linux-kernel - Re: [RFC PATCH 0/3] iommu: Add range flush operation

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Date:	Tue, 29 Sep 2015 14:20:05 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	Tomasz Figa <tfiga@...omium.org>
Cc:	iommu@...ts.linux-foundation.org, Hiroshi Doyu <hdoyu@...dia.com>,
	Stephen Warren <swarren@...dotorg.org>,
	Thierry Reding <thierry.reding@...il.com>,
	Alexandre Courbot <gnurou@...il.com>,
	Vince Hsu <vince.h@...dia.com>,
	Russell King <rmk+kernel@....linux.org.uk>,
	Paul Walmsley <paul@...an.com>,
	Tomeu Vizoso <tomeu.vizoso@...labora.com>,
	Mikko Perttunen <mperttunen@...dia.com>,
	Will Deacon <will.deacon@....com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Arnd Bergmann <arnd@...db.de>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Antonios Motakis <a.motakis@...tualopensystems.com>,
	Olav Haugan <ohaugan@...eaurora.org>,
	Nicolas Iooss <nicolas.iooss_linux@....org>,
	linux-kernel@...r.kernel.org, linux-tegra@...r.kernel.org
Subject: Re: [RFC PATCH 0/3] iommu: Add range flush operation

Hi Tomasz,

On Tue, Sep 29, 2015 at 02:25:23PM +0900, Tomasz Figa wrote:
> This series tries to mitigate the two issues above, while acknowledging
> the fact that the .map_sg() callback might be still necessary for some
> specific platforms, which could have the need to iterate over SG elements
> inside driver code. Proposed solution introduces a new .flush() callback,
> which expects IOVA range as its argument and is expected to flush all
> respective caches (be it CPU, IOMMU TLB or whatever) to make the given
> IOVA area mapping change visible to IOMMU clients. Then all the 3 basic
> map/unmap operations are modified to call the .flush() callback at the end
> of the operation. 
> 
> Advantages of proposed approach include:
>  1) ability to use default_iommu_map_sg() helper if all the driver needs
>     for performance optimization is batching the flush,
>  2) completely no effect on existing code - the .flush() callback is made
>     optional and if it isn't implemented drivers are expected to do
>     necessary flushes on a page by page basis in respective (un)mapping
>     callbakcs,
>  3) possibility of exporting the iommu_flush() operation and providing
>     unsynchronized map/unmap operations for subsystems with even higher
>     requirements for performance (e.g. drivers/gpu/drm).

Thanks for the patches, I really like the idea. The VT-d driver probably
also benefits a lot from this.

In the past I also proposed something like this, it was called a new
iommu_commit() API function for making changes from map/unmap visible to
the hardware. But this requires updating all callers first, so your
approach of doing an implicit flush at the end of map/unmap is better.



	Joerg

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