Re: interrupts disabled during DSRs on ARM

On 01/11/13 16:30, Andrew Parlane wrote:
> Hi,
> 
> Another question regarding the ARM interrupt handling code.
> 
> When an IRQ occurs we jump to IRQ: in hal\arm\arch\current\src\vectors.s 
> we save the previous state, potentially switch stack, take the scheduler 
> lock, find the IRQ number, call the ISR, and then call interrupt_end.
> 
> interrupt_end() in kernel\current\src\intr\intr.cxx queues up the DSR if 
> asked to, and then unlocks the scheduler. As long as the scheduler 
> wasn't locked before the IRQ, this causes the scheduler to call any DSRs 
> that are queued up. This happens in call_pending_DSRs_inner() in the 
> same file. We disable interrupts, get the pointer to the DSR to call, 
> restore interrupts and call the DSR.
> 
> In the case I've been describing we are still in the IRQ handler, and as 
> such the interrupts are disabled throughout the entirety of this, and 
> hence disabled in the DSR.
> 
> Looking at: http://ecos.sourceware.org/docs-1.3.1/ref/ecos-ref.c.html in 
> the first paragraph it states: "This separation explicitly allows for 
> the DSRs to be run with interrupts enabled, thus allowing other 
> potentially higher priority interrupts to occur and be processed while 
> processing a lower priority interrupt."
> 
> Am I missing something here?

call_pending_DSRs_inner() is called via
cyg_interrupt_call_pending_DSRs() which is itself called by
hal_interrupt_stack_call_pending_DSRs() in vectors.S. This last routine
switches to the interrupt stack and enables interrupts before calling
cyg_interrupt_call_pending_DSRs().


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Nick Garnett                                         Kernel Architect
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