cvs commit: src/sys/i386/isa clock.c src/sys/amd64/isa clock.c

[Suleiman Souhlal]

Bruce Evans wrote:
> bde         2006-12-03 03:49:28 UTC
> 
>   FreeBSD src repository
> 
>   Modified files:
>     sys/i386/isa         clock.c 
>     sys/amd64/isa        clock.c 
>   Log:
>   Optimized RTC accesses by avoiding null writes to the index register
>   and by only delaying when an RTC register is written to.  The delay
>   after writing to the data register is now not just a workaround.
>   
>   This reduces the number of ISA accesses in the usual case from 4 to
>   1.  The usual case is 2 rtcin()'s for each RTC interrupt.  The index
>   register is almost always RTC_INTR for this.  The 3 extra ISA accesses
>   were 1 for writing the index and 2 for delays.  Some delays are needed
>   in theory, but in practice they now just slow down slow accesses some
>   more since almost eveyone including us does them wrong so modern systems
>   enforce sufficient delays in hardware.  I used to have the delays ifdefed
>   out, but with the index register optimization the delays are rarely
>   executed so the old magic ones can be kept or even implemented non-
>   magically without significant cost.
>   
>   Optimizing RTC interrupt handling is more interesting than it used to
>   be because RTC interrupts are currently needed to fix the more efficient
>   apic timer interrupts on some systems.  apic_timer_hz is normally 2000
>   so the RTC interrupt rate needs to be 2048 to keep the apic timer
>   firing on such systems.  Without these changes, each RTC interrupt
>   normally took 10 ISA accesses (2 PIC accesses and 2 sets of 4 RTC
>   accesses).  Each ISA access takes 1-1.5uS so 10 of then at 2048 Hz
>   takes 2-3% of a CPU.  Now 4 of them take 0.8-1.2% of a CPU.
>   

Why do we have both RTC and LAPIC timer interrupts at once?
Also, why does having the RTC interrupt rate being the same as the LAPIC timers'
prevent the latter from firing?

-- Suleiman