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

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.

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.

In order for this patch to do anything useful, we also need Sonny's
patch at <https://patchwork.kernel.org/patch/4790921/>

Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Sonny Rao <redacted>
---
 Documentation/devicetree/bindings/arm/arch_timer.txt | 6 ++++++
 drivers/clocksource/arm_arch_timer.c                 | 3 +++
 2 files changed, 9 insertions(+)

diff --git a/Documentation/devicetree/bindings/arm/arch_timer.txt b/Documentation/devicetree/bindings/arm/arch_timer.txt
index 37b2caf..876d32b 100644
--- a/Documentation/devicetree/bindings/arm/arch_timer.txt
+++ b/Documentation/devicetree/bindings/arm/arch_timer.txt

@@ -22,6 +22,12 @@ to deliver its interrupts via SPIs.
 - always-on : a boolean property. If present, the timer is powered through an
   always-on power domain, therefore it never loses context.
 
+** Optional properties:
+
+- arm,use-physical-timer : Don't ever use the virtual timer, just use the
+  physical one.  Not supported for ARM64.
+
+
 Example:
 
 	timer {

diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
index 5163ec1..8ca07a9 100644
--- a/drivers/clocksource/arm_arch_timer.c
+++ b/drivers/clocksource/arm_arch_timer.c

@@ -649,6 +649,9 @@ static void __init arch_timer_init(struct device_node *np)
 		arch_timer_ppi[i] = irq_of_parse_and_map(np, i);
 	arch_timer_detect_rate(NULL, np);
 
+	if (of_property_read_bool(np, "arm,use-physical-timer"))
+		arch_timer_use_virtual = false;
+
 	/*
 	 * If HYP mode is available, we know that the physical timer
 	 * has been configured to be accessible from PL1. Use it, so