soc: apple: mailbox: Add ASC/M3 mailbox driver

This new driver is based on the existing apple-mailbox driver, but
replaces the usage of the mailbox subsystem with directly exported
symbols.

As part of this refactor, this adds support for using the hardware FIFOs
(not supported in mailbox) and implicitly fixes a bunch of bugs caused
by bad interactions with the mailbox subsystem. It also adds runtime-PM
support.

The new config symbol is APPLE_MBOX, while the module name remains
identical ("apple-mailbox"). The configs are mutually exclusive in
Kconfig, to avoid conflicts.

Acked-by: Eric Curtin <ecurtin@redhat.com>
Acked-by: Neal Gompa <neal@gompa.dev>
Acked-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Signed-off-by: Hector Martin <marcan@marcan.st>

drivers/soc/apple/Kconfig

@@ -17,6 +17,20 @@ config APPLE_PMGR_PWRSTATE
 	  controls for SoC devices. This driver manages them through the
 	  generic power domain framework, and also provides reset support.
 
+config APPLE_MBOX
+	tristate "Apple SoC mailboxes"
+	depends on PM
+	depends on ARCH_APPLE || (64BIT && COMPILE_TEST)
+	depends on !APPLE_MAILBOX
+	default ARCH_APPLE
+	help
+	  Apple SoCs have various co-processors required for certain
+	  peripherals to work (NVMe, display controller, etc.). This
+	  driver adds support for the mailbox controller used to
+	  communicate with those.
+
+	  Say Y here if you have an Apple SoC.
+
 config APPLE_RTKIT
 	tristate "Apple RTKit co-processor IPC protocol"
 	depends on MAILBOX

drivers/soc/apple/Makefile

@@ -1,6 +1,9 @@
 # SPDX-License-Identifier: GPL-2.0-only
 obj-$(CONFIG_APPLE_PMGR_PWRSTATE)	+= apple-pmgr-pwrstate.o
 
+obj-$(CONFIG_APPLE_MBOX) += apple-mailbox.o
+apple-mailbox-y = mailbox.o
+
 obj-$(CONFIG_APPLE_RTKIT) += apple-rtkit.o
 apple-rtkit-y = rtkit.o rtkit-crashlog.o