Date: Thu, 10 Jan 2002 21:22:11 +0100 From: Nils Holland <nils@tisys.org> To: freebsd-hackers@freebsd.org Subject: Two devices, one Interrupt Message-ID: <20020110212211.A59757@tisys.org>
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Hi folks, I already posted a similar question to -hardware yesterday but received no answer, so I'm trying again on this list. If you look at the below excerpt of my dmesg.boot, you will see that my soundcard and Brooktree / WinTV card share one IRQ: pcm0: <VIA VT82C686A> port 0xe400-0xe403,0xe000-0xe003,0xdc00-0xdcff irq 5 at device 7.5 on pci0 bktr0: <BrookTree 878> mem 0xdc000000-0xdc000fff irq 5 at device 15.0 on pci0 As far as I have read, this is something that PCI devices can do. However, I've been wondering if there was any advantage of manually assigning a signle interrupt to both of these devices. As far as I have read (but I'm no expert, so I might be wrong), when two PCI devices share the same IRQ and the IRQ gets triggered, the IRQ handler for both devices is called. I've been wondering if this may have some performance implications or some other side effects, although I have seen none (i.e. nothing seems to change if I assign distinctive interrupts to the devices). The only thing I noticed is that if I run "systat -vm 1" while both devices share IRQ 5, I do only get one device entry for IRQ 5, labelled "mux", obviously named after the soundcard. This "mux" entry now seems to count both the interrupts for the sound card and the WinTV board. If, however, both cards use their own interrupts, systat shows two seperate entries, one labelled "mux" and only counting soundcard interrupts, and the other called "bktr0", only counting the interrupts for the WinTV board. That's however the only difference I've seen when comparing the cards with a shared interrupt vs. non-sharred interrupts, but I still wonder if there's any technical reason why it would be beneficial to give both cards their own interrupts, or if I just don't need to care. Any hints are welcome! Nils -- Nils Holland Ti Systems - FreeBSD in Tiddische, Germany http://www.tisys.org * nils@tisys.org To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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