[Qemu-devel] [RFC 0/5] hw: arm: exynos: Bring up secondary CPU + CPUIDLE issue

[Krzysztof Kozlowski]

Hi,


The first patch fixes GIC and brings the secondary CPU. Unfortunately,
this brings up to light an annoying issue with CPUIDLE driver.


Overview of the problem
=======================
On Exynos4210, by default Linux kernel uses cpuidle driver which tries
to enter low power mode, called AFTR (Arm Off, Top Running).  On real
hardware this brings some power savings.  This AFTR mode requires second
CPU to be off, so the driver (coupled cpuidle driver) when system is idle:
1. Turns off second CPU,
2. Enters AFTR on CPU0.

However the QEMU system is then totally unresponsive (e.g. on serial console)
and RCU stalls appear from time to time.  I spent some time on it and did not
find the real cause behind the lag.  Maybe it is because the second CPU
does not really power down itself and system just burns the cycles under spin
locks?

Looking at recent stable kernels:
 - 3.10, 3.16 - works fine with two CPUs (no CPUIDLE driver),
 - 4.1 and newer - stalls and are unresponsive due to CPUIDLE being enabled.

The cpuidle driver is not relying on DTS. It is just enabled in exynos_defconfig
and works.


Question
========
I am quite new to QEMU.  I do not know the internals (yet), nor the design
how hardware should be emulated in such subtle details.

The questions I have are:
1. What is the preferred way to solve it? Maybe some a QEMU
   workaround to disable the cpuidle is okay?
2. Is it worth implementing a proper secondary CPU power down and 
   at the end proper AFTR cpuidle? It might be quite difficult...
3. How such issues with deep sleep modes were solved for other
   ARM targets?


Best regards,
Krzysztof


Krzysztof Kozlowski (5):
  hw/intc/exynos4210_gic: Fix GIC memory mappings for secondary CPU
  hw/intc/exynos4210_gic: Use more meaningful name for local variable
  hw/timer/exynos4210_mct: Cleanup indentation and empty new lines
  hw/timer/exynos4210_mct: Fix checkpatch style errors
  hw/timer/exynos4210_mct: Remove unused defines

 hw/intc/exynos4210_gic.c  | 36 ++++++++++++++++++++--------------
 hw/timer/exynos4210_mct.c | 50 ++++++++++++++++++++---------------------------
 2 files changed, 42 insertions(+), 44 deletions(-)

-- 
2.9.3