From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Wed, 2 Oct 2013 12:43:24 +1000 From: Michael Ellerman To: Alexander Gordeev Subject: Re: [PATCH v2 2/6] PCI/MSI: Factor out pci_get_msi_cap() interface Message-ID: <20131002024324.GC22748@concordia> References: <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> <20131001075133.GJ17966@concordia> <20131001103526.GA5053@dhcp-26-207.brq.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20131001103526.GA5053@dhcp-26-207.brq.redhat.com> Cc: linuxppc-dev@lists.ozlabs.org, Joerg Roedel , "x86@kernel.org" , "linux-kernel@vger.kernel.org" , "linux-ide@vger.kernel.org" , Jan Beulich , "linux-pci@vger.kernel.org" , Tejun Heo , Bjorn Helgaas , Ingo Molnar List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Tue, Oct 01, 2013 at 12:35:27PM +0200, Alexander Gordeev wrote: > On Tue, Oct 01, 2013 at 05:51:33PM +1000, Michael Ellerman wrote: > > The disadvantage is that any restriction imposed on us above the quota > > can only be reported as an error from pci_enable_msix(). > > > > The quota code, called from pci_get_msix_limit(), can only do so much to > > interogate firmware about the limitations. The ultimate way to check if > > firmware will give us enough MSIs is to try and allocate them. But we > > can't do that from pci_get_msix_limit() because the driver is not asking > > us to enable MSIs, just query them. > > If things are this way then pci_enable_msix() already exposed to this > problem internally on pSeries. > > I see that even successful quota checks in rtas_msi_check_device() and > rtas_setup_msi_irqs() do not guarantee (as you say) that firmware will > give enough MSIs. Hence, pci_enable_msix() might fail even though the > its quota checks succeeded. Yes, but it can report that failure to the caller, which can then retry. > Therefore, nothing will really change if we make pci_get_msix_limit() check > quota and hope the follow-up call to pci_enable_msix() succeeded. No that's not equivalent. Under your scheme if pci_enable_msix() fails then the caller just bails, it will never try again with a lower number. > (Of course, we could allocate-deallocate MSIs at check time, but I think it > is an overkill). It's not only overkill, it's messing with the device behind the drivers back, which is definitely a no-no in my opinion. > > You'll also need to add another arch hook, for the quota check, and > > we'll have to add it to our per-platform indirection as well. > > Already, in a branch, hidden from Bjorn & Tejun eyes ;) > > > All a lot of bother for no real gain IMHO. > > Well, I do not have a strong opinion here. I leave it to the ones who have :) > But few drivers have became clearer as result of this change (and messy ones > are still messy). Amen. cheers