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Date: Mon, 28 Apr 2025 15:20:06 +0200
From: Christoph Hellwig <hch@lst.de>
To: Leon Romanovsky <leon@kernel.org>
Cc: Luis Chamberlain <mcgrof@kernel.org>,
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Subject: Re: [PATCH v9 07/24] dma-mapping: Implement link/unlink ranges API
Message-ID: <20250428132006.GA30966@lst.de>
References: <cover.1745394536.git.leon@kernel.org> <2d6ca43ef8d26177d7674b9e3bdf0fe62b55a7ed.1745394536.git.leon@kernel.org> <aA1iRtCsPkuprI-X@bombadil.infradead.org> <20250427081312.GE5848@unreal>
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In-Reply-To: <20250427081312.GE5848@unreal>
User-Agent: Mutt/1.5.17 (2007-11-01)

On Sun, Apr 27, 2025 at 11:13:12AM +0300, Leon Romanovsky wrote:
> > So arch_sync_dma_for_device() is a no-op on some architectures, notably x86.
> > So since you're doing this work and given the above pattern is common on
> > the non iova case, we could save ourselves 2 branches checks on x86 on
> > __dma_iova_link() and also generalize savings for the non-iova case as
> > well. For the non-iova case we have two use cases, one with the attrs on
> > initial mapping, and one without on subsequent sync ops. For the iova
> > case the attr is always consistently used.
> 
> I want to believe that compiler will discards these "if (!coherent &&
> !(attrs & DMA_ATTR_SKIP_CPU_SYNC)))" branch if case is empty.

Yes, it is the poster child for dead code elimination using the
IS_ENABLED() helper.

> checks are scattered over all dma-iommu.c file with different
> combinations. While we can do new static functions for small number of
> use cases, it will be half-solution.

Don't bother.