mailbox: apple: Add driver for Apple IOP mailbox

This mailbox driver provides a communication channel with the
Apple IOP controllers found on Apple SoCs.  These IOP controllers
are used to implement various functions such as the System
Manegement Controller (SMC) and NVMe storage.  It allows sending
and receiving a 96-bit message over a single channel.

The header file with the struct used for mailbox messages is taken
straight from Linux.

Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
Signed-off-by: Sven Peter <sven@svenpeter.dev>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested on: Macbook Air M1
Tested-by: Simon Glass <sjg@chromium.org>

arch/arm/Kconfig

@@ -931,6 +931,7 @@ config ARCH_APPLE
 	select DM
 	select DM_GPIO
 	select DM_KEYBOARD
+	select DM_MAILBOX
 	select DM_SERIAL
 	select DM_USB
 	select DM_VIDEO

drivers/mailbox/Kconfig

@@ -10,6 +10,17 @@ config DM_MAILBOX
 	  the basis of a variety of inter-process/inter-CPU communication
 	  protocols.
 
+config APPLE_MBOX
+	bool "Enable Apple IOP controller support"
+	depends on DM_MAILBOX && ARCH_APPLE
+	default y
+	help
+	  Enable support for the mailboxes that provide a comminucation
+	  channel with Apple IOP controllers integrated on Apple SoCs.
+	  These IOP controllers are used to implement various functions
+	  such as the System Management Controller (SMC) and NVMe and this
+	  driver is required to get that functionality up and running.
+
 config SANDBOX_MBOX
 	bool "Enable the sandbox mailbox test driver"
 	depends on DM_MAILBOX && SANDBOX

drivers/mailbox/Makefile

@@ -4,6 +4,7 @@
 #
 
 obj-$(CONFIG_$(SPL_)DM_MAILBOX) += mailbox-uclass.o
+obj-$(CONFIG_APPLE_MBOX) += apple-mbox.o
 obj-$(CONFIG_SANDBOX_MBOX) += sandbox-mbox.o
 obj-$(CONFIG_SANDBOX_MBOX) += sandbox-mbox-test.o
 obj-$(CONFIG_STM32_IPCC) += stm32-ipcc.o

drivers/mailbox/apple-mbox.c

@@ -0,0 +1,92 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright (C) 2021 Mark Kettenis <kettenis@openbsd.org>
+ */
+
+#include <common.h>
+#include <dm.h>
+#include <mailbox-uclass.h>
+#include <asm/io.h>
+#include <linux/apple-mailbox.h>
+#include <linux/delay.h>
+
+#define REG_A2I_STAT	0x110
+#define  REG_A2I_STAT_EMPTY	BIT(17)
+#define  REG_A2I_STAT_FULL	BIT(16)
+#define REG_I2A_STAT	0x114
+#define  REG_I2A_STAT_EMPTY	BIT(17)
+#define  REG_I2A_STAT_FULL	BIT(16)
+#define REG_A2I_MSG0	0x800
+#define REG_A2I_MSG1	0x808
+#define REG_I2A_MSG0	0x830
+#define REG_I2A_MSG1	0x838
+
+struct apple_mbox_priv {
+	void *base;
+};
+
+static int apple_mbox_of_xlate(struct mbox_chan *chan,
+			       struct ofnode_phandle_args *args)
+{
+	if (args->args_count != 0)
+		return -EINVAL;
+
+	return 0;
+}
+
+static int apple_mbox_send(struct mbox_chan *chan, const void *data)
+{
+	struct apple_mbox_priv *priv = dev_get_priv(chan->dev);
+	const struct apple_mbox_msg *msg = data;
+
+	writeq(msg->msg0, priv->base + REG_A2I_MSG0);
+	writeq(msg->msg1, priv->base + REG_A2I_MSG1);
+	while (readl(priv->base + REG_A2I_STAT) & REG_A2I_STAT_FULL)
+		udelay(1);
+
+	return 0;
+}
+
+static int apple_mbox_recv(struct mbox_chan *chan, void *data)
+{
+	struct apple_mbox_priv *priv = dev_get_priv(chan->dev);
+	struct apple_mbox_msg *msg = data;
+
+	if (readl(priv->base + REG_I2A_STAT) & REG_I2A_STAT_EMPTY)
+		return -ENODATA;
+
+	msg->msg0 = readq(priv->base + REG_I2A_MSG0);
+	msg->msg1 = readq(priv->base + REG_I2A_MSG1);
+	return 0;
+}
+
+struct mbox_ops apple_mbox_ops = {
+	.of_xlate = apple_mbox_of_xlate,
+	.send = apple_mbox_send,
+	.recv = apple_mbox_recv,
+};
+
+static int apple_mbox_probe(struct udevice *dev)
+{
+	struct apple_mbox_priv *priv = dev_get_priv(dev);
+
+	priv->base = dev_read_addr_ptr(dev);
+	if (!priv->base)
+		return -EINVAL;
+
+	return 0;
+}
+
+static const struct udevice_id apple_mbox_of_match[] = {
+	{ .compatible = "apple,asc-mailbox-v4" },
+	{ /* sentinel */ }
+};
+
+U_BOOT_DRIVER(apple_mbox) = {
+	.name = "apple-mbox",
+	.id = UCLASS_MAILBOX,
+	.of_match = apple_mbox_of_match,
+	.probe = apple_mbox_probe,
+	.priv_auto = sizeof(struct apple_mbox_priv),
+	.ops = &apple_mbox_ops,
+};

include/linux/apple-mailbox.h

@@ -0,0 +1,19 @@
+/* SPDX-License-Identifier: GPL-2.0-only OR MIT */
+/*
+ * Apple mailbox message format
+ *
+ * Copyright (C) 2021 The Asahi Linux Contributors
+ */
+
+#ifndef _LINUX_APPLE_MAILBOX_H_
+#define _LINUX_APPLE_MAILBOX_H_
+
+#include <linux/types.h>
+
+/* encodes a single 96bit message sent over the single channel */
+struct apple_mbox_msg {
+	u64 msg0;
+	u32 msg1;
+};
+
+#endif