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

[Rob Clark]

On Sat, Nov 14, 2020 at 10:58 AM Jonathan Marek <jonathan@xxxxxxxx> wrote:
>
> 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.
>

I don't think userspace can *not* worry about details like that.
Consider a case where the cpu and gpu are simultaneously accessing
different parts of a buffer (for ex, sub-allocation).  There needs to
be cache-line separation between the two.

BR,
-R