linux-kernel - Re: [PATCH v9 07/24] dma-mapping: Implement link/unlink ranges API

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Message-ID: <20250428132006.GA30966@lst.de>
Date: Mon, 28 Apr 2025 15:20:06 +0200
From: Christoph Hellwig <hch@....de>
To: Leon Romanovsky <leon@...nel.org>
Cc: Luis Chamberlain <mcgrof@...nel.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@....de>,
	Keith Busch <kbusch@...nel.org>, Jake Edge <jake@....net>,
	Jonathan Corbet <corbet@....net>, Jason Gunthorpe <jgg@...pe.ca>,
	Zhu Yanjun <zyjzyj2000@...il.com>,
	Robin Murphy <robin.murphy@....com>, Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>, Sagi Grimberg <sagi@...mberg.me>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Logan Gunthorpe <logang@...tatee.com>,
	Yishai Hadas <yishaih@...dia.com>,
	Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>,
	Kevin Tian <kevin.tian@...el.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Jérôme Glisse <jglisse@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-block@...r.kernel.org, linux-rdma@...r.kernel.org,
	iommu@...ts.linux.dev, linux-nvme@...ts.infradead.org,
	linux-pci@...r.kernel.org, kvm@...r.kernel.org, linux-mm@...ck.org,
	Niklas Schnelle <schnelle@...ux.ibm.com>,
	Chuck Lever <chuck.lever@...cle.com>,
	Matthew Wilcox <willy@...radead.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Kanchan Joshi <joshi.k@...sung.com>,
	Chaitanya Kulkarni <kch@...dia.com>
Subject: Re: [PATCH v9 07/24] dma-mapping: Implement link/unlink ranges API

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.