linux-kernel - Re: Linux 2.6.20-rc6

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Date:	Tue, 30 Jan 2007 02:39:11 -0500
From:	Jeff Garzik <jeff@...zik.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Stephen Hemminger <shemminger@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 2.6.20-rc6 - sky2 resume breakage

Ingo Molnar wrote:
> i'm wondering, could we go with Thomas' temporary patch that disables 
> sky2 MSI if CONFIG_PM is enabled - we could revert that after 2.6.20. 
> It's not like MSI is a life and death feature. On IO-APIC systems 
> vectors are abundant and in any case we share irqs just fine. The true 
> advantage of MSI is minimal. (MSI-X has the potential to be better by 
> being message based, but in reality it still goes through the full IRQ 
> layer.) MSI might be useful on really, really large systems - but i 
> really hope those really large systems dont rely on CONFIG_PM. Meanwhile 
> Thomas' patch maximizes the amount of working hardware (it has the 
> chance to produce working systems in 100% of the cases) - which is a few 
> orders of magnitude more important than IRQ management micro-costs. Am i 
> missing anything?


Sharing irqs /sucks/.  I routinely have to fight a USB device dying, 
because the ATA device is causing an interrupt storm, or vice versa. 
/Very/ common headache.

Other than that, they use a tiny bit fewer CPU cycles, and allow 
simplification of the interrupt handler (saving another few CPU cycles).

The biggest benefit is (a) for hardware designers, where MSI means a 
cleaner h/w design, and (b) preparation of drivers and the kernel 
systems for MSI-only hardware.

At present only high end hardware is MSI-only (like infiniband), but 
that's the future direction.

	Jeff


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