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arc: fixes in context switch explanation comments

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There were some typos and some weird phrasing.

Change-Id: I7b183755058e5ffedca97d434c43f448aafd1926
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>
This commit is contained in:
Benjamin Walsh 2015-08-04 11:27:27 -04:00 • committed by Anas Nashif
commit add6b9794b
1 changed files with 12 additions and 12 deletions
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24
arch/arc/core/isr_wrapper.S
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@ -97,10 +97,10 @@ Not loading _nanokernel into r0 allows loading _nanokernel without stomping on
the parameter in r0 in _Swap(). the parameter in r0 in _Swap().
ARCv2 processor have two kinds of interrupts: fast (FIRQ) and regular. The ARCv2 processors have two kinds of interrupts: fast (FIRQ) and regular. The
official documentation calls them regular interrupts IRQ, but the internals of official documentation calls the regular interrupts 'IRQs', but the internals
the kernel calls them RIRQ to differentiate with the 'irq' subsystem, which is of the kernel call them 'RIRQs' to differentiate from the 'irq' subsystem,
the interrupt API/layer of abstraction. which is the interrupt API/layer of abstraction.
FIRQs can be used to allow ISRs to run without having to save any context, FIRQs can be used to allow ISRs to run without having to save any context,
since they work with a second register bank. However, they can be somewhat more since they work with a second register bank. However, they can be somewhat more
@ -129,11 +129,11 @@ o RIRQ
o FIRQ o FIRQ
First, a FIRQ can be interrupting a lower-priority RIRQ: if this is the case, First, a FIRQ can be interrupting a lower-priority RIRQ: if this is the case,
the first does not take a scheduling decision and leaves it the RIRQ to the FIRQ does not take a scheduling decision and leaves it the RIRQ to
handle. The limits the amount of code that has to run at interrupt-level. handle. This limits the amount of code that has to run at interrupt-level.
GPRs are banked, loop registers are saved in a global structure upon GPRs are banked, loop registers are saved in a global structure upon
interrupt entry. If returning to a fiber, loop are poppped and the interrupt entry. If returning to a fiber, loop registers are poppped and the
CPU switches back to bank 0 for the GPRs. If a context switch is CPU switches back to bank 0 for the GPRs. If a context switch is
needed, at this point only are all the registers saved. First, a needed, at this point only are all the registers saved. First, a
stack frame with the same layout as the automatic RIRQ one is created stack frame with the same layout as the automatic RIRQ one is created
@ -141,14 +141,14 @@ o FIRQ
ilink are saved in this case, not status32 and pc. ilink are saved in this case, not status32 and pc.
To create the stack frame, the FIRQ handling code must first go back to using To create the stack frame, the FIRQ handling code must first go back to using
bank0 of registers, since that is where the register containing the exiting bank0 of registers, since that is where the registers containing the exiting
thread are saved. Care must be taken not to touch any register before saving thread are saved. Care must be taken not to touch any register before saving
them: the only one usable at that point is the stack pointer. them: the only one usable at that point is the stack pointer.
o coop o coop
When a coop context switch is done, the callee-saved registers are When a coop context switch is done, the callee-saved registers are
saved in the CCS. The other GPRs do not have to be saved, since the saved in the CCS. The other GPRs do not need to be saved, since the
compiler has already placed them on the stack. compiler has already placed them on the stack.
For restoring the contexts, there are six cases. In all cases, the For restoring the contexts, there are six cases. In all cases, the
@ -159,7 +159,7 @@ From coop:
o to coop o to coop
Restore interrupt lock level and normal function call return. Restore interrupt lock level and do a normal function call return.
o to any irq o to any irq
@ -170,7 +170,7 @@ From coop:
From FIRQ: From FIRQ:
The processor is back to using bank0, not bank1 anymore, because it had to The processor is back to using bank0, not bank1 anymore, because it had to
save the outgoing context from bank0, and not has to load the incoming one save the outgoing context from bank0, and now has to load the incoming one
into bank0. into bank0.
o to coop o to coop
@ -198,7 +198,7 @@ From RIRQ:
caller-saved. This means that the processor pushes it in the stack frame caller-saved. This means that the processor pushes it in the stack frame
along with r12, but the compiler does not save it before entering a along with r12, but the compiler does not save it before entering a
function. So, it is saved as part of the callee-saved registers, and function. So, it is saved as part of the callee-saved registers, and
restored there, but the processor restores it *a second time* when popping restored there, but the processor restores it _a second time_ when popping
the IRQ stack frame. Thus, the correct value must also be put in the fake the IRQ stack frame. Thus, the correct value must also be put in the fake
stack frame when returning to a thread that context switched out stack frame when returning to a thread that context switched out
cooperatively. cooperatively.