docs: Add description of using OPENSBI_EXTERNAL_SBI_TYPES

Add description of using OPENSBI_EXTERNAL_SBI_TYPES in external
firmware code base.

Signed-off-by: Abner Chang <abner.chang@hpe.com>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Anup Patel <anup.patel@wdc.com>

1 files changed, 21 insertions, 0 deletions

diff --git a/docs/library_usage.md b/docs/library_usage.md
index d765fa9..ff99801 100644
--- a/docs/library_usage.md
+++ b/docs/library_usage.md
@@ -66,3 +66,24 @@ bootloader to service the following interrupts and traps:
 
 **Note:** external firmwares or bootloaders can be more conservative by
 forwarding all traps and interrupts to *sbi_trap_handler()*.
+
+Definitions of OpenSBI Data Types for the External Firmware
+-----------------------------------------------------------
+
+OpenSBI can be built as library using external firmware build system such as EDK2
+code base (The open source of UEFI firmware implementation) and linked with external
+firmware drivers based on the external firmware architecture.
+
+**OPENSBI_EXTERNAL_SBI_TYPES** identifier is introduced to *sbi_types.h* for selecting
+external header file during the build preprocess in order to define OpensSBI data types
+based on external firmware data type binding.
+For example, *bool* is declared as *int* in sbi_types.h. However in EDK2 build system,
+*bool* is declared as *BOOLEAN* which is defined as *unsigned char* data type.
+
+External firmware can define **OPENSBI_EXTERNAL_SBI_TYPES** in CFLAGS and specify it to the
+header file maintained in its code tree. However, the external build system has to address
+the additional include directory for the external header file based on its own build system.
+For example,
+*-D***OPENSBI_EXTERNAL_SBI_TYPES***=OpensbiTypes.h*
+Above tells *sbi_types.h* to refer to *OpensbiTypes.h* instead of using original definitions of
+data types.