linux-kernel - Re: Multiple MSI, take 3

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Message-Id: <20080713.174457.82768245.davem@davemloft.net>
Date:	Sun, 13 Jul 2008 17:44:57 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	benh@...nel.crashing.org
Cc:	ebiederm@...ssion.com, suresh.b.siddha@...el.com, matthew@....cx,
	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	grundler@...isc-linux.org, mingo@...e.hu, tglx@...utronix.de,
	jgarzik@...ox.com, linux-ide@...r.kernel.org,
	jbarnes@...tuousgeek.org, rdunlap@...otime.net,
	mtk.manpages@...il.com
Subject: Re: Multiple MSI, take 3

From: Benjamin Herrenschmidt <benh@...nel.crashing.org>
Date: Mon, 14 Jul 2008 10:17:39 +1000

> On Sun, 2008-07-13 at 16:29 -0700, Eric W. Biederman wrote:
> > Ben.  Multi-MSI is a crap hardware design.  Why do you think we have
> > MSI-X?  
> 
> I know and I agree. Which is why I'd rather keep the SW crap totally
> local to the MSI support code and not add new concepts to the generic
> IRQ API such as sub-channels, for which it's really not ready for imho.
> 
> They -are- separate IRQs, just badly implemented. Besides, a large part
> of the problem is purely due to the typical x86 implementation of them,
> since for example, on most PowerPC's (and possibly other archs), they
> tend to land in the PIC as normal sources, and as such benefit from all
> the "features" of such interrupts like HW masking, affinity control,
> etc... at the PIC level.

This is how it works on sparc64 too.

The x86 system designers decided to implement multi-MSI in an
inconvenient way, it is not a "crap hardware design", merely
some (unfortunately common) implementations of it happen to be.

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