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OpenSBI Library Usage
=====================
OpenSBI provides two types of static libraries:
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. *libsbiutils.a* - A static library that will contain all common code required
by any platform supported in OpenSBI. It will be built by default and included
in libplatsbi.a. This library is installed as
*<install_directory>/lib/libsbiutils.a*.
3. *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*
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 need to be provided.
The platform-specific example firmwares provided by OpenSBI are not mandatory.
An implementation may choose to link the OpenSBI generic static library together
with an M-mode firmware or bootloader providing the hardware-specific hooks.
Since OpenSBI is a statically linked library, users must ensure that the
license of these external components is compatible with the OpenSBI license.
Constraints on OpenSBI usage from external firmware
---------------------------------------------------
Users have to ensure that an external firmware or bootloader linking against
OpenSBI static libraries (*libsbi.a* or *libplatsbi.a*) is compiled with the
same GCC target options *-mabi*, *-march*, and *-mcmodel*.
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
(i.e. a *struct sbi_scratch* instance).
2. The RISC-V *SP* register (i.e. the stack pointer) must be set per-HART
pointing to distinct non-overlapping stacks.
The most important functions from an external firmware or bootloader
perspective are *sbi_init()* and *sbi_trap_handler()*.
In addition to the above constraints, the external firmware or bootloader must
ensure that interrupts are disabled in the *MSTATUS* and *MIE* CSRs when calling
the functions *sbi_init()* and *sbi_trap_handler()*.
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()* 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
3. Illegal instruction trap
4. Misaligned load trap
5. Misaligned store trap
6. Supervisor ecall trap
7. Hypervisor ecall trap
**Note:** external firmwares or bootloaders can be more conservative by
forwarding all traps and interrupts to *sbi_trap_handler()*.
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