Re: [PATCH] MSI interrupts: disallow when no LAPIC/IOAPIC support

[Grant Grundler <grundler@parisc-linux.org>]

On Mon, Sep 26, 2005 at 08:11:56PM -0700, Randy.Dunlap wrote:
> From: Randy Dunlap <rdunlap@xenotime.net>
> 
> MSI requires local APIC + IO APIC support (according to
> drivers/pci/Kconfig),

The purpose of MSI is to bypass the IO APIC.
I'm not aware of any dependency on IO APIC in HW.
I suspect the dependency in the Kconfig is historical
because of how the x86 source code is structured:
config X86_LOCAL_APIC
        bool
        depends on X86_UP_APIC || ((X86_VISWS || SMP) && !X86_VOYAGER)
        default y

essentially, LOCAL_APIC gets enabled if "UP_APIC or SMP".
I've no clue why folks thought it was better to ignore
the IO APIC on UP kernels.

I'm pretty sure one only needs a local APIC support.
Local APIC responds to 0xfee00000 range of addresses and
directs the interrupt to a CPU.


> but if a kernel is built with all of
> those (APICs + CONFIG_PCI_MSI) and then booted with "nosmp" or
> "max_cpus=0|1" or "noapic" or "nolapic", MSI also should be
> disabled, otherwise the interrupt routing is bad (so that only
> using "irqpoll" helps).

UP vs SMP is orthogonal to MSI support wrt HW.
I suppose this is another historical artifact of
APICs being used with SMP x86 boxes first.

thanks,
grant