misc: amd-sbi: Add document for AMD SB IOCTL description

- This document provides AMD side band IOCTL description defined
  for APML and its usage.
  Multiple AMD custom protocols defined for side band system
  management uses this IOCTL.
  User space C-APIs are made available by esmi_oob_library [1],
  which is provided by the E-SMS project [2].

Link: https://github.com/amd/esmi_oob_library
Link: https://www.amd.com/en/developer/e-sms.html
Reviewed-by: Naveen Krishna Chatradhi <naveenkrishna.chatradhi@amd.com>
Signed-off-by: Akshay Gupta <akshay.gupta@amd.com>
Link: https://lore.kernel.org/r/20250428063034.2145566-11-akshay.gupta@amd.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/Documentation/misc-devices/amd-sbi.rst b/Documentation/misc-devices/amd-sbi.rst
new file mode 100644 (file)
index 0000000..5648fc6
--- /dev/null
+++ b/Documentation/misc-devices/amd-sbi.rst
@@ -0,0 +1,99 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=======================
+AMD SIDE BAND interface
+=======================
+
+Some AMD Zen based processors supports system management
+functionality via side-band interface (SBI) called
+Advanced Platform Management Link (APML). APML is an I2C/I3C
+based 2-wire processor target interface. APML is used to
+communicate with the Remote Management Interface
+(SB Remote Management Interface (SB-RMI)
+and SB Temperature Sensor Interface (SB-TSI)).
+
+More details on the interface can be found in chapter
+"5 Advanced Platform Management Link (APML)" of the family/model PPR [1]_.
+
+.. [1] https://www.amd.com/content/dam/amd/en/documents/epyc-technical-docs/programmer-references/55898_B1_pub_0_50.zip
+
+
+SBRMI device
+============
+
+apml_sbrmi driver under the drivers/misc/amd-sbi creates miscdevice
+/dev/sbrmi-* to let user space programs run APML mailbox, CPUID,
+MCAMSR and register xfer commands.
+
+Register sets is common across APML protocols. IOCTL is providing synchronization
+among protocols as transactions may create race condition.
+
+$ ls -al /dev/sbrmi-3c
+crw-------    1 root     root       10,  53 Jul 10 11:13 /dev/sbrmi-3c
+
+apml_sbrmi driver registers hwmon sensors for monitoring power_cap_max,
+current power consumption and managing power_cap.
+
+Characteristics of the dev node:
+ * Differnet xfer protocols are defined:
+       * Mailbox
+       * CPUID
+       * MCA_MSR
+       * Register xfer
+
+Access restrictions:
+ * Only root user is allowed to open the file.
+ * APML Mailbox messages and Register xfer access are read-write,
+ * CPUID and MCA_MSR access is read-only.
+
+Driver IOCTLs
+=============
+
+.. c:macro:: SBRMI_IOCTL_MBOX_CMD
+.. kernel-doc:: include/uapi/misc/amd-apml.h
+   :doc: SBRMI_IOCTL_MBOX_CMD
+.. c:macro:: SBRMI_IOCTL_CPUID_CMD
+.. kernel-doc:: include/uapi/misc/amd-apml.h
+   :doc: SBRMI_IOCTL_CPUID_CMD
+.. c:macro:: SBRMI_IOCTL_MCAMSR_CMD
+.. kernel-doc:: include/uapi/misc/amd-apml.h
+   :doc: SBRMI_IOCTL_MCAMSR_CMD
+.. c:macro:: SBRMI_IOCTL_REG_XFER_CMD
+.. kernel-doc:: include/uapi/misc/amd-apml.h
+   :doc: SBRMI_IOCTL_REG_XFER_CMD
+
+User-space usage
+================
+
+To access side band interface from a C program.
+First, user need to include the headers::
+
+  #include <uapi/misc/amd-apml.h>
+
+Which defines the supported IOCTL and data structure to be passed
+from the user space.
+
+Next thing, open the device file, as follows::
+
+  int file;
+
+  file = open("/dev/sbrmi-*", O_RDWR);
+  if (file < 0) {
+    /* ERROR HANDLING */
+    exit(1);
+  }
+
+The following IOCTLs are defined:
+
+``#define SB_BASE_IOCTL_NR             0xF9``
+``#define SBRMI_IOCTL_MBOX_CMD         _IOWR(SB_BASE_IOCTL_NR, 0, struct apml_mbox_msg)``
+``#define SBRMI_IOCTL_CPUID_CMD                _IOWR(SB_BASE_IOCTL_NR, 1, struct apml_cpuid_msg)``
+``#define SBRMI_IOCTL_MCAMSR_CMD       _IOWR(SB_BASE_IOCTL_NR, 2, struct apml_mcamsr_msg)``
+``#define SBRMI_IOCTL_REG_XFER_CMD     _IOWR(SB_BASE_IOCTL_NR, 3, struct apml_reg_xfer_msg)``
+
+
+User space C-APIs are made available by esmi_oob_library, hosted at
+[2]_ which is provided by the E-SMS project [3]_.
+
+.. [2] https://github.com/amd/esmi_oob_library
+.. [3] https://www.amd.com/en/developer/e-sms.html