Re: [PATCH] drm/xen-front: Make shmem backed display buffer coherent

[Christoph Hellwig]

On Tue, Dec 18, 2018 at 08:20:22PM +0100, Noralf Trønnes wrote:
> > +	if (!dma_map_sg(dev->dev, xen_obj->sgt->sgl, xen_obj->sgt->nents,
> > +			DMA_BIDIRECTIONAL)) {
> 
> 
> Are you using the DMA streaming API as a way to flush the caches?

This looks rather broken.  Please send the whole patch series to
the iommu list for proper review.

> Does this mean that GFP_USER isn't making the buffer coherent?

How could GFP_USER make memory coherent in any way?