[LinuxBIOS] about cs5536 interrupt ack

Sergei Shtylyov sshtylyov at ru.mvista.com
Wed Jul 11 17:42:51 CEST 2007


Maciej W. Rozycki wrote:

>>"Control Logic
>>The INT output goes directly to the CPU interrupt input.
>>When an INT signal is activated, the CPU responds with an
>>Interrupt Acknowledge access that is translated to two
>>pulses on the INTA input of the PIC. At the first INTA pulse,
>>the highest priority IRR bit is loaded into the corresponding
>>ISR bit, and that IRR bit is reset. The second INTA pulse
>>instructs the PIC to present the 8-bit vector of the interrupt
>>handler onto the data bus."

>>Is it the responsibility of north bridge to reponse to intr with a PCI
>>Interrupt Ack cycle?

>  With an i386 system such a pair of INTA cycles would be generated by the 
> CPU itself and translated by the north bridge to a PCI Interrupt 
> Acknowledge cycle (see the PCI spec for a more elaborate description).

>  If the CPU does not generate INTA cycles, it is a common practice to let 
> it ask the north bridge for a PCI Interrupt Acknowledge in some other way, 
> typically by issuing a read cycle that returns the vector reported by the 
> interrupt controller.

>>it's a problem that my northbridge didn't implement that! Fortunately we use a
>>fpga as a northbridge.

>>it seem it's no way to fix this by software, for OCW3 didn't implemnt Poll

>  Huh?  Have you managed to find an 8259A clone *that* broken?  So what 

    It's not such a problem, believe me. ;-)
    Some PPC boards use such clones -- you can see the comment in 

> does it return if you write 0xc to the address 0x20 in the I/O port space 
> and then read back from that location?  You should complain to the 
> manufacturer -- they may be able to fix the problem in a later revision.

    Haha, here's an excerpt form CS5535 spec. update:

96. PIC does not support Polling mode


Implications: This mode is not normally used in x86 systems.
Resolution: None.

>>so I guess the the process is:
>>1) 8259 receive a int, a bit irr got set.
>>2) 8259 assert intr.
>>3) northbrige generate a int ack cycle.
>>4) cs5536 translate the ack into two INTA pulse, and the reponse northbridge
>>with a interrupt vector.
>>5) then my program can get the vector from northbridge?

>>Is that right?

    Indeed, this would seem right but one step skipped -- where CPU tells 
northbridge that it's accepted an interrupt (via INTA).

>  More or less -- 3-5 should probably be the outcome of a single read 
> transaction from the north bridge.  I.e. you issue a read to a "magic" 
> location, 3-5 happen, and the data value returned is the vector presented 
> by the interrupt controller on the PCI bus.

    Yeah, another way of doing the missed step.

>>Without int ack, generic linux-mips 8259 code can't work.

>  You can still dispatch interrupts manually by examining the IRR register, 
> but having a way to ask the 8259A's prioritiser would be nice.  Although 
> given such a lethal erratum you report I would not count on the 
> prioritiser to provide any useful flexibility...

    Why not? AMD just decided not to implement poll mode, that's all.

>   Maciej

WBR, Sergei

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