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

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Dec 13, 2018 at 12:17:54PM +0200, Oleksandr Andrushchenko wrote:
> Daniel, could you please comment?

Cross-revieweing someone else's stuff would scale better, I don't think
I'll get around to anything before next year.
-Daniel

> 
> Thank you
> 
> On 11/27/18 12:32 PM, Oleksandr Andrushchenko wrote:
> > From: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
> > 
> > When GEM backing storage is allocated with drm_gem_get_pages
> > the backing pages may be cached, thus making it possible that
> > the backend sees only partial content of the buffer which may
> > lead to screen artifacts. Make sure that the frontend's
> > memory is coherent and the backend always sees correct display
> > buffer content.
> > 
> > Fixes: c575b7eeb89f ("drm/xen-front: Add support for Xen PV display frontend")
> > 
> > Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
> > ---
> >   drivers/gpu/drm/xen/xen_drm_front_gem.c | 62 +++++++++++++++++++------
> >   1 file changed, 48 insertions(+), 14 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/xen/xen_drm_front_gem.c b/drivers/gpu/drm/xen/xen_drm_front_gem.c
> > index 47ff019d3aef..c592735e49d2 100644
> > --- a/drivers/gpu/drm/xen/xen_drm_front_gem.c
> > +++ b/drivers/gpu/drm/xen/xen_drm_front_gem.c
> > @@ -33,8 +33,11 @@ struct xen_gem_object {
> >   	/* set for buffers allocated by the backend */
> >   	bool be_alloc;
> > -	/* this is for imported PRIME buffer */
> > -	struct sg_table *sgt_imported;
> > +	/*
> > +	 * this is for imported PRIME buffer or the one allocated via
> > +	 * drm_gem_get_pages.
> > +	 */
> > +	struct sg_table *sgt;
> >   };
> >   static inline struct xen_gem_object *
> > @@ -77,10 +80,21 @@ static struct xen_gem_object *gem_create_obj(struct drm_device *dev,
> >   	return xen_obj;
> >   }
> > +struct sg_table *xen_drm_front_gem_get_sg_table(struct drm_gem_object *gem_obj)
> > +{
> > +	struct xen_gem_object *xen_obj = to_xen_gem_obj(gem_obj);
> > +
> > +	if (!xen_obj->pages)
> > +		return ERR_PTR(-ENOMEM);
> > +
> > +	return drm_prime_pages_to_sg(xen_obj->pages, xen_obj->num_pages);
> > +}
> > +
> >   static struct xen_gem_object *gem_create(struct drm_device *dev, size_t size)
> >   {
> >   	struct xen_drm_front_drm_info *drm_info = dev->dev_private;
> >   	struct xen_gem_object *xen_obj;
> > +	struct address_space *mapping;
> >   	int ret;
> >   	size = round_up(size, PAGE_SIZE);
> > @@ -113,10 +127,14 @@ static struct xen_gem_object *gem_create(struct drm_device *dev, size_t size)
> >   		xen_obj->be_alloc = true;
> >   		return xen_obj;
> >   	}
> > +
> >   	/*
> >   	 * need to allocate backing pages now, so we can share those
> >   	 * with the backend
> >   	 */
> > +	mapping = xen_obj->base.filp->f_mapping;
> > +	mapping_set_gfp_mask(mapping, GFP_USER | __GFP_DMA32);
> > +
> >   	xen_obj->num_pages = DIV_ROUND_UP(size, PAGE_SIZE);
> >   	xen_obj->pages = drm_gem_get_pages(&xen_obj->base);
> >   	if (IS_ERR_OR_NULL(xen_obj->pages)) {
> > @@ -125,8 +143,27 @@ static struct xen_gem_object *gem_create(struct drm_device *dev, size_t size)
> >   		goto fail;
> >   	}
> > +	xen_obj->sgt = xen_drm_front_gem_get_sg_table(&xen_obj->base);
> > +	if (IS_ERR_OR_NULL(xen_obj->sgt)){
> > +		ret = PTR_ERR(xen_obj->sgt);
> > +		xen_obj->sgt = NULL;
> > +		goto fail_put_pages;
> > +	}
> > +
> > +	if (!dma_map_sg(dev->dev, xen_obj->sgt->sgl, xen_obj->sgt->nents,
> > +			DMA_BIDIRECTIONAL)) {
> > +		ret = -EFAULT;
> > +		goto fail_free_sgt;
> > +	}
> > +
> >   	return xen_obj;
> > +fail_free_sgt:
> > +	sg_free_table(xen_obj->sgt);
> > +	xen_obj->sgt = NULL;
> > +fail_put_pages:
> > +	drm_gem_put_pages(&xen_obj->base, xen_obj->pages, true, false);
> > +	xen_obj->pages = NULL;
> >   fail:
> >   	DRM_ERROR("Failed to allocate buffer with size %zu\n", size);
> >   	return ERR_PTR(ret);
> > @@ -149,7 +186,7 @@ void xen_drm_front_gem_free_object_unlocked(struct drm_gem_object *gem_obj)
> >   	struct xen_gem_object *xen_obj = to_xen_gem_obj(gem_obj);
> >   	if (xen_obj->base.import_attach) {
> > -		drm_prime_gem_destroy(&xen_obj->base, xen_obj->sgt_imported);
> > +		drm_prime_gem_destroy(&xen_obj->base, xen_obj->sgt);
> >   		gem_free_pages_array(xen_obj);
> >   	} else {
> >   		if (xen_obj->pages) {
> > @@ -158,6 +195,13 @@ void xen_drm_front_gem_free_object_unlocked(struct drm_gem_object *gem_obj)
> >   							xen_obj->pages);
> >   				gem_free_pages_array(xen_obj);
> >   			} else {
> > +				if (xen_obj->sgt) {
> > +					dma_unmap_sg(xen_obj->base.dev->dev,
> > +						     xen_obj->sgt->sgl,
> > +						     xen_obj->sgt->nents,
> > +						     DMA_BIDIRECTIONAL);
> > +					sg_free_table(xen_obj->sgt);
> > +				}
> >   				drm_gem_put_pages(&xen_obj->base,
> >   						  xen_obj->pages, true, false);
> >   			}
> > @@ -174,16 +218,6 @@ struct page **xen_drm_front_gem_get_pages(struct drm_gem_object *gem_obj)
> >   	return xen_obj->pages;
> >   }
> > -struct sg_table *xen_drm_front_gem_get_sg_table(struct drm_gem_object *gem_obj)
> > -{
> > -	struct xen_gem_object *xen_obj = to_xen_gem_obj(gem_obj);
> > -
> > -	if (!xen_obj->pages)
> > -		return ERR_PTR(-ENOMEM);
> > -
> > -	return drm_prime_pages_to_sg(xen_obj->pages, xen_obj->num_pages);
> > -}
> > -
> >   struct drm_gem_object *
> >   xen_drm_front_gem_import_sg_table(struct drm_device *dev,
> >   				  struct dma_buf_attachment *attach,
> > @@ -203,7 +237,7 @@ xen_drm_front_gem_import_sg_table(struct drm_device *dev,
> >   	if (ret < 0)
> >   		return ERR_PTR(ret);
> > -	xen_obj->sgt_imported = sgt;
> > +	xen_obj->sgt = sgt;
> >   	ret = drm_prime_sg_to_page_addr_arrays(sgt, xen_obj->pages,
> >   					       NULL, xen_obj->num_pages);
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch