Re: [RESEND PATCH v2 4/5] drm/msm: add DRM_MSM_GEM_SYNC_CACHE for non-coherent cache maintenance

[Jonathan Marek]

On 11/14/20 1:46 PM, Rob Clark wrote:
> On Sat, Nov 14, 2020 at 8:24 AM Christoph Hellwig <hch@xxxxxx> 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?

The range is intended to be "sync at least this range", so that 
userspace doesn't have to worry about details like that.

> BR,
> -R