linux-kernel - Re: [PATCH 5/9] clocksource: tegra: Enable ARM arch

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Message-ID: <20121220122246.GA6819@tbergstrom-lnx.Nvidia.com>
Date:	Thu, 20 Dec 2012 14:22:46 +0200
From:	Peter De Schrijver <pdeschrijver@...dia.com>
To:	Marc Zyngier <marc.zyngier@....com>
CC:	Hiroshi Doyu <hdoyu@...dia.com>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"andrew@...n.ch" <andrew@...n.ch>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"jason@...edaemon.net" <jason@...edaemon.net>,
	"johnstul@...ibm.com" <johnstul@...ibm.com>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 5/9] clocksource: tegra: Enable ARM arch_timer with TSC

> >>> +
> >>> +	/* CNTFRQ */
> >>> +	asm("mcr p15, 0, %0, c14, c0, 0\n" : : "r" (freq));
> >>> +	asm("mrc p15, 0, %0, c14, c0, 0\n" : "=r" (val));
> >>> +	BUG_ON(val != freq);
> >>
> >> This is scary. CNTFRQ is only writable from secure mode, and will
> >> explode in any other situation.
> >>
> >> Also, writing to CNTFRQ doesn't change the timer frequency! This is just
> >> a way for secure mode to tell the rest of the world the frequency the
> >> timer is ticking at. Unless you've wired the input clock to be able to
> >> change the frequency?
> > 
> > ATM, our upstream kernel is expected in secure mode. This situation
> > may be changed later, though....
> 
> I appreciate this. But I expect this kernel to be also used on the
> non-secure side if someone tried to run KVM with it. And this would go
> bang right away.
> 

But the guest wouldn't necessarily have this peripheral, or any other Tegra114
peripheral for that matter?

Cheers,

Peter.
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