arm: kernel: initialize broadcast hrtimer based clock event device
diff mbox series

Message ID 20191112120625.20173-1-benjamin.gaignard@st.com
State Mainlined
Commit 022eb8ae8b5ee8c5c813923c69b5ebb1e9612c4c
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Series
  • arm: kernel: initialize broadcast hrtimer based clock event device
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Commit Message

Benjamin GAIGNARD Nov. 12, 2019, 12:06 p.m. UTC
On platforms implementing CPU power management, the CPUidle subsystem
can allow CPUs to enter idle states where local timers logic is lost on power
down. To keep the software timers functional the kernel relies on an
always-on broadcast timer to be present in the platform to relay the
interrupt signalling the timer expiries.

For platforms implementing CPU core gating that do not implement an always-on
HW timer or implement it in a broken way, this patch adds code to initialize
the kernel hrtimer based clock event device upon boot (which can be chosen as
tick broadcast device by the kernel).
It relies on a dynamically chosen CPU to be always powered-up. This CPU then
relays the timer interrupt to CPUs in deep-idle states through its HW local
timer device.

Having a CPU always-on has implications on power management platform
capabilities and makes CPUidle suboptimal, since at least a CPU is kept
always in a shallow idle state by the kernel to relay timer interrupts,
but at least leaves the kernel with a functional system with some working
power management capabilities.

The hrtimer based clock event device is unconditionally registered, but
has the lowest possible rating such that any broadcast-capable HW clock
event device present will be chosen in preference as the tick broadcast
device.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
---
Note:
- The same reasons lead to same patch than for arm64 so I have copy the
  commit message from: 9358d755bd5c ("arm64: kernel: initialize broadcast
  hrtimer based clock event device")
 arch/arm/kernel/time.c | 2 ++
 1 file changed, 2 insertions(+)

Comments

Thomas Gleixner Nov. 12, 2019, 2:48 p.m. UTC | #1
On Tue, 12 Nov 2019, Benjamin Gaignard wrote:

> On platforms implementing CPU power management, the CPUidle subsystem
> can allow CPUs to enter idle states where local timers logic is lost on power
> down. To keep the software timers functional the kernel relies on an
> always-on broadcast timer to be present in the platform to relay the
> interrupt signalling the timer expiries.
> 
> For platforms implementing CPU core gating that do not implement an always-on
> HW timer or implement it in a broken way, this patch adds code to initialize
> the kernel hrtimer based clock event device upon boot (which can be chosen as
> tick broadcast device by the kernel).
> It relies on a dynamically chosen CPU to be always powered-up. This CPU then
> relays the timer interrupt to CPUs in deep-idle states through its HW local
> timer device.
> 
> Having a CPU always-on has implications on power management platform
> capabilities and makes CPUidle suboptimal, since at least a CPU is kept
> always in a shallow idle state by the kernel to relay timer interrupts,
> but at least leaves the kernel with a functional system with some working
> power management capabilities.
> 
> The hrtimer based clock event device is unconditionally registered, but
> has the lowest possible rating such that any broadcast-capable HW clock
> event device present will be chosen in preference as the tick broadcast
> device.
> 
> Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Russell King - ARM Linux admin Nov. 12, 2019, 2:55 p.m. UTC | #2
On Tue, Nov 12, 2019 at 03:48:10PM +0100, Thomas Gleixner wrote:
> On Tue, 12 Nov 2019, Benjamin Gaignard wrote:
> 
> > On platforms implementing CPU power management, the CPUidle subsystem
> > can allow CPUs to enter idle states where local timers logic is lost on power
> > down. To keep the software timers functional the kernel relies on an
> > always-on broadcast timer to be present in the platform to relay the
> > interrupt signalling the timer expiries.
> > 
> > For platforms implementing CPU core gating that do not implement an always-on
> > HW timer or implement it in a broken way, this patch adds code to initialize
> > the kernel hrtimer based clock event device upon boot (which can be chosen as
> > tick broadcast device by the kernel).
> > It relies on a dynamically chosen CPU to be always powered-up. This CPU then
> > relays the timer interrupt to CPUs in deep-idle states through its HW local
> > timer device.
> > 
> > Having a CPU always-on has implications on power management platform
> > capabilities and makes CPUidle suboptimal, since at least a CPU is kept
> > always in a shallow idle state by the kernel to relay timer interrupts,
> > but at least leaves the kernel with a functional system with some working
> > power management capabilities.
> > 
> > The hrtimer based clock event device is unconditionally registered, but
> > has the lowest possible rating such that any broadcast-capable HW clock
> > event device present will be chosen in preference as the tick broadcast
> > device.
> > 
> > Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
> 
> Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

Thanks.  Benjamin, please put it in the patch system, thanks.

Patch
diff mbox series

diff --git a/arch/arm/kernel/time.c b/arch/arm/kernel/time.c
index b996b2cf0703..dddc7ebf4db4 100644
--- a/arch/arm/kernel/time.c
+++ b/arch/arm/kernel/time.c
@@ -9,6 +9,7 @@ 
  *  reading the RTC at bootup, etc...
  */
 #include <linux/clk-provider.h>
+#include <linux/clockchips.h>
 #include <linux/clocksource.h>
 #include <linux/errno.h>
 #include <linux/export.h>
@@ -107,5 +108,6 @@  void __init time_init(void)
 		of_clk_init(NULL);
 #endif
 		timer_probe();
+		tick_setup_hrtimer_broadcast();
 	}
 }