[LinuxBIOS] about cs5536 interrupt ack

Maciej W. Rozycki macro at linux-mips.org
Wed Jul 11 15:24:41 CEST 2007

On Wed, 11 Jul 2007, Songmao Tian 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
> command:(

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

 BTW, I have just found a bug (OK, a misfeature, perhaps) in 
include/asm-mips/i8259.h. ;-)  I'll cook a patch.

> 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?

 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.

> 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...


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