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57 lines
2.4 KiB
Markdown
57 lines
2.4 KiB
Markdown
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OpenSBI Library Usage Guideline
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===============================
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OpenSBI provides two types of static libraries:
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1. *libsbi.a* - A generic OpenSBI static library installed at
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*<install_directory>/lib/libsbi.a*
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2. *libplatsbi.a* - A platform specific OpenSBI static library, that is,
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libsbi.a plus platform specific hooks installed at
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*<install_directory>/platform/<platform_subdir>/lib/libplatsbi.a*
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The platform specific firmwares provided by OpenSBI are not mandatory. Users
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can always link OpenSBI as static library to their favorite M-mode firmware
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or bootloader provided that it has a license compatible with OpenSBI license.
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Users can choose either *libsbi.a* or *libplatsbi.a* to link with their
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firmware or bootloader but with *libsbi.a* platform specific hooks (i.e.
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*struct sbi_platform* instance) will have to be provided.
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Constraints on OpenSBI usage from external firmware
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---------------------------------------------------
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Users have to ensure that external firmware or bootloader and OpenSBI static
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library (*libsbi.a* or *libplatsbi.a*) are compiled with the same GCC target
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options *-mabi*, *-march*, and *-mcmodel*.
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There are only two constraints on calling any OpenSBI function from an
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external M-mode firmware or bootloader:
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1. The RISC-V *MSCRATCH* CSR must point to a valid OpenSBI scratch space
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(i.e. *struct sbi_scratch* instance)
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2. The RISC-V *SP* register (i.e. stack pointer) must be set per-HART
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pointing to distinct non-overlapping stacks
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The most important functions from an external firmware or bootloader
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perspective are *sbi_init()* and *sbi_trap_handler()*.
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In addition to above constraints, the external firmware or bootloader must
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ensure that interrupts are disabled in *MSTATUS* and *MIE* CSRs when calling
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*sbi_init()* and *sbi_trap_handler()* functions.
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The *sbi_init()* should be called by the external firmware or bootloader
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when a HART is powered-up at boot-time or in response to a CPU hotplug event.
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The *sbi_trap_handler()* should be called by the external firmware or
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bootloader for the following interrupts and traps:
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1. M-mode timer interrupt
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2. M-mode software interrupt
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3. Illegal instruction trap
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4. Misaligned load trap
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5. Misaligned store trap
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6. Supervisor ecall trap
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7. Hypervisor ecall trap
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**Note:** external firmwares or bootloaders can be more conservative by
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forwarding all traps and interrupts to *sbi_trap_handler()*
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