[Open-graphics] Video interrupt enable/disable

[Paul Brook]

On Thursday 19 April 2007 00:47, James Richard Tyrer wrote:
> Paul Brook wrote:
> > Really? I'd expect everything to be a single PCI device. DMA controllers
> > only tend to exist as separate entities on systems where the normal
> > devices can't be bus-masters. You may implement it as a separate
> > functional block in the FPGA, but the host system doesn't know or care
> > about that.
>
> And exactly how will the host system determine if two interrupts come
> from one device or from two separate devices?

In general the only way a host can tell where an interrupt came from is to ask 
all the devices (in a device specific fashion) and see which one has an 
active interrupt.

On high-end systems you may have a 1:1 mapping from device output to host 
input IRQ lines, so can determine an IRQ source without having to poll the 
individual devices.  However as a device designer/driver writer we can not 
assume this is the case.

I'm using the term "device" in the same sense as the PCI spec does. 
While you could have separate PCI devices for the DMA and video components 
this would at best be a strange configuration. Each PCI device requires its 
own OS driver, so you would then need some additional mechanism for matching 
up and communicating betwee those guest OS drivers. PCI devices are generally 
logically independent devices, which may happen to be on the same physical 
card/IC.

Paul