On Wed, Mar 15, 2017 at 02:41:59PM -0600, Mathieu Poirier wrote:

On 15 March 2017 at 10:44, Suzuki K Poulose [off-list ref] wrote:

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On 13/03/17 16:56, Mathieu Poirier wrote:

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On Fri, Mar 10, 2017 at 02:29:53PM +0000, Suzuki K Poulose wrote:

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Btw, I don't see any PM calls to make sure the power domain (at least the
debug domain)
is up, which could cause problems with accesses to some of these
registers (leave alone the
ones in CPU power domain), especially the EDPRSR. We could also do
pm_runtime_get on the
CPU's power domain, if the CPU is online, before we access the pcsr.

I thought about PM runtime operations a little while back but wondered if
it is
really a good thing to have them around.  When this code is called the
system
has crashed and as such making PM runtimes call isn't a good idea.

You are right. It is not safe to make such calls when we have crashed.
The other side effect is, if we don't have the debug power domain up,
we could possibly hang the system and prevent other registered notifiers
from running, which doesn't sound good either.

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One thing we could do is _not_ call pm_runtime_put() at the end of the
probe()
operation.  That way we wouldn't have to mess around with PM runtime
operations
on an unstable system.  This, of course, is costly in terms of power
consumption
but the system is under test/debug anyway.

May be control the behavior via kernel command line ? Something like
coresight_debug={on or 1} or
even use the "nohlt" ?

We need to deal with the debug and CPU power domains.

For the former I suggest we do what coresight does and use the
"power-domains" binding[1].  For the CPU power domain we can re-use
the "nohlt" flag.  In the probe function if the "nohlt" cmd line flag
is not set the code bails out.  If it is set pm_runtime_put() is _not_
called and the driver can be used without worries of hanging the
system when the panic handler is invoked.

Am I forgetting something?

I tested this drvier on Hikey and DB410c. For Hikey we must pass
"nohlt" to disable low power states, otherwise the kernel will hang
during initialization. But for DB410c, this driver even can work well
without "nohlt" and I checked the CPUIdle has been really enabled with
below sysfs entries:

root at linaro-developer:~# cat /sys/devices/system/cpu/cpu*/cpuidle/state*/usage
5992
6988
4225
4547
2790
23696
4202
3899

And from my previous experience, I'm quite sure some SoCs can access
the debug module registers with CPUIdle enabled, and it will read
back 0xFFFF_FFFF_FFFF_FFFF when the CPU stays in low power state.
So I prefer we could keep current method to suggest to use "nohlt" in
Kconfig's help description but it's not mandotory to check this in
the code. How about you think for this?

Thanks,
Leo Yan