linux-kernel - Re: [RESEND PATCH v2 4/5] drm/msm: add DRM_MSM_GEM_SYNC

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Date:   Mon, 16 Nov 2020 12:52:41 -0500
From:   Jonathan Marek <jonathan@...ek.ca>
To:     Rob Clark <robdclark@...il.com>, Christoph Hellwig <hch@....de>,
        Jordan Crouse <jcrouse@...eaurora.org>
Cc:     freedreno <freedreno@...ts.freedesktop.org>,
        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>
Subject: Re: [RESEND PATCH v2 4/5] drm/msm: add DRM_MSM_GEM_SYNC_CACHE for
 non-coherent cache maintenance

On 11/16/20 12:50 PM, Rob Clark wrote:
> On Mon, Nov 16, 2020 at 9:33 AM Christoph Hellwig <hch@....de> wrote:
>>
>> On Sat, Nov 14, 2020 at 03:07:20PM -0500, Jonathan Marek wrote:
>>> qcom's vulkan driver has nonCoherentAtomSize=1, and it looks like
>>> dma_sync_single_for_cpu() does deal in some way with the partial cache line
>>> case, although I'm not sure that means we can have a nonCoherentAtomSize=1.
>>
>> No, it doesn't.  You need to ensure ownership is managed at
>> dma_get_cache_alignment() granularity.
> 
> my guess is nonCoherentAtomSize=1 only works in the case of cache
> coherent buffers
> 

nonCoherentAtomSize doesn't apply to coherent memory (as the name 
implies), I guess qcom's driver is just wrong about having 
nonCoherentAtomSize=1.

Jordan just mentioned there is at least one conformance test for this, I 
wonder if it just doesn't test it well enough, or just doesn't test the 
non-coherent memory type?