linux-kernel - Re: [PATCH v1 2/2] dma: Add IOMMU static calls with clear default ops

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Message-ID: <20240717085652.GA18554@lst.de>
Date: Wed, 17 Jul 2024 10:56:52 +0200
From: Christoph Hellwig <hch@....de>
To: Leon Romanovsky <leon@...nel.org>
Cc: Christoph Hellwig <hch@....de>, Robin Murphy <robin.murphy@....com>,
	Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Easwar Hariharan <eahariha@...ux.microsoft.com>,
	linux-kernel@...r.kernel.org, iommu@...ts.linux.dev,
	Jason Gunthorpe <jgg@...dia.com>
Subject: Re: [PATCH v1 2/2] dma: Add IOMMU static calls with clear default
 ops

On Wed, Jul 17, 2024 at 11:47:13AM +0300, Leon Romanovsky wrote:
> It will cause to the situation where "struct device" memory footprint
> will increase, while we still need to keep dma_ops for archs that don't
> want to use default IOMMU.

It won't.  Even with the new bit we still have 28 more spare bits next
to it :)

> Because dma_ops pointer exists anyway and has already specific flags. I
> decided to take evolutive approach and add a new flag to it, instead of
> revolutionary approach and add a new field to struct device.

The point is that with a little more work we can actually kill that
for common configuration.  For non-Xen setups that basically just means
removing the dummy ops, so it's a pretty big quick win.