From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Tue, 1 Oct 2013 17:19:55 +1000 From: Michael Ellerman To: Alexander Gordeev Subject: Re: [PATCH v2 2/6] PCI/MSI: Factor out pci_get_msi_cap() interface Message-ID: <20131001071954.GG17966@concordia> References: <20130918142231.GA21650@mtj.dyndns.org> <20130918165045.GB2353@dhcp-26-207.brq.redhat.com> <20130920082458.GA10507@dhcp-26-207.brq.redhat.com> <20130920122736.GD7630@mtj.dyndns.org> <20130925180220.GB26273@google.com> <20130925205804.GA21737@dhcp-26-207.brq.redhat.com> <20130925210016.GA8926@htj.dyndns.org> <20130926074646.GA16774@dhcp-26-207.brq.redhat.com> <20130926131147.GA31249@mtj.dyndns.org> <20130926143901.GE16774@dhcp-26-207.brq.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20130926143901.GE16774@dhcp-26-207.brq.redhat.com> Cc: "linux-ide@vger.kernel.org" , "linux-pci@vger.kernel.org" , Joerg Roedel , "x86@kernel.org" , "linux-kernel@vger.kernel.org" , Bjorn Helgaas , Jan Beulich , Tejun Heo , linuxppc-dev@lists.ozlabs.org, Ingo Molnar List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Thu, Sep 26, 2013 at 04:39:02PM +0200, Alexander Gordeev wrote: > On Thu, Sep 26, 2013 at 09:11:47AM -0400, Tejun Heo wrote: > > > Because otherwise we will re-introduce a problem described by Michael: > > > "We have a small number of MSIs available, limited by hardware & > > > firmware, if we don't impose a quota then the first device that probes > > > will get most/all of the MSIs and other devices miss out." > > > > Still not following. Why wouldn't just letting the drivers request > > the optimal number they want and falling back to single interrupt mode > > work? ie. why can't we just have an all or nothing interface? > > I can imagine a scenario where the first device probes in, requests its > optimal number, acquires that number and exhausts MSIs in pSeries firmware. > The next few devices possibly end up with single MSI, since no MSIs left > to satisfy their optimal numbers. If one of those single-MSI'ed devices > happened to be a high-performance HBA hitting a degraded performance that > alone would force (IBM) to introduce the quotas. Yes that's exactly the scenario, and I didn't imagine it, our test people actually hit it and yelled at me. I don't remember exactly which adapters it was, I might be able to find the details if I looked hard, a quick search through my mail archive didn't find it - it might have come in via irc / bugzilla etc. cheers