linux-kernel - Re: [PATCH] clocksource: arch_timer: Fix code to use physical timers when requested

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Message-ID: <20140904174840.GB14822@e102568-lin.cambridge.arm.com>
Date:	Thu, 4 Sep 2014 18:48:40 +0100
From:	Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:	Sonny Rao <sonnyrao@...omium.org>
Cc:	Mark Rutland <Mark.Rutland@....com>,
	Doug Anderson <dianders@...omium.org>,
	Olof Johansson <olof@...om.net>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Russell King <linux@....linux.org.uk>,
	Sudeep Holla <Sudeep.Holla@....com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Will Deacon <Will.Deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] clocksource: arch_timer: Fix code to use physical timers
 when requested

On Thu, Sep 04, 2014 at 06:01:27PM +0100, Sonny Rao wrote:

[...]

> > If an OS is booted at PL2 it can access the physical counters, and
> > should do so in case something like KVM will be used later. The OS can
> > write to CNTVOFF at PL2, and if it sets CNTVOFF to zero the physical and
> > virtual counters are equivalent. Thus it can use the virtual counters
> > and doesn't need to have additional code in several places (including
> > the VDSO) where it needs to choose to read which counters to read.
> >
> > The problem only exists where PL2 exists and the firmware/bootloader
> > skipped PL2 without initialising the necessary PL2 state. This is in
> > general a stupid thing to do; it introduces a problem that need not
> > exist and throws away the option of using the features PL2 provides.
> > This is a firmware/bootloader bug.
> 
> Well it's not quite that simple, this is actually an issue with the
> hardware that the CNTVOFF comes up with different values on different
> cores.  This happens not only at boot, but any time the core is
> powered on, which could include deep sleep or CPU hotplug and suspend
> to ram.  The firmware may not be involved in all these cases, so we
> cannot rely on it to fix this problem.

And why isn't firmware involved in those cases if it _is_ involved in
cold boot ? Resuming from low-power means resuming the machine/core as it
was when it was running before power down, anything that deviates from that
behaviour is a programming bug.

Lorenzo

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