Re: the ppc_n_lost_interrupts thing...

[Benjamin Herrenschmidt <bh40@calva.net>]

On Thu, Jan 27, 2000, Jun Sun <jsun@mvista.com> wrote:

>I am a newbie to PowerPC-linux.  Hopefully this question is
>not too dumb.

Definitely not ;) Here's my understanding:

>From this, I can infer that ppc_n_lost_interrupts is used to
>"fake" a new interrupt at the end of an interrupt handling.

That's it.

>However,
>what I don't understand is - how could this possibly detect an
>interrupt that happens while the CPU is blocking external
>interrupt?
>In this case, ppc_n_lost_interrupts won't be incremented and when
>the external interrupt is re-enabled, we won't be able to get
>into interrupt mode.

The interrupt will be lost only when it's masked in the controller while
beeing issued by the device. It's a PIC bug.

So, we need to check it out only when unmasking the interrupt -in the
controller-. That's what we do in pmac-pic.c. If interrupts are masked on
the CPU but not on the controller, they are not lost. If they are masked
in both, it's not a problem neither, at least until someone tries to
umask one in the controller.

Note that our implementation if restore_flags() will check
lost_interrupts since the unmask call can be done while MSR_EE is 0.


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