[RFC PATCH 3/3] ARM: vexpress/TC2: Implement MCPM power_down_finish()

[Nicolas Pitre]

On Mon, 30 Sep 2013, Dave Martin wrote:

> This patch implements the power_down_finish() method for TC2, to
> enable the kernel to confirm when CPUs are safely powered down.
> 
> The information required for determining when a CPU is parked
> cannot be obtained from any single place, so a few sources of
> information must be combined:
> 
>   * mcpm_cpu_power_down() must be pending for the CPU, so that we
>     don't get confused by false STANDBYWFI positives arising from
>     CPUidle.  This is detected by waiting for the tc2_pm use count
>     for the target CPU to reach 0.
> 
>   * Either the SPC must report that the CPU has asserted
>     STANDBYWFI, or the TC2 tile's reset control logic must be
>     holding the CPU in reset.
> 
>     Just checking for STANDBYWFI is not sufficient, because this
>     signal is not latched when the the cluster is clamped off and
>     powered down: the relevant status bits just drop to zero.  This
>     means that STANDBYWFI status cannot be used for reliable
>     detection of the last CPU in a cluster reaching WFI.
> 
> This patch is required in order for kexec to work with MCPM on TC2.
> 
> Signed-off-by: Dave Martin <Dave.Martin at arm.com>
> ---
> 
> The mdelay(1) in tc2_pm_power_down_finish() is arbitrary.  The power
> controller can show millisecond response times in the worst case, and
> CPU hotplug is not expected to be performance-critical.
> 
> It may be wise to add a timeout to this function, but that's open to
> discussion.

That would be a good idea.  I'd suggest you reduce the polling loop to 
something like 10 ms and bail out after one second.   We've been 
affected by funny STANDBYWFI 
behaviors before.



Nicolas