Hello Rene, Thanks for the reply. > On 19-08-08 07:11, Rajat Jain wrote: > >> I want to understand who decides the IRQ number that a agiven PCI >> card will use. I understand that from a PCI device drivers point of >> view, it'll find the IRQ vector that it needs to attach by reading >> it from the configuration space of the device. > > Actually, a quick look at struct pci_dev suggests you should > be using pci_dev->irq. Note sure why that is; maybe odd > bridged setups. > Where in source code is pci_dev->irq getting populated? I tried but could not exactly locate it... > >> 1) My first question is WHO writes that IRQ value in the device >> configuration space? Is it hardwired on the card? Is it written by >> the PCI controller driver or some other kernel component? Or some >> other piece of software takes care of it? > > The thing hardwired on the card is the interrupt PIN(s) it > uses; A, B, C and/or D (one pin per device and thus per > config space). The thing that writes the LINE value into the > register is generally the BIOS; being motherboard specific it > knows which IRQ line it is that pin X (A, B, C or D) from > slot N is routed to and it writes those values back into the device. > OK. But who does it in an embedded environment (PPC for eg) where there is no POST software. The first piece of code that gets executed is U-boot and then the kernel. So who writes the LINE value into the config space? > >> 2) Secondly, irrespective of whoever writes it, what determines the >> irq vector VALUE that will be written? So given that a PCI card uses >> PCI INTA, what determines the IRQ vector associated with it? My >> understanding is that the board specifications say something like >> "The INTA from this PCI slot goes to IRQx input of interrupt >> controller". And then from interrupt controller dosument we can find >> out which IRQ vector is associated with input IRQx. Is this right? > > That's the basic form but in reality things are not hardwired > on the motherboard either; there's a programmable interrupt > router (part of the PCI chipset) between the lines coming > from the bus and the lines going into the interrupt > controller that enables software control of the routing. > Thanks, I got it. > >> 3) Lastly, if we boot linux and a different OS, on the same board >> with the same PCI card plugged in, are they bound to use the same >> IRQ number? Why or why not? > > Not bound then. The programmable interrupt router could be > set up differently. Ok. Thanks. Thank you very much for the informative reply, Rajat -- To unsubscribe from this list: send the line "unsubscribe linux-newbie" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.linux-learn.org/faqs
