[Open-graphics] Video interrupt enable/disable

[Paul Brook]

> > 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.
>
> We should not presume that the card will be plugged into low end
> hardware -- a cheep motherboard with AT style interrupt controllers.

Huh? So you're saying that we should produce a card that doesn't work on most 
commodity PC hardware? Including the chipset you mention below?

95% of drivers will be independent of the host IRQ routing. Se even if you 
happen to have a dedicated IRQ line, the driver will query the device 
interrupt state anyway.

I expect the efficiency benefits from having multiple host IRQ come because 
the OS only has to poll a small subset of devices.

Saving a few extra bus cycles from the final poll in the case of a dedicated 
IRQ line is probably going to be lost in the noise. I don't believe that a 
single read and conditional branch to figure out whether we're processing a 
sync or other service request is going to have any measurable benefit.

> My current entertainment is reading Intel: 31305602.pdf a southbridge
> chip.  So far, I find that it has *8* PCI interrupt inputs for 4 slots.

That's still sharing IRQ lines then. 4 slots give you a total of 16 PCI 
interrupt sources.

> I will probably read through this to get a better understanding of it,
> but it appears to me that we have no way of determining how many
> hardware vectored interrupts the card will have available.  So, I still
> suggest that we use 2 (or more) and the worst case is that they will all
> wind up connected to the same vector, but it would preform better if we
> could find more than 1.  This would also mean that at the PCI interface
> level that we need to be able to disable the interrupts individually for
> each IRQ output.

Single-function PCI cards may use a single IRQ pin. No more. This is 
explicitly specified by the PCI standard. A graphics card is a single function 
device. Except for SLI/Crossfire-like setups, which are just two devices in a 
single slot.

I guess we could expose the 3D and basic framebuffer components as separate 
PCI devices, but this requires implementing a pair of PCI interfaces on the 
card and complicates driver development for very marginal gain.

IIUC this thread started because someone wanted a vertical blank interrupt. 
When all said, 100Hz isn't a particularly high-frequency interrupt.
Also, if something is sufficiently latency sensitive to require a dedicated 
interrupt vector then a general purpose OS (windows, linux, BSD) isn't going 
to be sufficient either. You're going to need a specialised RTOS.

> Next question: does the Linux Kernel have support for APICs on Intel
> hardware?

Of course it does. This is a completely unrelated to the task of designing a 
PCI add-on card though.

Paul