Re: [PATCH 2/3] drm/msm: add DRM_MSM_GEM_SYNC_CACHE for non-coherent cache maintenance

[Christoph Hellwig]

On Fri, Oct 02, 2020 at 08:46:35AM -0400, 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);
> > > +
> > > +	/* TODO: sync only the required range, and don't invalidate on clean */
> > > +
> > > +	if (flags & MSM_GEM_SYNC_CACHE_CLEAN)
> > > +		sync_for_device(msm_obj);
> > > +
> > > +	if (flags & MSM_GEM_SYNC_CACHE_INVALIDATE)
> > > +		sync_for_cpu(msm_obj);
> > 
> > And make to these ones as well.  They are complete abuses of the DMA
> > API, and while we had to live with that for now to not cause regressions
> > they absoutely must not be exposed in a userspace ABI like this.
> > 
> 
> How do you propose that cached non-coherent memory be implemented? It is a
> useful feature for userspace.

If the driver is using the DMA API you need to use dma_alloc_noncoherent
and friends as of 5.10 (see the iommu list for the discussion).

If you use the raw IOMMU API (which I think the msm drm driver does) you
need to work with the maintainers to implement a cache synchronization
API that is not tied to the DMA API.