【华为云技术分享】漫谈LiteOS-LiteOS SDK支持RISC-V架构
作者：互联网
【摘要】 本文首先对RISC-V的架构做了简要的介绍，在此基础上实现了LiteOS在RISC-V架构上的适配过程的具体步骤，希望对你有所帮助。
1 RISC-V架构简介
RISC-V是一个基于精简指令集（RISC）原则的开源指令集架构(ISA)。
与大多数指令集相比，RISC-V指令集可以自由地用于任何目的，允许任何人设计、制造和销售RISC-V芯片和软件而不必支付给任何公司专利费。RISC-V指令集的设计考虑了小型、快速、低功耗的现实情况来实做，但并没有对特定的微架构做过度的设计。
RISC-V的Spec文档可以在RISC-C官网https://riscv.org/specifications/ 上下载。主要看riscv-privileged.pdf和riscv-spec.pdf。
主要精读的内容包括：
RV32ICM Instruction Set
I：RV32I Base Integer Instruction Set
C：Standard Extension for Compressed Instructions
M：Standard Extension for Integer Multiplication and Division
Privilege Levels
Control and Status Registers (CSRs)
Machine-Level ISA
在了解通用的RV32架构之后，由于RV32是开源的ISA架构，所以实际芯片都会在此基础上做一些定制化，因此需要再读一下芯片手册，LiteOS的RISC-V架构支持使用的芯片是GD32VF103，请下载GD32VF103 的Spec进行阅览。
2 LiteOS支持一种处理器
RTOS支持一种新的处理器架构，最主要的修改有以下几个方面：
1.启动汇编的适配
2.适配系统调度汇编
3.Tick的适配
4.根据芯片设置系统相关参数
5.适配中断管理模块
6.编译链接脚本的调整
那么，对应到LiteOS，主要修改的目录和文件是：
LiteOS_Lab\iot_link\os\liteos\arch\riscv\src中
los_dispatch.S
los_hw.c
los_hw_tick.c
los_hwi.c
和对应芯片target目录下的start.S启动汇编以及ld链接脚本。
步骤如下：
1. start.S
A. 和RISC-V的异常中断处理密切相关，注意向量表的对齐
vector_base:
j _start
.align 2
.word 0
.word 0
.word osInterrupt #eclic_msip_handler
.word 0
.word 0
.word 0
.word osInterrupt #eclic_mtip_handlerB. 设置中断，异常等的入口地址
_start0800:
/* Set the the NMI base to share with mtvec by setting CSR_MMISC_CTL */
li t0, 0x200
csrs CSR_MMISC_CTL, t0
/* Intial the mtvt*/
la t0, vector_base
csrw CSR_MTVT, t0
/* Intial the mtvt2 and enable it*/
la t0, irq_entry
csrw CSR_MTVT2, t0
csrs CSR_MTVT2, 0x1
/* Intial the CSR MTVEC for the Trap ane NMI base addr*/
la t0, trap_entry
csrw CSR_MTVEC, t0C.设置gp，sp，初始化data和bss section，然后跳转到main函数
.option push
.option norelax
la gp, __global_pointer$
.option pop
la sp, _sp
/* Load data section */
la a0, _data_lma
la a1, _data
la a2, _edata
bgeu a1, a2, 2f
1:
lw t0, (a0)
sw t0, (a1)
addi a0, a0, 4
addi a1, a1, 4
bltu a1, a2, 1b
2:
/* Clear bss section */
la a0, __bss_start
la a1, _end
bgeu a0, a1, 2f
1:
sw zero, (a0)
addi a0, a0, 4
bltu a0, a1, 1b2. 适配系统调度汇编（los_dispatch.s），主要修改函数LOS_StartToRun、LOS_IntLock、LOS_IntUnLock、TaskSwitch等；
任务栈的设计，在osTskStackInit中针对RISC-V的寄存器的定义，做出context的设计：
pstContext->ra = (UINT32)osTaskExit;
pstContext->sp = 0x02020202L;
pstContext->gp = 0x03030303L;
pstContext->tp = 0x04040404L;
pstContext->t0 = 0x05050505L;
pstContext->t1 = 0x06060606L;
pstContext->t2 = 0x07070707L;
pstContext->s0 = 0x08080808L;
pstContext->s1 = 0x09090909L;
pstContext->a0 = pstTaskCB->uwTaskID; //a0 first argument
pstContext->a1 = 0x11111111L;
pstContext->a2 = 0x12121212L;
pstContext->a3 = 0x13131313L;
pstContext->a4 = 0x14141414L;
pstContext->a5 = 0x15151515L;
pstContext->a6 = 0x16161616L;
pstContext->a7 = 0x17171717L;
pstContext->s2 = 0x18181818L;
pstContext->s3 = 0x19191919L;
pstContext->s4 = 0x20202020L;
pstContext->s5 = 0x21212121L;
pstContext->s6 = 0x22222222L;
pstContext->s7 = 0x23232323L;
pstContext->s8 = 0x24242424L;
pstContext->s9 = 0x25252525L;
pstContext->s10 = 0x26262626L;
pstContext->s11 = 0x27272727L;
pstContext->t3 = 0x28282828L;
pstContext->t4 = 0x29292929L;
pstContext->t5 = 0x30303030L;
pstContext->t6 = 0x31313131L;
pstContext->mepc =(UINT32)osTaskEntry;LOS_IntLock的实现：
LOS_IntLock:
csrr a0, mstatus
li t0, 0x08
csrrc zero, mstatus, t0
retLOS_IntUnLock的实现：
LOS_IntUnLock:
csrr a0, mstatus
li t0, 0x08
csrrs zero, mstatus, t0
retTaskSwitch的实现：
TaskSwitch:
la t0, g_stLosTask
lw t1, 0(t0)
csrr t2, mscratch
sw t2, 0(t1)
//Clear the task running bit of pstRunTask.
la t0, g_stLosTask
lw t1, (t0)
lb t2, 0x4(t1)
andi t2, t2, OS_TASK_STATUS_NOT_RUNNING
sb t2, 0x4(t1)
//copy pstNewTask into pstRunTask
la t0, g_stLosTask
lw t1, 0x4(t0)
sw t1, 0x0(t0)
//set the task running bit=1
lh t2, 0x4(t1)
ori t2, t2, OS_TASK_STATUS_RUNNING
sh t2, 0x4(t1)
//retireve stack pointer
lw sp, (t1)
//retrieve the address at which exception happened
lw t0, 31 * 4(sp)
csrw mepc, t0
li t0, 0x1800
csrs mstatus, t0
//retrieve the registers
lw ra, 0 * 4(sp)
lw t0, 4 * 4(sp)
lw t1, 5 * 4(sp)
lw t2, 6 * 4(sp)
lw s0, 7 * 4(sp)
lw s1, 8 * 4(sp)
lw a0, 9 * 4(sp)
lw a1, 10 * 4(sp)
lw a2, 11 * 4(sp)
lw a3, 12 * 4(sp)
lw a4, 13 * 4(sp)
lw a5, 14 * 4(sp)
lw a6, 15 * 4(sp)
lw a7, 16 * 4(sp)
lw s2, 17 * 4(sp)
lw s3, 18 * 4(sp)
lw s4, 19 * 4(sp)
lw s5, 20 * 4(sp)
lw s6, 21 * 4(sp)
lw s7, 22 * 4(sp)
lw s8, 23 * 4(sp)
lw s9, 24 * 4(sp)
lw s10, 25 * 4(sp)
lw s11, 26 * 4(sp)
lw t3, 27 * 4(sp)
lw t4, 28 * 4(sp)
lw t5, 29 * 4(sp)
lw t6, 30 * 4(sp)
addi sp, sp, 4 * 32
mret3. Tick的适配
osTickStart的启动：
MTIMECMP和MTIME寄存器的设定，TIMER中断的使能，TIMER中断处理函数的注册
LITE_OS_SEC_TEXT_INIT UINT32 osTickStart(VOID)
{
UINT32 uwRet;
g_uwCyclesPerTick = OS_SYS_CLOCK / LOSCFG_BASE_CORE_TICK_PER_SECOND;
g_ullTickCount = 0;
*(UINT64 *)(TIMER_CTRL_ADDR + TIMER_MTIMECMP) = OS_SYS_CLOCK / LOSCFG_BASE_CORE_TICK_PER_SECOND / 4;
*(UINT64 *)(TIMER_CTRL_ADDR + TIMER_MTIME) = 0;
eclic_irq_enable(CLIC_INT_TMR, 1, 1);
LOS_HwiCreate(CLIC_INT_TMR, 3, 0, eclic_mtip_handler, 0);
g_bSysTickStart = TRUE;
return LOS_OK;
}4. 根据芯片设置系统相关参数（时钟频率，tick中断配置，los_config.h系统参数配置（内存池大小、信号量、队列、互斥锁，软件定时器数量等））；
根据实际开发板的资源和实际使用需求，配置target_config.h的参数和选项。
5. 适配中断管理模块，LiteOS的中断向量表由m_pstHwiForm[OS_VECTOR_CNT]数组管理，需要根据芯片配置中断使能，重定向等；
A.在los_hwi.c和los_hwi.h中根据实际芯片的中断向量数目和驱动做一些调整
B.在entry.S中设计irq_entry的处理,需要注意的是需要单独在irq stack中处理中断嵌套：
irq_entry: // -------------> This label will be set to MTVT2 register
// Allocate the stack space
SAVE_CONTEXT// Save 16 regs
//------This special CSR read operation, which is actually use mcause as operand to directly store it to memory
csrrwi x0, CSR_PUSHMCAUSE, 17
//------This special CSR read operation, which is actually use mepc as operand to directly store it to memory
csrrwi x0, CSR_PUSHMEPC, 18
//------This special CSR read operation, which is actually use Msubm as operand to directly store it to memory
csrrwi x0, CSR_PUSHMSUBM, 19
la t0, g_int_cnt
lw t1, 0(t0)
addi t1, t1, 1
sw t1, 0(t0)
li t2, 1
bgtu t1,t2,service_loop
csrw mscratch, sp
la sp, __irq_stack_topservice_loop:
//------This special CSR read/write operation, which is actually Claim the CLIC to find its pending highest
// ID, if the ID is not 0, then automatically enable the mstatus.MIE, and jump to its vector-entry-label, and
// update the link register
csrrw ra, CSR_JALMNXTI, ra
//RESTORE_CONTEXT_EXCPT_X5
la t0, g_int_cnt
lw t1, 0(t0)
addi t1, t1, -1
sw t1, 0(t0)
bnez t1, _rfi
csrr sp, mscratch
DISABLE_MIE # Disable interrupts
LOAD x5, 19*REGBYTES(sp)
csrw CSR_MSUBM, x5
LOAD x5, 18*REGBYTES(sp)
csrw CSR_MEPC, x5
LOAD x5, 17*REGBYTES(sp)
csrw CSR_MCAUSE, x5
la t0, g_usLosTaskLock
lw t1, 0(t0)
bnez t1, _rfi
la t0, g_stLosTask
lw t1, 0x4(t0)
lw t2, 0x0(t0)
beq t1, t2, _rfi
RESTORE_CONTEXT
push_reg
csrr t0, mepc
sw t0, 31*4(sp)
csrw mscratch, sp
j TaskSwitch_rfi:
RESTORE_CONTEXT
// Return to regular code
mret6. 编译链接脚本的调整
几个关键的设置:
irq stack内存区域：
__stack_size = DEFINED(__stack_size) ? __stack_size : 2K;
__irq_stack_size = DEFINED(__irq_stack_size) ? __irq_stack_size : 2K;
__heap_size = DEFINED(__heap_size) ? __heap_size : 0xc00;gp初始值的设定：gp用于代码的优化，因为请合理选择__global_pointer的初值：
PROVIDE( __global_pointer$ = . + 0x800);堆栈的设定：
.stack : ALIGN(0x10)
{
. += __stack_size;
PROVIDE( _sp = . );
. = ALIGN(0x10);
PROVIDE( __irq_stack_bottom = . );
. += __irq_stack_size;
PROVIDE( __irq_stack_top = . );
} >ram AT>ram
.heap : ALIGN(0x10)
{
PROVIDE( __los_heap_addr_start__ = . );
. = __heap_size;
. = __heap_size == 0 ? 0 : ORIGIN(ram) + LENGTH(ram);
PROVIDE( __los_heap_addr_end__ = . );
PROVIDE( _heap_end = . );
} >ram AT>ram主要的步骤已经整体讲述了，顺利移植的主要前提条件是对RISC-V处理器架构的全面理解和LiteOS任务调度的设计，所以再次提醒精读riscv-privileged.pdf和riscv-spec.pdf的相关章节。在移植过程中，会遇到很多问题，建议使用IoT Studio的开发调试环境，方便的进行汇编级的单步调试，另外把串口驱动和printf打印调通，也是一种较重要的调试手段。
作者：星辰27
标签：__,LiteOS,sp,RISC,t0,t1,pstContext,lw,SDK 来源： https://www.cnblogs.com/huaweicloud/p/12383658.html