From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Tue, 1 Oct 2013 17:35:48 +1000 From: Michael Ellerman To: Tejun Heo Subject: Re: [PATCH v2 2/6] PCI/MSI: Factor out pci_get_msi_cap() interface Message-ID: <20131001073548.GI17966@concordia> References: <20130905185440.GA13175@dhcp-26-207.brq.redhat.com> <20130905200608.GA3846@htj.dyndns.org> <20130906160621.GF22763@mtj.dyndns.org> <20130906233205.GF12956@google.com> <20130909152044.GA24962@dhcp-26-207.brq.redhat.com> <20130916102210.GA14102@dhcp-26-207.brq.redhat.com> <20130917143022.GA7707@concordia> <20130918094759.GA2353@dhcp-26-207.brq.redhat.com> <20130918142231.GA21650@mtj.dyndns.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20130918142231.GA21650@mtj.dyndns.org> Cc: Joerg Roedel , "x86@kernel.org" , "linux-kernel@vger.kernel.org" , "linux-ide@vger.kernel.org" , Alexander Gordeev , Jan Beulich , "linux-pci@vger.kernel.org" , Bjorn Helgaas , 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 Wed, Sep 18, 2013 at 09:22:31AM -0500, Tejun Heo wrote: > Hello, > > On Wed, Sep 18, 2013 at 11:48:00AM +0200, Alexander Gordeev wrote: > > On Wed, Sep 18, 2013 at 12:30:23AM +1000, Michael Ellerman wrote: > > > How about no? > > > > > > 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. > > > > Out of curiosity - how pSeries has had done it without quotas before > > 448e2ca ("powerpc/pseries: Implement a quota system for MSIs")? > > Hmmm... do we need to treat this any differently? If the platform > can't allocate full range of requested MSIs, just failing should be > enough regardless of why such allocation can't be met, no? > > > > Anyway I don't see what problem you're trying to solve? I agree the > > > -ve/0/+ve return value pattern is ugly, but it's hardly the end of the > > > world. > > > > Well, the interface recently has been re-classified from "ugly" to > > "unnecessarily complex and actively harmful" in Tejun's words ;) > > LOL. :) > > > Indeed, I checked most of the drivers and it is incredible how people > > are creative in misusing the interface: from innocent pci_disable_msix() > > calls when if pci_enable_msix() failed to assuming MSI-Xs were enabled > > if pci_enable_msix() returned a positive value (apparently untested). > > > > Roughly third of the drivers just do not care and bail out once > > pci_enable_msix() has not succeeded. Not sure how many of these are > > mandated by the hardware. > > Yeah, I mean, this type of interface is a trap. People have to > actively resist to avoid doing silly stuff which is a lot to ask. I really think you're overstating the complexity here. Functions typically return a boolean -> nothing to see here This function returns a tristate value -> brain explosion! > > /* > > * Retrieving 'nvec' by means other than pci_msix_table_size() > > */ > > > > rc = pci_get_msix_limit(pdev); > > if (rc < 0) > > return rc; > > > > /* > > * nvec = min(rc, nvec); > > */ > > > > for (i = 0; i < nvec; i++) > > msix_entry[i].entry = i; > > > > rc = pci_enable_msix(pdev, msix_entry, nvec); > > if (rc) > > return rc; > > I really think what we should do is > > * Determine the number of MSIs the controller wants. Don't worry > about quotas or limits or anything. Just determine the number > necessary to enable enhanced interrupt handling. > > * Try allocating that number of MSIs. If it fails, then just revert > to single interrupt mode. It's not the end of the world and mostly > guaranteed to work. Let's please not even try to do partial > multiple interrupts. I really don't think it's worth the risk or > complexity. It will potentially break existing setups on our hardware. Can I make that any clearer? cheers