On Sun, Feb 18, 2007 at 03:07:29PM +0100, Udo van den Heuvel wrote: > Is there some howto information available about using PCI riser cards > (with multiple PCI slots) under Linux? > Several incarnations exist of PCI riser cards with two PCI slots where > the Device Number of one slot can be changed. > How, based on what information, does Linux assign an IRQ to each card, > plugged into the riser? > How can one tweak/influence the irq routing? > How can I make a dual riser card work so that both cards have a working > interrupt? > Or if stuff should work all by itself, what could be wrong?
The PCI specifications cover how pci to pci bridges should work. My understanding (which is better of verified against the specs) is: PCI interrupts (PCI INTA to INTD) are rotated for every slot by one. So slot 0, 4, 8, etc see INTA->realINTA, INTB->realINTB. INTC->realINTC, INTD->realINTD slot 1, 5, 9, etc see INTA->realINTB, INTB->realINTC. INTC->realINTD, INTD->realINTA slot 2, 6, 10, etc see INTA->realINTC, INTB->realINTD. INTC->realINTA, INTD->realINTB slot 3, 7, 11, etc see INTA->realINTD, INTB->realINTA. INTC->realINTB, INTD->realINTC The same rules apply behind a PCI bridge, so whatever INTA is on the slot with the pci bridge chip, should make to INTA on slots 0, 4, 8, etc behind the bridge, while INTB is seen as INTA on slots 1, 5, 9, etc. On a PC, the BIOS is supposed to assign interrupts to devices based on those rules, since that is how the hardware must be done according to the PCI specifications. On other platforms the firmware may or may not handle it. On ARM for example, the kernel takes care of assigning this (or it least it should). I have seen pci interrupt swizzle routines in the kernel code for an arm system which went through, looking for devices, and assigning IRQs based on their slot number, and when it found a bridge it would do a mapping past the bridge and continue assigning interrupts to devices behind the bridge. So as long as the riser board is wired according to the PCI bridge rules, then interrupts assignment should simply work. Of course if the riser card is NOT a proper pci bridge, but rather some weird device, well then it probably isn't even PCI complient and who knows how it shold be handled. Interrupts for PCI are assigned based on the 4 shared interrupts line on PCI, not really per device. A PCI device may use up to 4 interrupts if it wants to, and is supposed to always use interrupt A (as seen from it's slot) as the first interrupt. The rotating assignment of the 4 interrupts to the slots are supposed to help balance the distribution of the interrupts between devices in the system, so that although they are shared interrupts, as few devices as possible get assigned to each interrupt. On some systems some of the 4 PCI interrupts may in fact get mapped to the same interrupts if not enough are available (for example a system I work with only has two PCI interrupts available, so PCI INTA and C are the same interrupt line, and INTB and D are the same, but the system is still wired as if there were 4 interrupts, and the BIOS assigns the interrupts properly based on that, but using IRQ 11 for A and C and IRQ 10 for B and D.) -- Len Sorensen - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
