linux-kernel - Re: [PATCH v5 6/9] coresight: add support for CPU debug module

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Date:   Thu, 30 Mar 2017 09:18:39 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     Mike Leach <mike.leach@...aro.org>
Cc:     Suzuki K Poulose <Suzuki.Poulose@....com>,
        Jonathan Corbet <corbet@....net>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Wei Xu <xuwei5@...ilicon.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Andy Gross <andy.gross@...aro.org>,
        David Brown <david.brown@...aro.org>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...eaurora.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Guodong Xu <guodong.xu@...aro.org>,
        John Stultz <john.stultz@...aro.org>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-arm-msm@...r.kernel.org, linux-soc@...r.kernel.org,
        linux-clk@...r.kernel.org, Sudeep Holla <sudeep.holla@....com>
Subject: Re: [PATCH v5 6/9] coresight: add support for CPU debug module

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