From mboxrd@z Thu Jan 1 00:00:00 1970 From: Arjan van de Ven Subject: Re: hwo to adjust interrupt? Date: Fri, 12 Aug 2005 19:51:10 +0200 Message-ID: <1123869071.3218.35.camel@laptopd505.fenrus.org> References: Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Return-path: Received: from pentafluge.infradead.org ([213.146.154.40]:25319 "EHLO pentafluge.infradead.org") by vger.kernel.org with ESMTP id S1750760AbVHLRvQ (ORCPT ); Fri, 12 Aug 2005 13:51:16 -0400 In-Reply-To: Sender: linux-scsi-owner@vger.kernel.org List-Id: linux-scsi@vger.kernel.org To: Bryan Henderson Cc: linux-scsi , mingz@ele.uri.edu On Fri, 2005-08-12 at 10:35 -0700, Bryan Henderson wrote: > >>How much extra work does Linux have to do for each interrupt? > > > >usually 1 pci mmio read; the rest is negligible. > > I was hoping you would cater better to my ignorance of how PCI interrupt > handling works in Linux. > > Is it the case that Linux invokes the registered interrupt handler of each > of the drivers for the devices that share the interrupt, and each does an > mmio read of its device to find out if it had reason to generate an > interrupt? So the waste is that extra call, and you're saying the CPU > instructions involved are negligible compared to the mmio read? yes. A function call is like half a cycle (a function pointer call is maybe 40) an mmio read is a lot more > Are these level-sensitive interrupts, so that if both devices need service > at the same time, they generate just one interrupt and neither device > driver call is wasted? ok this is more complex, but if 2 cards raise it quickly after eachother, before the ISR has run, then you only get the handler called once afaik.