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

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Message-ID: <50D305A6.2080904@arm.com>
Date:	Thu, 20 Dec 2012 12:33:42 +0000
From:	Marc Zyngier <marc.zyngier@....com>
To:	Peter De Schrijver <pdeschrijver@...dia.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

On 20/12/12 12:22, Peter De Schrijver wrote:
>>>>> +
>>>>> +	/* 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?

The problem is not so much the guest but the host. The host has to be
booted in non-secure, so just saying "we do not support non-secure" is
not a very convincing argument.

Unless of course you've already decided that you don't want to support
KVM on this SoC...

	M.
-- 
Jazz is not dead. It just smells funny...

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