docs: improve library usage document

Clarify the libraries descriptions.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>

1 files changed, 30 insertions, 22 deletions

diff --git a/docs/library_usage.md b/docs/library_usage.md
index 0eb74ef..579411f 100644
--- a/docs/library_usage.md
+++ b/docs/library_usage.md
@@ -1,30 +1,36 @@
-OpenSBI Library Usage Guideline
-===============================
+OpenSBI Library Usage
+=====================
 
 OpenSBI provides two types of static libraries:
 
-1. *libsbi.a* - A generic OpenSBI static library installed at
+1. *libsbi.a* - A platform independent generic static library implementing the
+   interface defined by the SBI specifications. Platform specific processing
+   hooks for the execution of this interface must be provided by the firmware or
+   bootloader linking with this library. This library is installed as
    *<install_directory>/lib/libsbi.a*
-2. *libplatsbi.a* - A platform specific OpenSBI static library, that is,
-   libsbi.a plus platform specific hooks installed at
+2. *libplatsbi.a* - An example platform specific static library integrating 
+   *libsbi.a* with platform specific hooks. This library is available only for
+   the platforms supported by OpenSBI. This library is installed as
    *<install_directory>/platform/<platform_subdir>/lib/libplatsbi.a*
 
-The platform specific firmwares provided by OpenSBI are not mandatory. Users
-can always link OpenSBI as static library to their favorite M-mode firmware
-or bootloader provided that it has a license compatible with OpenSBI license.
+Implementations may choose either *libsbi.a* or *libplatsbi.a* to link with
+their firmware or bootloader. In the case of *libsbi.a*, platform specific
+hooks in the form of a *struct sbi_platform* instance needs to be provided.
 
-Users can choose either *libsbi.a* or *libplatsbi.a* to link with their
-firmware or bootloader but with *libsbi.a* platform specific hooks (i.e.
-*struct sbi_platform* instance) will have to be provided.
+The platform specific example firmwares provided by OpenSBI are not mandatory.
+An implementation may choose to link OpenSBI generic static library together
+with an M-mode firmware or bootloader providing hardware specific hooks. Since
+OpenSBI is a statically linked library, users must ensure that the license of
+these external components is compatible with OpenSBI license.
 
 Constraints on OpenSBI usage from external firmware
 ---------------------------------------------------
 
-Users have to ensure that external firmware or bootloader and OpenSBI static
-library (*libsbi.a* or *libplatsbi.a*) are compiled with the same GCC target
-options *-mabi*, *-march*, and *-mcmodel*.
+Users have to ensure that an external firmware or bootloader linking against
+OpenSBI static libraries (*libsbi.a* or *libplatsbi.a*) are compiled with the
+same GCC target options *-mabi*, *-march*, and *-mcmodel*.
 
-There are only two constraints on calling any OpenSBI function from an
+There are only two constraints on calling any OpenSBI library function from an
 external M-mode firmware or bootloader:
 
 1. The RISC-V *MSCRATCH* CSR must point to a valid OpenSBI scratch space
@@ -35,15 +41,16 @@ external M-mode firmware or bootloader:
 The most important functions from an external firmware or bootloader
 perspective are *sbi_init()* and *sbi_trap_handler()*.
 
-In addition to above constraints, the external firmware or bootloader must
+In addition to the above constraints, the external firmware or bootloader must
 ensure that interrupts are disabled in *MSTATUS* and *MIE* CSRs when calling
-*sbi_init()* and *sbi_trap_handler()* functions.
+the functions *sbi_init()* and *sbi_trap_handler()*.
 
-The *sbi_init()* should be called by the external firmware or bootloader
-when a HART is powered-up at boot-time or in response to a CPU hotplug event.
+The *sbi_init()* function should be called by the external firmware or
+bootloader for each HART that is powered-up at boot-time or in response to a
+CPU hotplug event.
 
-The *sbi_trap_handler()* should be called by the external firmware or
-bootloader for the following interrupts and traps:
+The *sbi_trap_handler()* function should be called by the external firmware or
+bootloader to service the following interrupts and traps:
 
 1. M-mode timer interrupt
 2. M-mode software interrupt
@@ -54,4 +61,5 @@ bootloader for the following interrupts and traps:
 7. Hypervisor ecall trap
 
 **Note:** external firmwares or bootloaders can be more conservative by
-forwarding all traps and interrupts to *sbi_trap_handler()*
\ No newline at end of file
+forwarding all traps and interrupts to *sbi_trap_handler()*.
+