linux-kernel - Re: [Xen-devel] [PATCH 09/11] swiotlb-xen: simplify cache maintainance

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Date:   Fri, 6 Sep 2019 15:07:42 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Christoph Hellwig <hch@....de>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
CC:     Stefano Stabellini <sstabellini@...nel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        <x86@...nel.org>, <linux-kernel@...r.kernel.org>,
        <iommu@...ts.linux-foundation.org>, <gross@...e.com>,
        <xen-devel@...ts.xenproject.org>,
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [Xen-devel] [PATCH 09/11] swiotlb-xen: simplify cache
 maintainance

On 06/09/2019 15:01, Christoph Hellwig wrote:
> On Fri, Sep 06, 2019 at 09:52:12AM -0400, Boris Ostrovsky wrote:
>> We need nop definitions of these two for x86.
>>
>> Everything builds now but that's probably because the calls are under
>> 'if (!dev_is_dma_coherent(dev))' which is always false so compiler
>> optimized is out. I don't think we should rely on that.
> That is how a lot of the kernel works.  Provide protypes only for code
> that is semantically compiled, but can't ever be called due to
> IS_ENABLED() checks.  It took me a while to get used to it, but it
> actually is pretty nice as the linker does the work for you to check
> that it really is never called.  Much better than say a BUILD_BUG_ON().

Yeah - its a weird concept to get used to, but it results in much
clearer code.

~Andrew