linux-kernel - Re: [PATCH] ARM: hw_breakpoint: Clear breakpoints before enabling monitor mode

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Date:	Thu, 20 Sep 2012 18:35:56 +0100
From:	Will Deacon <will.deacon@....com>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] ARM: hw_breakpoint: Clear breakpoints before enabling
 monitor mode

Hi Stephen,

On Thu, Sep 20, 2012 at 05:57:40PM +0100, Stephen Boyd wrote:
> The reset value of the BCR, BVR, WCR, and WVR registers are all
> UNKNOWN on ARMv7. Unfortunately, reset_ctrl_regs() clears these
> registers *after* enabling monitor mode, not before, and so some
> implementations may experience UNPREDICTABLE behavior if the
> reset values of these registers are non-zero. Clear the
> breakpoints before enabling monitor mode so that we don't
> experience boot hangs/loops due to breakpoints being enabled
> out of reset.
> 
> Signed-off-by: Stephen Boyd <sboyd@...eaurora.org>

Aha, thanks for the patch. We should definitely zero these registers before
enabling monitor mode. However...

> +/* Determine if halting mode is enabled */
> +static int halting_mode_enabled(void)
> +{
> +	u32 dscr;
> +
> +	ARM_DBG_READ(c1, 0, dscr);
> +
> +	if (WARN_ONCE(dscr & ARM_DSCR_HDBGEN,
> +		"halting debug mode enabled. Unable to access hardware resources.\n")) {
> +		return -EPERM;
> +	}
> +	return 0;
> +}

...it looks like debug arch 7.1 defines this bit as UNKNOWN when the OS lock
is clear, so we probably shouldn't be reading it at all. I'll pour myself a
stiff drink and start reading the debug arch docs to work out what on Earth
we should do.

Stay tuned.

Will
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