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SubjectRe: [PATCH] drm/xen-front: Make shmem backed display buffer coherent
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Daniel, could you please comment?

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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