cd83e85edc
RISC-V is an open-source instruction set architecture.
Added support for the 32bit version of RISC-V to Zephyr.
1) exceptions/interrupts/faults are handled at the architecture
level via the __irq_wrapper handler. Context saving/restoring
of registers can be handled at both architecture and SOC levels.
If SOC-specific registers need to be saved, SOC level needs to
provide __soc_save_context and __soc_restore_context functions
that shall be accounted by the architecture level, when
corresponding config variable RISCV_SOC_CONTEXT_SAVE is set.
2) As RISC-V architecture does not provide a clear ISA specification
about interrupt handling, each RISC-V SOC handles it in its own
way. Hence, at the architecture level, the __irq_wrapper handler
expects the following functions to be provided by the SOC level:
__soc_is_irq: to check if the exception is the result of an
interrupt or not.
__soc_handle_irq: handle pending IRQ at SOC level (ex: clear
pending IRQ in SOC-specific IRQ register)
3) Thread/task scheduling, as well as IRQ offloading are handled via
the RISC-V system call ("ecall"), which is also handled via the
__irq_wrapper handler. The _Swap asm function just calls "ecall"
to generate an exception.
4) As there is no conventional way of handling CPU power save in
RISC-V, the default nano_cpu_idle and nano_cpu_atomic_idle
functions just unlock interrupts and return to the caller, without
issuing any CPU power saving instruction. Nonetheless, to allow
SOC-level to implement proper CPU power save, nano_cpu_idle and
nano_cpu_atomic_idle functions are defined as __weak
at the architecture level.
Change-Id: I980a161d0009f3f404ad22b226a6229fbb492389
Signed-off-by: Jean-Paul Etienne <fractalclone@gmail.com>
85 lines
2.5 KiB
ArmAsm
85 lines
2.5 KiB
ArmAsm
/*
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* Copyright (c) 2016 Jean-Paul Etienne <fractalclone@gmail.com>
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#define _ASMLANGUAGE
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#include <irq.h>
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#include <kernel_structs.h>
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#include <offsets_short.h>
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/* exports */
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GTEXT(_Swap)
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GTEXT(_thread_entry_wrapper)
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/* Use ABI name of registers for the sake of simplicity */
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/*
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* unsigned int _Swap(unsigned int key)
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*
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* Always called with interrupts locked
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* key is stored in a0 register
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*/
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SECTION_FUNC(exception.other, _Swap)
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/* Make a system call to perform context switch */
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ecall
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/*
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* when thread is rescheduled, unlock irq and return.
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* Restored register a0 contains IRQ lock state of thread.
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*
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* Prior to unlocking irq, load return value of
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* _Swap to temp register t2 (from _thread_offset_to_swap_return_value).
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* Normally, it should be -EAGAIN, unless someone has previously
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* called _set_thread_return_value(..).
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*/
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la t0, _kernel
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/* Get pointer to _kernel.current */
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lw t1, _kernel_offset_to_current(t0)
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/* Load return value of _Swap function in temp register t2 */
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lw t2, _thread_offset_to_swap_return_value(t1)
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/*
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* Unlock irq, following IRQ lock state in a0 register.
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* Use atomic instruction csrrs to do so.
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*/
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andi a0, a0, SOC_MSTATUS_IEN
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csrrs t0, mstatus, a0
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/* Set value of return register a0 to value of register t2 */
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addi a0, t2, 0
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/* Return */
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jalr x0, ra
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/*
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* void _thread_entry_wrapper(_thread_entry_t, void *, void *, void *)
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*/
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SECTION_FUNC(TEXT, _thread_entry_wrapper)
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/*
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* _thread_entry_wrapper is called for every new thread upon the return
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* of _Swap or ISR. Its address, as well as its input function arguments
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* thread_entry_t, void *, void *, void * are restored from the thread
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* stack (initialized via function _thread).
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* In this case, thread_entry_t, * void *, void * and void * are stored
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* in registers a0, a1, a2 and a3. These registers are used as arguments
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* to function _thread_entry. Hence, just call _thread_entry with
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* return address set to 0 to indicate a non-returning function call.
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*/
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jal x0, _thread_entry
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