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

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>