Should message signaled interrupts (MSI) work with Intel GMA 950?

[Jesse Barnes]

On Tuesday, September 4, 2007 1:56 pm Roland Dreier wrote:
> I have a Lenovo X60s laptop with Intel GMA 950 (945GM) graphics, and
> for the fun of it I decided to try enabling message signaled
> interrupts for the graphics controller in the i915 drm module, since
> the internal graphics device's PCI header contains an MSI capability.
> (And MSI is nice with powertop, because it avoids shared interrupts
> that make it impossible to see who is really causing a wakeup)
>
> Unfortunately, although my patch seems to work enough so MSI gets
> enabled, and X also seems to work OK, I don't see any interrupts.
> Using 2.6.23-rc5 without the patch (so using INTx) I see a steady
> stream of (vblank?)  interrupts.  (My patch testing is on top of an
> otherwise completely vanilla 2.6.23-rc5 kernel)
>
> Does anyone (probably from Intel) know whether MSI is supposed to
> work with this graphics controller and if there's something else that
> needs to be poked to get it to work?  I couldn't find anything in the
> 945GM datasheet or spec update that indicated that MSI was broken,
> but I also couldn't find a definitive statement in the datasheet that
> said that MSI is supposed to work for the internal graphics.

We don't know of any interrupt breakage related to MSI, but OTOH we 
haven't tested it either.  For sending vblanks, it doesn't seem like 
there's much of an advantage to using MSIs, but (assuming they work) if 
we modified the driver to use MSIs for other events it could simplify 
the interrupt handler a bit, which might be nice...

Jesse