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

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Date:	Tue, 14 Nov 2006 23:11:54 -0500
From:	Jeff Garzik <jeff@...zik.org>
To:	Linus Torvalds <torvalds@...l.org>
CC:	David Miller <davem@...emloft.net>, linux-kernel@...r.kernel.org,
	tiwai@...e.de
Subject: Re: [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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