[PATCH v2] clocksource: arch_timer: Allow the device tree to specify the physical timer

Hi,

On Thu, Sep 11, 2014 at 10:00 AM, Marc Zyngier [off-list ref] wrote:

On 11/09/14 17:47, Will Deacon wrote:

quoted

On Thu, Sep 11, 2014 at 05:16:44PM +0100, Doug Anderson wrote:

quoted

Some 32-bit (ARMv7) systems are architected like this:

* The firmware doesn't know and doesn't care about hypervisor mode and
  we don't want to add the complexity of hypervisor there.

* The firmware isn't involved in SMP bringup or resume.

* The ARCH timer come up with an uninitialized offset between the
  virtual and physical counters.  Each core gets a different random
  offset.

On systems like the above, it doesn't make sense to use the virtual
counter.  There's nobody managing the offset and each time a core goes
down and comes back up it will get reinitialized to some other random
value.

You probably need to rephrase this slightly, as there *is* still a
requirement on the hypervisor/firmware (actually, two!). See below.

quoted

Let's add a property to the device tree to say that we shouldn't use
the virtual timer.  Firmware could potentially remove this property
before passing the device tree to the kernel if it really wants the
kernel to use a virtual timer.

Note that it's been said that ARM64 (ARMv8) systems the firmware and
kernel really can't be architected as described above.  That means
using the physical timer like this really only makes sense for ARMv7
systems.

I'd go further: this only makes sense if you're booting in secure SVC
mode.

If that's the case, what's the problem? Enter monitor mode, set SCR.NS
to one, nuke CNTVOFF, revert, job done.

What am I missing?

Stuff like this was talked about in the thread about Sonny's patch at
<https://patchwork.kernel.org/patch/4790921/>

...in that case we were always talking about HYP mode, though.  I
don't think anyone has explicitly talked about just switching to
monitor mode and then leaving ourselves in Secure SVC after we're
done.  It would be nice (especially for the VDSO guys) if we could
just init the virtual offset...

We would need to run this code potentially at processor bringup and
after suspend/resume, but that seems possible too.

Is the transition to monitor mode and back simple?  Where would you
suggest putting this code?  It would definitely need to be pretty
early.  We'd also need to be able to detect that we're in Secure SVC
and not mess up anyone else who happened to boot in Non Secure SVC.

-Doug