linux-kernel - Re: [PATCH v1 4/4] iommu/tegra: gart: Optimize map/unmap

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Message-ID: <20180507080420.GB18595@8bytes.org>
Date:   Mon, 7 May 2018 10:04:20 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     Dmitry Osipenko <digetx@...il.com>
Cc:     Robin Murphy <robin.murphy@....com>,
        Thierry Reding <thierry.reding@...il.com>,
        linux-tegra@...r.kernel.org, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org,
        Jonathan Hunter <jonathanh@...dia.com>
Subject: Re: [PATCH v1 4/4] iommu/tegra: gart: Optimize map/unmap

On Mon, May 07, 2018 at 12:19:01AM +0300, Dmitry Osipenko wrote:
> Probably the best variant would be to give an explicit control over syncing to a
> user of the IOMMU API, like for example device driver may perform multiple
> mappings / unmappings and then sync/flush in the end. I'm not sure that it's
> really worth the hassle to shuffle the API right now, maybe we can implement it
> later if needed. Joerg, do you have objections to a 'compound page' approach?

Have you measured the performance difference on both variants? The
compound-page approach only works for cases when the physical memory you
map contiguous and correctly aligned.

If it is really needed I would prefer a separate iotlb_sync_map()
call-back that is just NULL when not needed. This way all users that
don't need it only get a minimal penalty in the mapping path and you
don't have any requirements on the physical memory you map to get good
performance.

	Joerg