powerpc/rtas: handle extended delays safely in early boot

powerpc/rtas: handle extended delays safely in early boot

Some code that runs early in boot calls RTAS functions that can return
-2 or 990x statuses, which mean the caller should retry. An example is
pSeries_cmo_feature_init(), which invokes ibm,get-system-parameter but
treats these benign statuses as errors instead of retrying.

pSeries_cmo_feature_init() and similar code should be made to retry
until they succeed or receive a real error, using the usual pattern:

	do {
		rc = rtas_call(token, etc...);
	} while (rtas_busy_delay(rc));

But rtas_busy_delay() will perform a timed sleep on any 990x
status. This isn't safe so early in boot, before the CPU scheduler and
timer subsystem have initialized.

The -2 RTAS status is much more likely to occur during single-threaded
boot than 990x in practice, at least on PowerVM. This is because -2
usually means that RTAS made progress but exhausted its self-imposed
timeslice, while 990x is associated with concurrent requests from the
OS causing internal contention. Regardless, according to the language
in PAPR, the OS should be prepared to handle either type of status at
any time.

Add a fallback path to rtas_busy_delay() to handle this as safely as
possible, performing a small delay on 990x. Include a counter to
detect retry loops that aren't making progress and bail out. Add __ref
to rtas_busy_delay() since it now conditionally calls an __init
function.

This was found by inspection and I'm not aware of any real
failures. However, the implementation of rtas_busy_delay() before
commit 38f7b7067dae ("powerpc/rtas: rtas_busy_delay() improvements")
was not susceptible to this problem, so let's treat this as a
regression.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Fixes: 38f7b7067dae ("powerpc/rtas: rtas_busy_delay() improvements")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230125-b4-powerpc-rtas-queue-v3-1-26929c8cce78@linux.ibm.com