ARM: 8428/1: kgdb: Fix registers on sleeping tasks

ARM: 8428/1: kgdb: Fix registers on sleeping tasks

Dumping registers from other sleeping tasks in KGDB was totally
failing for me.  All registers were reported as 0 in many cases.

The code was using task_pt_regs(task) to try to get other thread
registers.  This doesn't appear to be the right place to look.  From
my tests, I saw non-zero values in this structure when we were looking
at a kernel thread that had a userspace task associated with it, but
it contained the register values from the userspace task.  So even in
the cases where registers weren't reported as 0 we were still not
showing the right thing.

Instead of using task_pt_regs(task) let's use task_thread_info(task).
This is the same place that is referred to when doing a dump of all
sleeping task stacks (kdb_show_stack() -> show_stack() ->
dump_backtrace() -> unwind_backtrace() -> thread_saved_sp()).

As further evidence that this is the right thing to do, you can find
the following comment in "gdbstub.c" right before it calls
sleeping_thread_to_gdb_regs():
  Pull stuff saved during switch_to; nothing else is accessible (or
  even particularly relevant).  This should be enough for a stack
  trace.
...and if you look at switch_to() it only saves r4-r11, sp and lr.
Those are the same registers that I'm getting out of the
task_thread_info().

With this change you can use "info thread" to see all tasks in the
kernel and you can switch to other tasks and examine them in gdb.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Tested-by: Stephen Boyd <sboyd@codeurora.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>