From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pz0-f46.google.com ([209.85.210.46]:50841 "EHLO mail-pz0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756703Ab2EDBSC (ORCPT ); Thu, 3 May 2012 21:18:02 -0400 Received: by dady13 with SMTP id y13so2668427dad.5 for ; Thu, 03 May 2012 18:18:02 -0700 (PDT) Message-ID: <4FA32E47.7020406@gmail.com> Date: Thu, 03 May 2012 18:17:59 -0700 From: David Daney MIME-Version: 1.0 To: Bjorn Helgaas CC: John Crispin , Ralf Baechle , Grant Likely , linux-pci@vger.kernel.org, devicetree-discuss@lists.ozlabs.org, linux-mips@linux-mips.org Subject: Re: [PATCH] OF: PCI: const usage needed by MIPS References: <1335808019-24502-1-git-send-email-blogic@openwrt.org> <4F9ED1DC.3050007@gmail.com> <4F9FE4F6.5070909@openwrt.org> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Sender: linux-pci-owner@vger.kernel.org List-ID: On 05/03/2012 05:30 PM, Bjorn Helgaas wrote: > On Tue, May 1, 2012 at 7:28 AM, John Crispin wrote: >> On 30/04/12 19:54, David Daney wrote: >>> On 04/30/2012 10:46 AM, John Crispin wrote: >>>> On MIPS we want to call of_irq_map_pci from inside >>>> >>>> arch/mips/include/asm/pci.h:extern int pcibios_map_irq( >>>> const struct pci_dev *dev, u8 slot, u8 pin); >>>> >>>> For this to work we need to change several functions to const usage. >>> >>> I think there is a mismatch on this throughout the kernel. >>> >>> Properly fixing it requires touching many more places than these. >>> Although I haven't tried it, I wouldn't be surprised if doing this >>> caused warnings to appear in non-MIPS code. >>> >>> Ralf had a patch at one point that tried to make this consistent >>> tree-wide, but it is not yet applied. >>> >>> David Daney >> >> Hi, >> >> Ok, lets see what Ralf has to say. >> >> I just tested the patch on x86 with OF enabled and drivers turned on >> that use the API. I did not see any errors appear. > > I'm far from a const expert, but I think this should be safe. > Here's my reasoning: > > We're changing pci_swizzle_interrupt_pin() to take a pointer to a > constant struct pci_dev. pci_swizzle_interrupt_pin() only reads the > struct pci_dev; it doesn't modify it. It is legal to pass either > "struct pci_dev *" or "const struct pci_dev *" to a function expecting > "const struct pci_dev *"; the callee just won't be able to modify the > struct, even if the caller can. > The problem is when you start declaring function pointers in various ops vectors. Consider: void (*foo)(const struct pci_dev *) void (*bar)(struct pci_dev *) foo and bar are not type compatible, and you will get compiler warnings if you use one where the other is expected. So the question is: Are we ever going to the address of any of the functions that are being modified? If so, we have created a problem. > Similar reasoning applies to of_irq_map_pci(). > > So I'm fine with this. You sent it to Grant, so I'll assume he'll > merge it unless I hear otherwise. > > Acked-by: Bjorn Helgaas >