On Sat, Jan 26, 2019 at 09:00:03PM +0000, Paul Walmsley wrote:

On Fri, 25 Jan 2019, Tony Lindgren wrote:

quoted

* Russell King [off-list ref] [190125 21:04]:

quoted

Executing loops such as:

	while (1)
		cpu_relax();

with interrupts disabled results in a livelock of the entire system,
as other CPUs are prevented making progress.  This is most noticable
as a failure of crashdump kexec, which stops just after issuing:

	Loading crashdump kernel...

to the system console.  A workaround for this is to use 10 nops in
cpu_relax().

We also use wfe() in while (1) loops to avoid burning cycles in a
tight loop, giving the CPU a hint that we're not doing anything
useful.

Signed-off-by: Russell King <redacted>
---
It's been a while since this was posted, Will's suggestion was to use
10 nops in cpu_relax() last time around.  I still prefer wfe() in
these infinite-not-doing-anything-ever loops.

Works for me:

Tested-by: Tony Lindgren <tony@atomide.com>

There was some concern in the past that WFE, like WFI, might cause the 
core to assert an external signal that might cause the SoC integration to 
place the core into a low-power mode from which it might not be able to 
wake up.  This could happen on OMAP, for example, with WFI.

I don't recall the outcome of those discussions.  Was a conclusion ever 
reached?

First, we use WFE in spinlocks.  If WFE were to place the CPU in a
low power state that it may not be able to wake up from, all our
spinlocks would be unsafe.

Next, in all of the situations in this patch, we're executing an
infinite loop.  If it were to cause the core to go into a low power
mode, surely that's a good thing, rather than the core endlessly
executing NOPs?  The only way out of that is for the core to receive
a reset _anyway_.

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