Re: RFC: add MSI/MSI-X support to NetBSD

[Matt Thomas <matt%3am-software.com@localhost>]

On Aug 29, 2014, at 1:27 PM, David Young <dyoung%pobox.com@localhost> wrote:

> bus_msi(9) gives MI code access to doorbells: MI code uses it to
> establish a doorbell -> interrupt handler mapping and find out the
> doorbell's physical address.
> 
> All the code to map the doorbell's physaddr into a PCI busaddr, to
> program the IOMMU if there is one, to establish the MSI address/data in
> the PCI device, and to enable MSI is MI code using bus_dma(9), pci(9),
> and bus_space(9).  Even if it's 100 lines or fewer, why duplicate it
> across platforms?
> 
> Also, doorbells look to me like a potentially useful facility to make
> generally available, even apart from their use with PCI MSI.  Anyway,
> I'm curious what uses people would come up with.

Most non-X86 MSI implementations can't use them since their MSIs don't
work that way.  They need to write a specific h/w address with a 
particular value to trigger an MSI.  That address may be PCI attachment
specific.