echi-dbgp: Add kernel debugger support for the usb debug port

This patch adds the capability to use the usb debug port with the
kernel debugger.  It is also still possible to use this functionality
with or without the earlyprintk=dbgpX.  It is possible to use the
kgdbwait boot argument to debug very early in the kernel start up code.

There are two ways to use this driver extension with a kernel boot argument.

1) kgdbdbgp=#   -- Where # is the number of the usb debug controller

   You must use sysrq-g to break into the kernel debugger on another
   connection type other than the dbgp.

2) kgdbdbgp=#debugControlNum#,#Seconds#

   In this mode, the usb debug port is polled every #Seconds# for
   character input.  It is possible to use gdb or press control-c to
   break into the kernel debugger.

From the implementation perspective there are 3 high level changes.

1) Allow variable retries for the the hardware via dbgp_bulk_read().

   The amount of retries for the dbgp_bulk_read() needed to be
   variable instead of fixed.  We do not want to poll at all when the
   kernel is operating in interrupt driven mode.  The polling only
   occurs if the kernel was booted when specifying some number of
   seconds via the kgdbdbgp boot argument (IE kgdbdbgp=0,1).  In this
   case the loop count is reduced to 1 so as introduce the smallest
   amount of latency as possible.

2) Save the bulk IN endpoint address for use by the kgdb code.

3) The addition of the kgdb interface code.

   This consisted of adding in a character read function for the dbgp
   as well as a polling thread to allow the dbgp to interrupt the
   kernel execution.  The rest is the typical kgdb I/O api.

CC: Eric Biederman <ebiederm@xmission.com>
CC: Yinghai Lu <yhlu.kernel@gmail.com>
CC: linux-usb@vger.kernel.org
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 11 insertions, 0 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 4c44d01c67f4..f5fce483930c 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1127,6 +1127,17 @@ and is between 256 and 4096 characters. It is defined in the file
                         use the HighMem zone if it exists, and the Normal
                         zone if it does not.
 
+        kgdbdbgp=       [KGDB,HW] kgdb over EHCI usb debug port.
+                        Format: <Controller#>[,poll interval]
+                        The controller # is the number of the ehci usb debug
+                        port as it is probed via PCI.  The poll interval is
+                        optional and is the number seconds in between
+                        each poll cycle to the debug port in case you need
+                        the functionality for interrupting the kernel with
+                        gdb or control-c on the dbgp connection.  When
+                        not using this parameter you use sysrq-g to break into
+                        the kernel debugger.
+
         kgdboc=         [KGDB,HW] kgdb over consoles.
                         Requires a tty driver that supports console polling,
                         or a supported polling keyboard driver (non-usb).