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Date: Fri, 27 Feb 2004 22:51:05 +0800
From: "Michael Frank" <mhf@linuxmail.org>
To: "Russell King" <rmk+lkml@arm.linux.org.uk>
Subject: Re: Why no interrupt priorities?
Cc: "Benjamin Herrenschmidt" <benh@kernel.crashing.org>,
       "Grover, Andrew" <andrew.grover@intel.com>,
       "Mark Gross" <mgross@linux.co.intel.com>, arjanv@redhat.com,
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On Fri, 27 Feb 2004 13:50:19 +0000, Russell King <rmk+lkml@arm.linux.org.uk> wrote:

> On Fri, Feb 27, 2004 at 09:31:43PM +0800, Michael Frank wrote:
>> On Fri, 27 Feb 2004 09:05:48 +0000, Russell King <rmk+lkml@arm.linux.org.uk> wrote:
>> > On Fri, Feb 27, 2004 at 02:26:31PM +0800, Michael Frank wrote:
>> >> Is this to imply that edge triggered shared interrupts are used anywhere?
>> >
>> > It is (or used to be) rather common with serial ports.  Remember that
>> > COM1 and COM3 were both defined to use IRQ4 and COM2 and COM4 to use
>> > IRQ3.
>> >
>> >> Never occured to me to use shared IRQ's edge triggered as this mode
>> >> _cannot_ work reliably for HW limitations.
>> >
>> > The serial driver takes great care with this - when we service such an
>> > interrupt, we keep going until we have scanned all the devices until
>> > such time that we can say "all devices are no longer signalling an
>> > interrupt".
>> >
>> > This is something it has always done - it's nothing new.
>> >
>>
>> Sorry, i think the serial driver IRQ is level triggered :)
>
> That's actually incorrect.  Serial devices are (were) connected to the
> old ISA PICs which are definitely edge triggered.
>

I was under the impression that the PIC's are historically set to
level triggered, certainly was the case with (IBM) PC's/AT's and
with embedded system I am working with.

At least it explains why I was never able to share IRQ's on hardware
with PIC's under linux.

Regards
Michael