linux-kernel - Re: [PATCH v3 1/1] drm: msm: Replace dma_map_sg with dma_sync

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Date:   Thu, 29 Nov 2018 15:14:29 +0100
From:   Christoph Hellwig <hch@....de>
To:     Vivek Gautam <vivek.gautam@...eaurora.org>
Cc:     airlied@...ux.ie, robdclark@...il.com,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        freedreno@...ts.freedesktop.org, tfiga@...omium.org,
        architt@...eaurora.org, linux-arm-msm@...r.kernel.org,
        Christoph Hellwig <hch@....de>,
        Robin Murphy <robin.murphy@....com>,
        Jordan Crouse <jcrouse@...eaurora.org>,
        Sean Paul <seanpaul@...omium.org>
Subject: Re: [PATCH v3 1/1] drm: msm: Replace dma_map_sg with dma_sync_sg*

On Thu, Nov 29, 2018 at 07:33:15PM +0530, Vivek Gautam wrote:
> dma_map_sg() expects a DMA domain. However, the drm devices
> have been traditionally using unmanaged iommu domain which
> is non-dma type. Using dma mapping APIs with that domain is bad.
> 
> Replace dma_map_sg() calls with dma_sync_sg_for_device{|cpu}()
> to do the cache maintenance.

As I told you before:  hell no.  If you spent the slightest amount of
actually trying to understand what you are doing here you'd know this
can't work.  Just turn on dma debugging and this will blow up in your
face.

Either you use the DMA API properly, that is you use it to map and
to sync, or you don't use it at all.  Mix and match between iommu
APIs and DMA APIs is simply not possible.