linux-kernel - Re: [PATCH] powerpc/8xx: fix single

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Message-ID: <20160818101647.GA16503@visitor2.iram.es>
Date:	Thu, 18 Aug 2016 12:16:47 +0200
From:	Gabriel Paubert <paubert@...m.es>
To:	Christophe Leroy <christophe.leroy@....fr>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Scott Wood <oss@...error.net>, linuxppc-dev@...ts.ozlabs.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc/8xx: fix single_step debug

On Thu, Aug 18, 2016 at 12:13:21PM +0200, Christophe Leroy wrote:
> 
> 
> Le 18/08/2016 à 11:58, Gabriel Paubert a écrit :
> >On Thu, Aug 18, 2016 at 11:44:20AM +0200, Christophe Leroy wrote:
> >>SPRN_ICR must be read for clearing the internal freeze signal which
> >>is asserted by the single step exception, otherwise the timebase and
> >>decrementer remain freezed
> >
> >Minor nit: s/freezed/frozen/
> >
> >If the timebase and decrementer are frozen even for a few cycles, this
> >probably upsets timekeeping. I consider this a completely stupid design
> >decision, and maybe I'm not alone.
> >
> >    Gabriel
> 
> We could also unset TBF bit (TimeBase Freeze enable) in TBSCR
> register (today it is set in
> arch/powerpc/platforms/8xx/m8xx_setup.c) but then it would impact
> debug done with an external BDM system which expects the decrementer
> and TB frozen when it freezes the execution.

Ok, I believe that systematically setting it is a mistake, but then I'm
always a bit nervous about screwing up timekeeping (it certainly is
always a very bad idea when you are driving telescopes).

    Gabriel