[PATCH v5 6/9] coresight: add support for CPU debug module

[Leo Yan]

On Wed, Mar 29, 2017 at 04:17:19PM +0100, Mike Leach wrote:

[...]

> >> >+out_powered_up:
> >> >+    debug_os_unlock(drvdata);
> >>
> >> Question: Do we need a matching debug_os_lock() once we are done ?
> >
> > I have checked ARM ARMv8, but there have no detailed description for
> > this. I refered coresight-etmv4 code and Mike's pseudo code, ther have
> > no debug_os_lock() related operations.
> >
> > Mike, Mathieu, could you also help confirm this?
> >
> 
> Debug OS lock / unlock allows the power management code running on the
> core to lock out the external debugger while the debug registers are
> saved/restored during a core power event.
> 
> e.g. A sequence such as this might occur in a correctly programmed system....
> 
> debug_os_lock()
> save_debug_regs() // visible from core power domain - incl breakpoints etc
> save_etm_regs()
> ... // other stuff prior to core power down,
> <power_down_core>
> 
> Followed by...
> 
> <power_up_core>
> restore_etm_regs()
> restore_debug_regs() // visible from core power domain - incl breakpoints etc
> debug_os_unlock()
> 
> The value is 1 (locked) if cold resetting into AArch64 - it is
> expected that some system software will set this to 0 as part of the
> boot process.
> The lock prevents write access to the external debug registers so we
> need to clear it to set up the external debug registers we are using.

This description is conflict with upper restoring flows. During
restore_debug_regs(), the os lock is locked so how it can write
external debug register to restore context?

> This suggests that it should be restored as we found it when done.

Thanks,
Leo Yan