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docs: improve library usage document

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Clarify the libraries descriptions.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
This commit is contained in:
Damien Le Moal
2019-02-07 18:41:50 +09:00
committed by Anup Patel
parent b1b7e49c63
commit 01fe779113
1 changed files with 30 additions and 22 deletions
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52
docs/library_usage.md
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@@ -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()*
forwarding all traps and interrupts to *sbi_trap_handler()*.