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

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Message-ID: <CANLsYkycNPFafpet+qGv7LhCVN1qj+GfhVuNyW=1WpOc07S7ng@mail.gmail.com>
Date:	Wed, 1 Oct 2014 08:38:58 -0600
From:	Mathieu Poirier <mathieu.poirier@...aro.org>
To:	Geert Uytterhoeven <geert@...ux-m68k.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

On 1 October 2014 01:41, Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> Hi Will,
>
> 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.

>
>> 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.

>
> Thanks for your understanding!
>
> 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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