三个workaround的具体做法，原文见：
http://infocenter.arm.com/help/topic/com.arm.doc.faqs/ka3783.html
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Workaround:
There are 3 suggested workarounds. Which of these is most applicable will depend upon the requirements of the particular system.
Add code similar to the following at the start of the interrupt routine.
SUB lr, lr, #4 ; Adjust LR to point to return
STMFD sp!, {..., lr} ; Get some free regs
MRS lr, SPSR ; See if we got an interrupt while
TST lr, #I_Bit ; interrupts were disabled.
LDMNEFD sp!, {..., pc}^ ; If so, just return immediately.
; The interrupt will remain
; pending since we haven't
; acknowledged it and will be
; reissued when interrupts are next
; enabled.
; Rest of interrupt routine
This code will test for the situation where the IRQ was received during a write to disable IRQs. If this is the case, the code returns immediately - resulting in the IRQ not being acknowledged (cleared), and further IRQs being disabled.
Similar code may also be applied to the FIQ handler, in order to resolve the first issue.
This is the recommended workaround, as it overcomes both problems mentioned above. However, in the case of problem two, it does add several cycles to the maximum length of time FIQs will be disabled.
Disable IRQs and FIQs using separate writes to the CPSR, eg:
MRS r0, cpsr
ORR r0, r0, #I_Bit ;disable IRQs
MSR cpsr_c, r0
ORR r0, r0, #F_Bit ;disable FIQs
MSR cpsr_c, r0
This is the best workaround where the maximum time for which FIQs are disabled is critical (it does not increase this time at all). However, it does not solve problem one, and requires extra instructions at every point where IRQs and FIQs are disabled together.
Re-enable FIQs at the beginning of the IRQ handler. As the required state of all bits in the c field of the CPSR are known, this can be most efficiently be achieved by writing an immediate value to CPSR_c, for example:
MSR cpsr_c, #I_Bit:OR:irq_MODE ;IRQ should be disabled
;FIQ enabled
;ARM state, IRQ mode
This requires only the IRQ handler to be modified, and FIQs may be re-enabled more quickly than by using workaround 1. However, this should only be used if the system can guarantee that FIQs are never disabled while IRQs are enabled. It does not address problem one. |
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