linux-kernel - Re: [PATCH/RFC v2 1/4] ARM: hw_breakpoint: Add arm_dbg_regs

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Date:	Wed, 1 Oct 2014 22:26:25 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Mathieu Poirier <mathieu.poirier@...aro.org>
Cc:	Will Deacon <will.deacon@....com>,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	Russell King <linux@....linux.org.uk>,
	Simon Horman <horms@...ge.net.au>,
	Magnus Damm <magnus.damm@...il.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-sh@...r.kernel.org" <linux-sh@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH/RFC v2 1/4] ARM: hw_breakpoint: Add arm_dbg_regs_available flag

Hi Mathieu,

On Wed, Oct 1, 2014 at 4:38 PM, Mathieu Poirier
<mathieu.poirier@...aro.org> wrote:
> On 1 October 2014 01:41, Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
>> On Tue, Sep 30, 2014 at 6:03 PM, Will Deacon <will.deacon@....com> wrote:
>>> On Tue, Sep 30, 2014 at 01:26:24PM +0100, Geert Uytterhoeven wrote:
>>>> If power area D4, which contains the Coresight-ETM hardware block, is
>>>> powered down on R-Mobile A1 (r8a7740), the kernel crashes when
>>>> suspending from s2ram with:
>>>>
>>>>     Internal error: Oops - undefined instruction: 0 [#1] ARM
>>>>
>>>> This happens because dbg_cpu_pm_notify() calls reset_ctrl_regs(), which
>>>> can't access the debug registers as the debug module is powered down.
>>>>
>>>> As suggested by Russell King, track whether the ETM block is powered down
>>>> to fix this.
>>>>
>>>> The availability of the debug registers depends on the platform and its
>>>> state.  Hence provide a mechanism for platform code to indicate that the
>>>> debug registers are available or not, using a boolean flag that defaults
>>>> to true.
>
> I'm afraid the usage of the handle "arm,coresight-etm3x" is arbitrary
> here as what the code isn't related to an embedded trace module, but
> rather to the power domain that trace module happens to be in.  If we
> are to take that solution a handle name relate to the debug power
> domain is likely to be more appropriate.

W.r.t. PM domains, the datasheet only mentions "Coresight-ETM" being
in the D4 power area. That's why I used the "arm,coresight-etm3x" handle.
It's still to be discovered what else is hidden there...

>>> Whilst I guess this solves your problem, it doesn't feel like a scalable fix
>>> for something that can/will assumedly happen elsewhere in an SoC (e.g. PMU
>>> registers in perf). I'd much rather have a generic abstraction for power
>>> domains, which subsystems such as hw_breakpoint can attempt to take a
>>> reference on when they want to access registers in that domain.
>>
>> I agree this is a bit hackish, and not a long-term solution.
>>
>> As soon as the ARM debug/perf subsystem starts using devices instantiated
>> from DT, implementing PM runtime support, this will work out-of-the-box.
>> Until then, we cannot have proper support for the D4 PM domain on R-Mobile
>> SoCs, without hacks like this (or like never powering down the D4 PM domain
>> --- perhaps that's the way to go).
>>
>> I believe Mathieu is working on the former, but so far without converting
>> hw_breakpoint.c nor adding PM runtime support?
>
> Humm... At this time the coresight framework doesn't deal with PM
> runtime support.  It is something that's been on the list of things to
> do for a while now but I never had the time to get to it.  On the flip
> side I currently have two boards on my desk that need to interact with
> different debug domains to work properly and as such, the feature is
> bound to appear at some point.

OK. We'll see...

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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