misc: amd-sbi: Add support for register xfer

- Provide user register access over IOCTL.
  Both register read and write are supported.
- APML interface does not provide a synchronization method. By defining,
  a register access path, we use APML modules and library for
  all APML transactions. Without having to use external tools such as
  i2c-tools, which may cause race conditions.

Reviewed-by: Naveen Krishna Chatradhi <naveenkrishna.chatradhi@amd.com>
Signed-off-by: Akshay Gupta <akshay.gupta@amd.com>
Link: https://lore.kernel.org/r/20250428063034.2145566-10-akshay.gupta@amd.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: akky16

drivers/misc/amd-sbi/rmi-core.c

@@ -350,6 +350,33 @@ int rmi_mailbox_xfer(struct sbrmi_data *data,
 	return ret;
 }
 
+static int apml_rmi_reg_xfer(struct sbrmi_data *data,
+			     struct apml_reg_xfer_msg __user *arg)
+{
+	struct apml_reg_xfer_msg msg = { 0 };
+	unsigned int data_read;
+	int ret;
+
+	/* Copy the structure from user */
+	if (copy_from_user(&msg, arg, sizeof(struct apml_reg_xfer_msg)))
+		return -EFAULT;
+
+	mutex_lock(&data->lock);
+	if (msg.rflag) {
+		ret = regmap_read(data->regmap, msg.reg_addr, &data_read);
+		if (!ret)
+			msg.data_in_out = data_read;
+	} else {
+		ret = regmap_write(data->regmap, msg.reg_addr, msg.data_in_out);
+	}
+
+	mutex_unlock(&data->lock);
+
+	if (msg.rflag && !ret)
+		return copy_to_user(arg, &msg, sizeof(struct apml_reg_xfer_msg));
+	return ret;
+}
+
 static int apml_mailbox_xfer(struct sbrmi_data *data, struct apml_mbox_msg __user *arg)
 {
 	struct apml_mbox_msg msg = { 0 };
@@ -414,6 +441,8 @@ static long sbrmi_ioctl(struct file *fp, unsigned int cmd, unsigned long arg)
 		return apml_cpuid_xfer(data, argp);
 	case SBRMI_IOCTL_MCAMSR_CMD:
 		return apml_mcamsr_xfer(data, argp);
+	case SBRMI_IOCTL_REG_XFER_CMD:
+		return apml_rmi_reg_xfer(data, argp);
 	default:
 		return -ENOTTY;
 	}

include/uapi/misc/amd-apml.h

@@ -58,6 +58,21 @@ struct apml_mcamsr_msg {
 	__u32 pad;
 };
 
+struct apml_reg_xfer_msg {
+	/*
+	 * RMI register address offset
+	 */
+	__u16 reg_addr;
+	/*
+	 * Register data for read/write
+	 */
+	__u8 data_in_out;
+	/*
+	 * Register read or write
+	 */
+	__u8 rflag;
+};
+
 /*
  * AMD sideband interface base IOCTL
  */
@@ -118,4 +133,20 @@ struct apml_mcamsr_msg {
  */
 #define SBRMI_IOCTL_MCAMSR_CMD		_IOWR(SB_BASE_IOCTL_NR, 2, struct apml_mcamsr_msg)
 
+/**
+ * DOC: SBRMI_IOCTL_REG_XFER_CMD
+ *
+ * @Parameters
+ *
+ * @struct apml_reg_xfer_msg
+ *	Pointer to the &struct apml_reg_xfer_msg that will contain the protocol
+ *	information
+ *
+ * @Description
+ * IOCTL command for APML messages using generic _IOWR
+ * The IOCTL provides userspace access to AMD sideband register xfer protocol
+ * - Register xfer protocol to get/set hardware register for given offset
+ */
+#define SBRMI_IOCTL_REG_XFER_CMD	_IOWR(SB_BASE_IOCTL_NR, 3, struct apml_reg_xfer_msg)
+
 #endif /*_AMD_APML_H_*/