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In the multi console driver, allowing to register the same console more
than once may result in an infinte loop when putc is called.

If, for example, a boot message is trying to be printed, but the
consoles in the loop in the linked list are runtime consoles, putc will
iterate forever looking for a console that can print boot messages (or
a NULL pointer that will never come).

This loop in the linked list can occur after restoring the system from a
system suspend. The boot console is registered during the cold boot in
BL31, but the runtime console is registered even in the warm boot path.
Consoles are always added to the start of the linked list when they are
registered, so this it what should happen if they were actually
different structures:

   console_list -> NULL
   console_list -> BOOT -> NULL
   console_list -> RUNTIME -> BOOT -> NULL
   console_list -> RUNTIME -> RUNTIME -> BOOT -> NULL

In practice, the two runtime consoles are the same one, so they create
this loop:

   console_list -> RUNTIME -.    X -> BOOT -> NULL
                       ^    |
                       `----'

This patch adds an assertion to detect this problem. The assertion will
fail whenever the same structure tries to be registered while being on
the list.

In order to assert this, console_is_registered() has been implemented.
It returns 1 if the specified console is registered, 0 if not.

Change-Id: I922485e743775ca9bd1af9cbd491ddd360526a6d
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>

This patch overhauls the console API to allow for multiple console
instances of different drivers that are active at the same time. Instead
of binding to well-known function names (like console_core_init),
consoles now provide a register function (e.g. console_16550_register())
that will hook them into the list of active consoles. All console
operations will be dispatched to all consoles currently in the list.

The new API will be selected by the build-time option MULTI_CONSOLE_API,
which defaults to ${ERROR_DEPRECATED} for now. The old console API code
will be retained to stay backwards-compatible to older platforms, but
should no longer be used for any newly added platforms and can hopefully
be removed at some point in the future.

The new console API is intended to be used for both normal (bootup) and
crash use cases, freeing platforms of the need to set up the crash
console separately. Consoles can be individually configured to be active
active at boot (until first handoff to EL2), at runtime (after first
handoff to EL2), and/or after a crash. Console drivers should set a sane
default upon registration that can be overridden with the
console_set_scope() call. Code to hook up the crash reporting mechanism
to this framework will be added with a later patch.

This patch only affects AArch64, but the new API could easily be ported
to AArch32 as well if desired.

Change-Id: I35c5aa2cb3f719cfddd15565eb13c7cde4162549
Signed-off-by: Julius Werner <jwerner@chromium.org>