Re: [PATCH] ALSA: hda-intel - Disable MSI support by default

From: Jeff Garzik
Date: Tue Nov 14 2006 - 23:56:26 EST


Roland Dreier wrote:
> 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.

Huh? The device can't generate any legacy interrupts once MSI is
enabled. As the PCI spec says:

"While enabled for MSI or MSI-X operation, a function is prohibited
from using its INTx# pin (if implemented) to request service (MSI,
MSI-X, and INTx# are mutually exclusive)."

Although the MSI core does do pci_intx() for PCIe devices only, for
some reason I can't grok.

Probably the same reason I just mentioned in a reply to DaveM. Wildly speculating, some chips may depend on software to disable INTX -- i.e. depend on software to provide this aspect of PCI spec compliance.


> 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')

Actually NVidia/AMD was working on some systems long before that -- I
had it working at least 2 years ago.

Well, just noting that AMD/NV seems to be a common pattern in the referenced bug reports, and the set of Linux platforms which claim to support MSI had recent churn.

Jeff


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