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SubjectRe: [PATCH] ALSA: hda-intel - Disable MSI support by default
Linus Torvalds wrote:
> HOWEVER - that's only true on systems with no other PCI bridges. Even if
> you have an Intel NB/SB, what about other bridges in that same system, and
> the devices behind them?
>
> Now, I think that a MSI thing should look like a PCI write to a magic
> address (I'm really not very up on it, so correct me if I'm wrong), and
> thus maybe bridges are bound to get it right, and the only thing we really
> need to worry about is the host bridge. Maybe. In that case, it might be
> sensible to have a host-bridge white-table, and if we know all Intel
> bridges that claim to support MSI do so correctly, then maybe we can just
> say "ok, always enable it for Intel host bridges".

That's pretty much the idea behind MSI... it looks like any other PCI
bus transaction, rather than needing a separate pin.


> But right now I'm not convinced we really know what all goes wrong. Maybe
> it's just broken NVidia and AMD bridges. But maybe it's also individual
> devices that continue to (for example) raise _both_ the legacy IRQ line
> _and_ send an MSI request.

That reminds me of a potential driver bug -- MSI-aware drivers need to
call pci_intx(pdev,0) to turn off the legacy PCI interrupt, before
enabling MSI interrupts.

So far, MSI history on x86 has always followed these rules:
* it works on Intel
* it doesn't work [well | at all] on AMD/NV

The only thing that has changed recently is that people are trying to
get it working on AMD/NV as well. (Brice Goglin's stuff starting at
6397c75cbc4d7dbc3d07278b57c82a47dafb21b5 in 'git log')

Seems to me this hda-intel driver patch is fallout from
pci_msi_enabled() not failing on enough systems (all AMD/NV?).

Anyway, if you want your master switch, put it there, not in each driver...

Jeff


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