Re: MIPS Interrupts.

[Jun Sun <jsun@mvista.com>]

On Tue, Nov 25, 2003 at 04:52:20PM -0500, Kapoor, Pankaj wrote:
> All,
> 
> While studying the implementation of tasklets and softirq processing I came 
> across certain issues which I have outlined below. 
> 
> The function mipsIRQ in the file mipsIRQ.s is the registered interrupt 
> handler for all general purpose interrupts. 
> 
> The first thing that the function does is that it saves all registers. It 
> then checks the CAUSE register to check the source of the interrupt.
> Currently 
> all we are interested in is INT5 (Timer) and INT0 (i.e. all other devices) 
> 
> Consider a timer interrupt which would cause the code to jump to 0x8000:0180
> 
> and cause all the registers to be saved (SAVE_ALL). It would then jump to
> the 
> mips_timer_interrupt function in the file time.c 
> 
> The function services the timer interrupt. At the end of the function there 
> is an irq_exit and a check to see if there are any SOFT IRQ pending. 
> If there are any the function jumps to the do_softirq function defined in 
> the softirq.c. The function gets the softirq pending list, enables
> interrupts 
> and cycles through all pending soft irq's calling the appropriate handlers. 
> 
> Remember that the interrupts are enabled while executing the various bottom 
> half handlers. 
> 
> Now there are 2 cases that can happen 
> 
> 1. Since we have not exited the ISR and the exception level has still not 
>    been restored there can be no more interrupts that are generated in the 
>    system. In such a case does that mean that the all bottom half handlers 
>    pending execution will run with interrupts disabled. 
>    NOTE: This does not seem likely because the local_irq_enable routine
> calls 
>    _sti which clears the exception level in the status register and also 
>    sets the IE bit. 
>

Refer to your own note.  It is more correct.  :)
 
> 2. If we have large number of tasklets or if the bottom half handlers take
> time 
>    to execute, then we could get another timer interrupt or other device 
>    interrupts causing context saves which would cause the stack to grow and 
>    CRASH the system. 
>   
> Context is restored only when the code returns from do_softirq and uses the 
> ret_from_irq. 
> 

The nested interrupt call, do_IRQ(), may still try to call do_softirq() but
that it will return immediately as it discovers another instance of do_softirq()
is running.  No further nesting occurs as a result. 

Jun