Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754521AbaLHHOI (ORCPT ); Mon, 8 Dec 2014 02:14:08 -0500 Received: from mail-wg0-f43.google.com ([74.125.82.43]:65533 "EHLO mail-wg0-f43.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754457AbaLHHOD (ORCPT ); Mon, 8 Dec 2014 02:14:03 -0500 Message-ID: <54854FB2.8080206@linaro.org> Date: Mon, 08 Dec 2014 08:13:54 +0100 From: Tomasz Nowicki User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.2.0 MIME-Version: 1.0 To: Arnd Bergmann , Bjorn Helgaas CC: linux-arm-kernel@lists.infradead.org, Myron Stowe , rjw@rjwysocki.net, lorenzo.pieralisi@arm.com, linaro-acpi@lists.linaro.org, linux-pci , catalin.marinas@arm.com, x86 , will.deacon@arm.com, LKML , "linux-acpi@vger.kernel.org" , Ingo Molnar , hanjun.guo@linaro.org, "H. Peter Anvin" , wangyijing@huawei.com, Liviu.Dudau@arm.com, Thomas Gleixner Subject: Re: [PATCH 6/6] pci, acpi: Share ACPI PCI config space accessors. References: <1416413091-13452-1-git-send-email-tomasz.nowicki@linaro.org> <3957412.IooRKpIXjW@wuerfel> <20141121180825.GD6578@google.com> <2178540.dTppAIY2rE@wuerfel> In-Reply-To: <2178540.dTppAIY2rE@wuerfel> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org W dniu 24.11.2014 o 11:41, Arnd Bergmann pisze: > On Friday 21 November 2014 11:08:25 Bjorn Helgaas wrote: >> On Fri, Nov 21, 2014 at 01:24:52PM +0100, Arnd Bergmann wrote: >>> On Thursday 20 November 2014 21:00:17 Myron Stowe wrote: >>>> On Thu, Nov 20, 2014 at 3:26 PM, Bjorn Helgaas wrote: >> >>>>> That's interesting. I would have said exactly the opposite -- I think the >>>>> extra Kconfiggery is harder to follow than weak/strong functions >>>>> >>>>> But consistency is better than my personal opinion. Is there a consensus >>>>> that we should use the Kconfig strategy instead of __weak? >>>> >>>> I too find weak/strong functions easier to follow than "Kconfiggery" (nice term >>>> invention there). >>> >>> I don't think there is a universal consensus, but the majority of >>> maintainers seems to avoid them for the same reasons that I think >>> __weak is problematic. >>> >>> We have some uses of __weak in the core kernel, but there is >>> basically none in drivers outside of PCI, and the most common >>> uses are all providing an empty __weak function that can be >>> overridden with a function that actually does something, unlike >>> the code above. >> >> One thing I like better about __weak (when used correctly) is that you have >> exactly one declaration, and the role of each definition (weak default >> implementation or strong override) is obvious from looking at it. > > Right. > >> In your #ifdef example, the extern declaration and the inline definition >> are never compiled together, so you have to repeat the signature and the >> compiler doesn't enforce that they match. So you end up with the extern >> and the inline in one file, a #define in an arch header file or Kconfig, >> and an arch definition in a third file. >> >> But it's certainly true that everybody knows how #ifdef works, and the fact >> that __weak on a declaration affects all in-scope definitions is definitely >> a land mine (multiple weak definitions with no strong one is a disaster). >> >>> My pragmatic approach so far has been to advocate __weak for >>> drivers/pci patches but discourage it elsewhere when I review >>> patches, in order to maintain consistency. I also think it >>> would be nice to change the way that PCI handles architecture >>> specific overrides in the process of unifying the host bridge >>> handling. >>> >>> I wouldn't use Kconfig symbols in most cases though. My preferred >>> choice would be to turn a lot of the __weak symbols into function >>> pointers within a per-hostbridge structure. As an example, we could >>> replace pcibios_add_device() with a pointer in pci_host_bridge->ops >>> that gets set by all the architectures and host drivers that currently >>> override it, and replace the one caller with >>> >>> if (pci_host_bridge->ops->add_device) >>> pci_host_bridge->ops->add_device(dev); >> >> I definitely agree with this part, but I think it's orthogonal to the >> __weak question. In this case, we'd like to support multiple host bridges, >> each with a different flavor of add_device(). We can't do that at all with >> either __weak or #ifdef. > > What we currently have though is a a __weak definition of add_device, > which some architectures override, and some of them (ARM in particular) > by implementing their own abstraction. I suspect for the majority of > what we currently define as __weak functions, we could use a similar > approach and kill off the global symbols entirely. What would be next steps regarding this patch set? I am not sure we have reached a consensus on weak vs #ifdef choice. Regards, Tomasz -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/