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

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Date:   Sat, 14 Nov 2020 10:46:55 -0800
From:   Rob Clark <robdclark@...il.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Jonathan Marek <jonathan@...ek.ca>,
        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 Sat, Nov 14, 2020 at 8:24 AM Christoph Hellwig <hch@....de> wrote:
>
> On Sat, Nov 14, 2020 at 10:17:12AM -0500, Jonathan Marek wrote:
> > +void msm_gem_sync_cache(struct drm_gem_object *obj, uint32_t flags,
> > +             size_t range_start, size_t range_end)
> > +{
> > +     struct msm_gem_object *msm_obj = to_msm_bo(obj);
> > +     struct device *dev = msm_obj->base.dev->dev;
> > +
> > +     /* exit early if get_pages() hasn't been called yet */
> > +     if (!msm_obj->pages)
> > +             return;
> > +
> > +     /* TODO: sync only the specified range */
> > +
> > +     if (flags & MSM_GEM_SYNC_FOR_DEVICE) {
> > +             dma_sync_sg_for_device(dev, msm_obj->sgt->sgl,
> > +                             msm_obj->sgt->nents, DMA_TO_DEVICE);
> > +     }
> > +
> > +     if (flags & MSM_GEM_SYNC_FOR_CPU) {
> > +             dma_sync_sg_for_cpu(dev, msm_obj->sgt->sgl,
> > +                             msm_obj->sgt->nents, DMA_FROM_DEVICE);
> > +     }
>
> Splitting this helper from the only caller is rather strange, epecially
> with the two unused arguments.  And I think the way this is specified
> to take a range, but ignoring it is actively dangerous.  User space will
> rely on it syncing everything sooner or later and then you are stuck.
> So just define a sync all primitive for now, and if you really need a
> range sync and have actually implemented it add a new ioctl for that.

We do already have a split of ioctl "layer" which enforces valid ioctl
params, etc, and gem (or other) module code which is called by the
ioctl func.  So I think it is fine to keep this split here.  (Also, I
think at some point there will be a uring type of ioctl alternative
which would re-use the same gem func.)

But I do agree that the range should be respected or added later..
drm_ioctl() dispatch is well prepared for extending ioctls.

And I assume there should be some validation that the range is aligned
to cache-line?  Or can we flush a partial cache line?

BR,
-R