that is, the same

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