linux-kernel - Re: [PATCH 5/7] I/OAT: Add support for MSI and MSI-X

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Date:	Sat, 21 Jul 2007 01:59:01 +0400
From:	"Manu Abraham" <abraham.manu@...il.com>
To:	"Nelson, Shannon" <shannon.nelson@...el.com>
Cc:	"Roland Dreier" <rdreier@...co.com>,
	"Leech, Christopher" <christopher.leech@...el.com>,
	"Waskiewicz Jr, Peter P" <peter.p.waskiewicz.jr@...el.com>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	davem@...emloft.net, jeff@...zik.org,
	"Williams, Dan J" <dan.j.williams@...el.com>
Subject: Re: [PATCH 5/7] I/OAT: Add support for MSI and MSI-X

On 7/21/07, Nelson, Shannon <shannon.nelson@...el.com> wrote:
> Manu Abraham [mailto:abraham.manu@...il.com]
> >Sorry for being not clear. What i was asking is thus:
> >
> >A device that has legacy interrupts and MSI-X. I was thinking that if
> >MSI-X failed one should fall back to MSI mode (single message), that's
> >what i was assuming.
> >
> >In such a case, i do enable MSI-X mode on the device, when the request
> >for 2 ^ n  number of messages (where messages can be a max of 32), If
> >the request fails one falls into a single message mode, ie MSI ?
> >
> >
> >> What device do you have in mind?
> >
> >
> >The device that i have in mind is a SAA7160.
>
> Notice our code looks at the return from pci_enable_msix() - it will
> give you a hint whether MSI-X is not supported (returns < 0) or you
> simply asked for too many (returns > 0).  If the former, then fallback
> to legacy; if the latter, try MSI-X with only one interrupt, which
> essentially emulates MSI mode.

Ok. Thanks for clearing it up. So the idea would be that if
pci_enable_msix() for "n" number of messages failed, then settle down
for 1 message, which is equivalent to MSI mode
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