firmware: scmi: support Arm SMCCC transport

This change implements a SMCCC transport for SCMI exchanges. This
implementation follows the Linux kernel as references implementation
for SCMI message processing, using the SMT format for communication
channel meta-data.

Use of SMCCC transport in SCMI FDT bindings are defined in the Linux
kernel DT bindings since v5.8. SMCCC with SMT is implemented in OP-TEE
from tag 3.9.0 [2].

Links: [2] https://github.com/OP-TEE/optee_os/commit/a58c4d706d23Signed-off-by: NEtienne Carriere <etienne.carriere@linaro.org>
Cc: Simon Glass <sjg@chromium.org>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Sudeep Holla <sudeep.holla@arm.com>
Reviewed-by: NSimon Glass <sjg@chromium.org>

drivers/firmware/scmi/Kconfig

@@ -2,7 +2,7 @@ config SCMI_FIRMWARE
 	bool "Enable SCMI support"
 	select FIRMWARE
 	select OF_TRANSLATE
-	depends on SANDBOX || DM_MAILBOX
+	depends on SANDBOX || DM_MAILBOX || ARM_SMCCC
 	help
 	  System Control and Management Interface (SCMI) is a communication
 	  protocol that defines standard interfaces for power, performance
@@ -15,5 +15,5 @@ config SCMI_FIRMWARE
 
 	  Communications between agent (client) and the SCMI server are
 	  based on message exchange. Messages can be exchange over tranport
-	  channels as a mailbox device with some piece of identified shared
-	  memory.
+	  channels as a mailbox device or an Arm SMCCC service with some
+	  piece of identified shared memory.

drivers/firmware/scmi/Makefile

 obj-y	+= scmi_agent-uclass.o
 obj-y	+= smt.o
+obj-$(CONFIG_ARM_SMCCC) 	+= smccc_agent.o
 obj-$(CONFIG_DM_MAILBOX)	+= mailbox_agent.o
 obj-$(CONFIG_SANDBOX)		+= sandbox-scmi_agent.o

drivers/firmware/scmi/smccc_agent.c

+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright (C) 2020 Linaro Limited.
+ */
+
+#include <common.h>
+#include <dm.h>
+#include <errno.h>
+#include <scmi_agent.h>
+#include <scmi_agent-uclass.h>
+#include <dm/devres.h>
+#include <dm/device-internal.h>
+#include <linux/arm-smccc.h>
+#include <linux/compat.h>
+
+#include "smt.h"
+
+#define SMCCC_RET_NOT_SUPPORTED         ((unsigned long)-1)
+
+/**
+ * struct scmi_smccc_channel - Description of an SCMI SMCCC transport
+ * @func_id:	SMCCC function ID used by the SCMI transport
+ * @smt:	Shared memory buffer
+ */
+struct scmi_smccc_channel {
+	ulong func_id;
+	struct scmi_smt smt;
+};
+
+static int scmi_smccc_process_msg(struct udevice *dev, struct scmi_msg *msg)
+{
+	struct scmi_smccc_channel *chan = dev_get_priv(dev);
+	struct arm_smccc_res res;
+	int ret;
+
+	ret = scmi_write_msg_to_smt(dev, &chan->smt, msg);
+	if (ret)
+		return ret;
+
+	arm_smccc_smc(chan->func_id, 0, 0, 0, 0, 0, 0, 0, &res);
+	if (res.a0 == SMCCC_RET_NOT_SUPPORTED)
+		ret = -ENXIO;
+	else
+		ret = scmi_read_resp_from_smt(dev, &chan->smt, msg);
+
+	scmi_clear_smt_channel(&chan->smt);
+
+	return ret;
+}
+
+static int scmi_smccc_probe(struct udevice *dev)
+{
+	struct scmi_smccc_channel *chan = dev_get_priv(dev);
+	u32 func_id;
+	int ret;
+
+	if (dev_read_u32(dev, "arm,smc-id", &func_id)) {
+		dev_err(dev, "Missing property func-id\n");
+		return -EINVAL;
+	}
+
+	chan->func_id = func_id;
+
+	ret = scmi_dt_get_smt_buffer(dev, &chan->smt);
+	if (ret) {
+		dev_err(dev, "Failed to get smt resources: %d\n", ret);
+		return ret;
+	}
+
+	return 0;
+}
+
+static const struct udevice_id scmi_smccc_ids[] = {
+	{ .compatible = "arm,scmi-smc" },
+	{ }
+};
+
+static const struct scmi_agent_ops scmi_smccc_ops = {
+	.process_msg = scmi_smccc_process_msg,
+};
+
+U_BOOT_DRIVER(scmi_smccc) = {
+	.name		= "scmi-over-smccc",
+	.id		= UCLASS_SCMI_AGENT,
+	.of_match	= scmi_smccc_ids,
+	.priv_auto_alloc_size = sizeof(struct scmi_smccc_channel),
+	.probe		= scmi_smccc_probe,
+	.ops		= &scmi_smccc_ops,
+};