From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sheng Yang Subject: Re: [PATCH] msi-x: let drivers retry when not enough vectors Date: Thu, 7 May 2009 18:28:32 +0800 Message-ID: <200905071828.33593.sheng@linux.intel.com> References: <20090507082841.GA31751@redhat.com> <20090507095302.GI8112@parisc-linux.org> <1241691830.24385.6.camel@concordia> Mime-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-15" Content-Transfer-Encoding: 7bit Cc: Matthew Wilcox , kvm@vger.kernel.org, "Michael S. Tsirkin" , jbarnes@virtuousgeek.org, linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org, Matthew Wilcox , virtualization@lists.linux-foundation.org To: michael@ellerman.id.au Return-path: In-Reply-To: <1241691830.24385.6.camel@concordia> Content-Disposition: inline Sender: linux-pci-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On Thursday 07 May 2009 18:23:50 Michael Ellerman wrote: > On Thu, 2009-05-07 at 03:53 -0600, Matthew Wilcox wrote: > > On Thu, May 07, 2009 at 05:40:15PM +0800, Sheng Yang wrote: > > > It's indeed weird. Why the semantic of pci_enable_msix can be changed > > > to "enable msix, or tell me how many vector do you have"? You can > > > simply call pci_msix_table_size() to get what you want, also without > > > any more work, no? I can't understand... > > > > Here's a good example. Let's suppose you have a driver which supports > > two different models of cards, one has 16 MSI-X interrupts, the other > > has 10. You can call pci_enable_msix() asking for 16 vectors. If your > > card is model A, you get 16 interrupts. If your card is model B, it says > > "you can have 10". > > > > This is less work in the driver (since it must implement falling back to > > a smaller number of interrupts *anyway*) than interrogating the card to > > find out how many interrupts there are, then requesting the right number, > > and still having the fallback path which is going to be less tested. > > Not to mention that there's no guarantee that you'll get as many > interrupts as the device supports, so you should really be coding to > cope with that anyway. Like the example in MSI-HOWTO.txt: > > 197 static int foo_driver_enable_msix(struct foo_adapter *adapter, int > nvec) 198 { > 199 while (nvec >= FOO_DRIVER_MINIMUM_NVEC) { > 200 rc = pci_enable_msix(adapter->pdev, > 201 adapter->msix_entries, nvec); > 202 if (rc > 0) > 203 nvec = rc; > 204 else > 205 return rc; > 206 } > 207 > 208 return -ENOSPC; > 209 } > > So I agree, this patch is an improvement. > Oh yeah. Forgot irq counts can also be changed from time to time. OK, there should be a loop, so that's fine. :) -- regards Yang, Sheng