linux-kernel - Re: [PATCH 2/3] drm/msm: add DRM_MSM_GEM_SYNC_CACHE for non-coherent cache maintenance

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Message-ID: <20201008082757.GB3107@8bytes.org>
Date:   Thu, 8 Oct 2020 10:27:57 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Jonathan Marek <jonathan@...ek.ca>,
        freedreno@...ts.freedesktop.org, Rob Clark <robdclark@...il.com>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <linux-arm-msm@...r.kernel.org>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <dri-devel@...ts.freedesktop.org>,
        open list <linux-kernel@...r.kernel.org>,
        iommu@...ts.linux-foundation.org,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH 2/3] drm/msm: add DRM_MSM_GEM_SYNC_CACHE for non-coherent
 cache maintenance

On Tue, Oct 06, 2020 at 08:23:06AM +0100, Christoph Hellwig wrote:
> If people want to use the "raw" IOMMU API with not cache coherent
> devices we'll need a cache maintainance API that goes along with it.
> It could either be formally part of the IOMMU API or be separate.

The IOMMU-API does not care about the caching effects of DMA, is manages
IO address spaces for devices. I also don't know how this would be going
to be implemented, the IOMMU-API does not have the concept of handles
for mapped ranges and does not care about CPU virtual addresses (which
are needed for cache flushes) of the memory it maps into IO page-tables.

So I think a cache management API should be separate from the IOMMU-API.

Regards,

	Joerg