From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from ozlabs.org (ozlabs.org [103.22.144.67]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id E29051A0003 for ; Tue, 8 Jul 2014 14:01:34 +1000 (EST) Message-ID: <1404792090.26459.1.camel@concordia> Subject: Re: [PATCH 1/3] PCI/MSI: Add pci_enable_msi_partial() From: Michael Ellerman To: Bjorn Helgaas Date: Tue, 08 Jul 2014 14:01:30 +1000 In-Reply-To: <20140702202201.GA28852@google.com> References: <4fef62a2e647a7c38e9f2a1ea4244b3506a85e2b.1402405331.git.agordeev@redhat.com> <20140702202201.GA28852@google.com> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Cc: linux-mips@linux-mips.org, linux-s390@vger.kernel.org, linux-doc@vger.kernel.org, linux-pci@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org, linux-ide@vger.kernel.org, iommu@lists.linux-foundation.org, Alexander Gordeev , xen-devel@lists.xenproject.org, linuxppc-dev@lists.ozlabs.org List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Wed, 2014-07-02 at 14:22 -0600, Bjorn Helgaas wrote: > On Tue, Jun 10, 2014 at 03:10:30PM +0200, Alexander Gordeev wrote: > > There are PCI devices that require a particular value written > > to the Multiple Message Enable (MME) register while aligned on > > power of 2 boundary value of actually used MSI vectors 'nvec' > > is a lesser of that MME value: > > > > roundup_pow_of_two(nvec) < 'Multiple Message Enable' > > > > However the existing pci_enable_msi_block() interface is not > > able to configure such devices, since the value written to the > > MME register is calculated from the number of requested MSIs > > 'nvec': > > > > 'Multiple Message Enable' = roundup_pow_of_two(nvec) > > For MSI, software learns how many vectors a device requests by reading > the Multiple Message Capable (MMC) field. This field is encoded, so a > device can only request 1, 2, 4, 8, etc., vectors. It's impossible > for a device to request 3 vectors; it would have to round up that up > to a power of two and request 4 vectors. > > Software writes similarly encoded values to MME to tell the device how > many vectors have been allocated for its use. For example, it's > impossible to tell the device that it can use 3 vectors; the OS has to > round that up and tell the device it can use 4 vectors. > > So if I understand correctly, the point of this series is to take > advantage of device-specific knowledge, e.g., the device requests 4 > vectors via MMC, but we "know" the device is only capable of using 3. > Moreover, we tell the device via MME that 4 vectors are available, but > we've only actually set up 3 of them. > > This makes me uneasy because we're lying to the device, and the device > is perfectly within spec to use all 4 of those vectors. If anything > changes the number of vectors the device uses (new device revision, > firmware upgrade, etc.), this is liable to break. It also adds more complexity into the already complex MSI API, across all architectures, all so a single Intel chipset can save a couple of MSIs. That seems like the wrong trade off to me. cheers