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SubjectRe: PC speaker driver patch for 2.4.0-test10-pre3
Date

kernel@blackhole.compendium-tech.com said:
> Apparently Linus doesn't like the way it handles interrupts or
> something, and is therefore 'wrong.' ::shrug:: As long as it's
> available though, I'll use it ;p

It's not just 'wrong'. It's 'sick'. But it works. The correct fix is to
shift the system timer onto the RTC, leaving the 8253 timer available for us
to abuse for the PC speaker.

See arch/i386/kernel/time.c:

/* This function must be called with interrupts disabled
* It was inspired by Steve McCanne's microtime-i386 for BSD. -- jrs
*
* However, the pc-audio speaker driver changes the divisor so that
* it gets interrupted rather more often - it loads 64 into the
* counter rather than 11932! This has an adverse impact on
* do_gettimeoffset() -- it stops working! What is also not
* good is that the interval that our timer function gets called
* is no longer 10.0002 ms, but 9.9767 ms. To get around this
* would require using a different timing source. Maybe someone
* could use the RTC - I know that this can interrupt at frequencies
* ranging from 8192Hz to 2Hz. If I had the energy, I'd somehow fix
* it so that at startup, the timer code in sched.c would select
* using either the RTC or the 8253 timer. The decision would be
* based on whether there was any other device around that needed
* to trample on the 8253. I'd set up the RTC to interrupt at 1024 Hz,
* and then do some jiggery to have a version of do_timer that
* advanced the clock by 1/1024 s. Every time that reached over 1/100
* of a second, then do all the old code. If the time was kept correct
* then do_gettimeoffset could just return 0 - there is no low order
* divider that can be accessed.
*
* Ideally, you would be able to use the RTC for the speaker driver,
* but it appears that the speaker driver really needs interrupt more
* often than every 120 us or so.
*
* Anyway, this needs more thought.... pjsg (1993-08-28)
*
* If you are really that interested, you should be reading
* comp.protocols.time.ntp!
*/


--
dwmw2


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