pldm: Document BIOS support
The BIOS attributes supported by OpenBMC systems are added in JSON
files, adhering to the specifications outlined in the PLDM BIOS
Specification [1]. Since a single BIOS attribute may have different
values for different variants of systems, creating different json files
for each of the system type supported. To enable this optional feature
within PLDM, creating meson option system-specific-bios-json.
System specific bios attributes support in PLDM is added in below
commits [2], [3], [4]
[1]: https://www.dmtf.org/sites/default/files/standards/documents/DSP0247_1.0.0.pdf
[2]: https://github.com/openbmc/pldm/tree/963c0fdf1aef305f266e00b2321fc244f89d0fa7
[3]: https://github.com/openbmc/pldm/tree/62dd8ff2f106c0eb80e8c1e79adfa99e093d1cf6
[4]: https://github.com/openbmc/pldm/tree/46f352ed2426368185f3bd36fd6bf88f15fa5111
Change-Id: Ieffdca541d1ff8489911b02c178f85d3b1b84a5c
Signed-off-by: Archana Kakani <archana.kakani@ibm.com>
diff --git a/README.md b/README.md
index 33ede85..46983f6 100644
--- a/README.md
+++ b/README.md
@@ -191,3 +191,119 @@
JSON updates. New PDR type support would require JSON updates as well as PDR
generation code. The PDR generator is a map of PDR Type -> C++ lambda to create
PDR entries for that type based on the JSON, and to update the central PDR repo.
+
+###### BIOS Support
+
+Redfish supports the BIOS Attribute Registry, which provides users with a list
+of BIOS attributes supported in the BIOS configuration. To incorporate BIOS
+attribute registry support in openBMC, BmcWeb is designed to read data from the
+Base BIOS Table. PLDM populates the Base BIOS Table for the BIOS Config Manager
+based on BIOS JSON files. BIOS functionality is integrated into PLDM according
+to the guidelines in the
+[PLDM BIOS Specification](https://www.dmtf.org/sites/default/files/standards/documents/DSP0247_1.0.0.pdf).
+BIOS attributes, also referred to as BIOS parameters or configuration settings,
+are structured within JSON files. Each attribute is defined by its name, type,
+and type-specific metadata. PLDM parses
+[BIOS JSON file](https://github.com/openbmc/pldm/tree/master/oem/ibm/configurations/bios/com.ibm.Hardware.Chassis.Model.Rainier2U)
+and creates the
+[Base BIOS Table](https://github.com/openbmc/phosphor-dbus-interfaces/blob/master/yaml/xyz/openbmc_project/BIOSConfig/Manager.interface.yaml#L62)
+hosted by [BIOS Config Manager](https://github.com/openbmc/bios-settings-mgr).
+The design is documented in
+[DesignDoc](https://github.com/openbmc/docs/blob/master/designs/remote-bios-configuration.md).
+Redfish supports
+[BIOS Attribute registry](https://redfish.dmtf.org/schemas/v1/AttributeRegistry.v1_3_8.json),
+which provides users with the list of BIOS attributes supported in the BIOS
+configuration. The BIOS Attribute registry data is supposed to be derived from
+[Base BIOS Table](https://github.com/openbmc/phosphor-dbus-interfaces/blob/master/yaml/xyz/openbmc_project/BIOSConfig/Manager.interface.yaml#L62).
+PLDM populates the Base BIOS Table for BIOS Config Manager based on BIOS Json
+files.
+
+After the JSON files are parsed, pldm would also create the string table,
+attribute table, and attribute value table as per Section7 in
+[PLDM BIOS Specification](https://www.dmtf.org/sites/default/files/standards/documents/DSP0247_1.0.0.pdf).
+Those BIOS tables are exchanged with remote PLDM terminus using the
+`GetBiosTable` command as defined in DSP0247_1.0.0.pdf Section 8.1. All the
+`bios attribute json files are kept under OEM`
+[Path](https://github.com/openbmc/pldm/tree/master/oem). BIOS json configuration
+is provided in bios_attr.json file which contains attributes of type enum,
+integer and string.
+
+Integer Attribute details as documented at Table9 of
+[PLDM BIOS Specification](https://www.dmtf.org/sites/default/files/standards/documents/DSP0247_1.0.0.pdf):
+
+```json
+{
+ "entries": [
+ {
+ "attribute_type": "integer",
+ "attribute_name": "Attribute Name",
+ "lower_bound": "The lower bound on the integer value",
+ "upper_bound": "The upper bound on the integer value",
+ "scalar_increment": "The scalar value that is used for the increments to this integer ",
+ "default_value": "The default value of the integer",
+ "help_text": "Help text about attribute usage",
+ "display_name": "Attribute Display Name",
+ "read_only": "Read only Attribute"
+ }
+ ]
+}
+```
+
+Enum Attribute details as documented at Table6 of
+[PLDM BIOS Specification](https://www.dmtf.org/sites/default/files/standards/documents/DSP0247_1.0.0.pdf):
+
+```json
+{
+ "entries": [
+ {
+ "attribute_type": "enum",
+ "attribute_name": "Attribute Name",
+ "possible_values": [
+ "An array of character strings of variable to indicate the possible values of the BIOS attribute"
+ ],
+ "default_values": "Default value",
+ "help_text": "Help text about attribute usage",
+ "display_name": "Display Name",
+ "read_only": "Read only Attribute"
+ }
+ ]
+}
+```
+
+String Attribute details as documented at Table7 of
+[PLDM BIOS Specification](https://www.dmtf.org/sites/default/files/standards/documents/DSP0247_1.0.0.pdf):
+
+```json
+{
+ "entries": [
+ {
+ "attribute_type": "string",
+ "attribute_name": "Attribute Name",
+ "string_type": "It specifies the character encoding format, which can be ASCII, Hex, UTF-8, UTF-16LE, or UTF-16BE. Currently, only ASCII is supported",
+ "minimum_string_length": "The minimum length of the string in bytes",
+ "maximum_string_length": "The maximum length of the string in bytes",
+ "default_string": "The default string itself",
+ "help_text": "Help text about attribute usage",
+ "display_name": "Attribute Display Name",
+ "read_only": "Read only Attribute"
+ }
+ ]
+}
+```
+
+As PLDM BIOS Attributes may differ across platforms and systems, supporting
+system-specific BIOS attributes is crucial. To achieve this, BIOS JSON files are
+organized under folders named after the system type. System type information is
+retrieved from the Entity Manager service, which hosts the
+[Compatible Interface](https://github.com/openbmc/phosphor-dbus-interfaces/blob/master/yaml/xyz/openbmc_project/Inventory/Decorator/Compatible.interface.yaml).
+
+This interface dynamically populates the Names property with system type
+information. However, determining the system type in the application space may
+take some time since the compatible interface and the Names property are
+dynamically created by the Entity Manager. Consequently, BIOS tables are lazily
+constructed upon receiving the system type.
+
+To enable system-specific BIOS attribute support within PLDM, the meson option
+`system-specific-bios-json` can be utilized. With `system-specific-bios-json`
+option `enabled` BIOS JSON files specific to the system type are fetched during
+runtime.