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

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Message-ID: <455A9DE2.8080908@garzik.org>
Date:	Tue, 14 Nov 2006 23:56:02 -0500
From:	Jeff Garzik <jeff@...zik.org>
To:	Roland Dreier <rdreier@...co.com>
CC:	Linus Torvalds <torvalds@...l.org>,
	David Miller <davem@...emloft.net>,
	linux-kernel@...r.kernel.org, tiwai@...e.de
Subject: Re: [PATCH] ALSA: hda-intel - Disable MSI support by default

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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