Re: [PATCH 8/9] drm/xen-front: Implement GEM operations

[Daniel Vetter]

On Wed, Feb 21, 2018 at 10:03:41AM +0200, Oleksandr Andrushchenko wrote:
> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
> 
> Implement GEM handling depending on driver mode of operation:
> depending on the requirements for the para-virtualized environment, namely
> requirements dictated by the accompanying DRM/(v)GPU drivers running in both
> host and guest environments, number of operating modes of para-virtualized
> display driver are supported:
>  - display buffers can be allocated by either frontend driver or backend
>  - display buffers can be allocated to be contiguous in memory or not
> 
> Note! Frontend driver itself has no dependency on contiguous memory for
> its operation.
> 
> 1. Buffers allocated by the frontend driver.
> 
> The below modes of operation are configured at compile-time via
> frontend driver's kernel configuration.
> 
> 1.1. Front driver configured to use GEM CMA helpers
>      This use-case is useful when used with accompanying DRM/vGPU driver in
>      guest domain which was designed to only work with contiguous buffers,
>      e.g. DRM driver based on GEM CMA helpers: such drivers can only import
>      contiguous PRIME buffers, thus requiring frontend driver to provide
>      such. In order to implement this mode of operation para-virtualized
>      frontend driver can be configured to use GEM CMA helpers.
> 
> 1.2. Front driver doesn't use GEM CMA
>      If accompanying drivers can cope with non-contiguous memory then, to
>      lower pressure on CMA subsystem of the kernel, driver can allocate
>      buffers from system memory.
> 
> Note! If used with accompanying DRM/(v)GPU drivers this mode of operation
> may require IOMMU support on the platform, so accompanying DRM/vGPU
> hardware can still reach display buffer memory while importing PRIME
> buffers from the frontend driver.
> 
> 2. Buffers allocated by the backend
> 
> This mode of operation is run-time configured via guest domain configuration
> through XenStore entries.
> 
> For systems which do not provide IOMMU support, but having specific
> requirements for display buffers it is possible to allocate such buffers
> at backend side and share those with the frontend.
> For example, if host domain is 1:1 mapped and has DRM/GPU hardware expecting
> physically contiguous memory, this allows implementing zero-copying
> use-cases.
> 
> Note! Configuration options 1.1 (contiguous display buffers) and 2 (backend
> allocated buffers) are not supported at the same time.
> 
> Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>

Some suggestions below for some larger cleanup work.
-Daniel

> ---
>  drivers/gpu/drm/xen/Kconfig                 |  13 +
>  drivers/gpu/drm/xen/Makefile                |   6 +
>  drivers/gpu/drm/xen/xen_drm_front.h         |  74 ++++++
>  drivers/gpu/drm/xen/xen_drm_front_drv.c     |  80 ++++++-
>  drivers/gpu/drm/xen/xen_drm_front_drv.h     |   1 +
>  drivers/gpu/drm/xen/xen_drm_front_gem.c     | 360 ++++++++++++++++++++++++++++
>  drivers/gpu/drm/xen/xen_drm_front_gem.h     |  46 ++++
>  drivers/gpu/drm/xen/xen_drm_front_gem_cma.c |  93 +++++++
>  8 files changed, 667 insertions(+), 6 deletions(-)
>  create mode 100644 drivers/gpu/drm/xen/xen_drm_front_gem.c
>  create mode 100644 drivers/gpu/drm/xen/xen_drm_front_gem.h
>  create mode 100644 drivers/gpu/drm/xen/xen_drm_front_gem_cma.c
> 
> diff --git a/drivers/gpu/drm/xen/Kconfig b/drivers/gpu/drm/xen/Kconfig
> index 4cca160782ab..4f4abc91f3b6 100644
> --- a/drivers/gpu/drm/xen/Kconfig
> +++ b/drivers/gpu/drm/xen/Kconfig
> @@ -15,3 +15,16 @@ config DRM_XEN_FRONTEND
>  	help
>  	  Choose this option if you want to enable a para-virtualized
>  	  frontend DRM/KMS driver for Xen guest OSes.
> +
> +config DRM_XEN_FRONTEND_CMA
> +	bool "Use DRM CMA to allocate dumb buffers"
> +	depends on DRM_XEN_FRONTEND
> +	select DRM_KMS_CMA_HELPER
> +	select DRM_GEM_CMA_HELPER
> +	help
> +	  Use DRM CMA helpers to allocate display buffers.
> +	  This is useful for the use-cases when guest driver needs to
> +	  share or export buffers to other drivers which only expect
> +	  contiguous buffers.
> +	  Note: in this mode driver cannot use buffers allocated
> +	  by the backend.
> diff --git a/drivers/gpu/drm/xen/Makefile b/drivers/gpu/drm/xen/Makefile
> index 4fcb0da1a9c5..12376ec78fbc 100644
> --- a/drivers/gpu/drm/xen/Makefile
> +++ b/drivers/gpu/drm/xen/Makefile
> @@ -8,4 +8,10 @@ drm_xen_front-objs := xen_drm_front.o \
>  		      xen_drm_front_shbuf.o \
>  		      xen_drm_front_cfg.o
>  
> +ifeq ($(CONFIG_DRM_XEN_FRONTEND_CMA),y)
> +	drm_xen_front-objs += xen_drm_front_gem_cma.o
> +else
> +	drm_xen_front-objs += xen_drm_front_gem.o
> +endif
> +
>  obj-$(CONFIG_DRM_XEN_FRONTEND) += drm_xen_front.o
> diff --git a/drivers/gpu/drm/xen/xen_drm_front.h b/drivers/gpu/drm/xen/xen_drm_front.h
> index 9ed5bfb248d0..c6f52c892434 100644
> --- a/drivers/gpu/drm/xen/xen_drm_front.h
> +++ b/drivers/gpu/drm/xen/xen_drm_front.h
> @@ -34,6 +34,80 @@
>  
>  struct xen_drm_front_drm_pipeline;
>  
> +/*
> + *******************************************************************************
> + * Para-virtualized DRM/KMS frontend driver
> + *******************************************************************************
> + * This frontend driver implements Xen para-virtualized display
> + * according to the display protocol described at
> + * include/xen/interface/io/displif.h
> + *
> + *******************************************************************************
> + * Driver modes of operation in terms of display buffers used
> + *******************************************************************************
> + * Depending on the requirements for the para-virtualized environment, namely
> + * requirements dictated by the accompanying DRM/(v)GPU drivers running in both
> + * host and guest environments, number of operating modes of para-virtualized
> + * display driver are supported:
> + *  - display buffers can be allocated by either frontend driver or backend
> + *  - display buffers can be allocated to be contiguous in memory or not
> + *
> + * Note! Frontend driver itself has no dependency on contiguous memory for
> + *       its operation.
> + *
> + *******************************************************************************
> + * 1. Buffers allocated by the frontend driver.
> + *******************************************************************************
> + *
> + * The below modes of operation are configured at compile-time via
> + * frontend driver's kernel configuration.
> + *
> + * 1.1. Front driver configured to use GEM CMA helpers
> + *      This use-case is useful when used with accompanying DRM/vGPU driver in
> + *      guest domain which was designed to only work with contiguous buffers,
> + *      e.g. DRM driver based on GEM CMA helpers: such drivers can only import
> + *      contiguous PRIME buffers, thus requiring frontend driver to provide
> + *      such. In order to implement this mode of operation para-virtualized
> + *      frontend driver can be configured to use GEM CMA helpers.
> + *
> + * 1.2. Front driver doesn't use GEM CMA
> + *      If accompanying drivers can cope with non-contiguous memory then, to
> + *      lower pressure on CMA subsystem of the kernel, driver can allocate
> + *      buffers from system memory.
> + *
> + * Note! If used with accompanying DRM/(v)GPU drivers this mode of operation
> + *   may require IOMMU support on the platform, so accompanying DRM/vGPU
> + *   hardware can still reach display buffer memory while importing PRIME
> + *   buffers from the frontend driver.
> + *
> + *******************************************************************************
> + * 2. Buffers allocated by the backend
> + *******************************************************************************
> + *
> + * This mode of operation is run-time configured via guest domain configuration
> + * through XenStore entries.
> + *
> + * For systems which do not provide IOMMU support, but having specific
> + * requirements for display buffers it is possible to allocate such buffers
> + * at backend side and share those with the frontend.
> + * For example, if host domain is 1:1 mapped and has DRM/GPU hardware expecting
> + * physically contiguous memory, this allows implementing zero-copying
> + * use-cases.
> + *
> + *******************************************************************************
> + * Driver limitations
> + *******************************************************************************
> + * 1. Configuration options 1.1 (contiguous display buffers) and 2 (backend
> + *    allocated buffers) are not supported at the same time.
> + *
> + * 2. Only primary plane without additional properties is supported.
> + *
> + * 3. Only one video mode supported which is configured via XenStore.
> + *
> + * 4. All CRTCs operate at fixed frequency of 60Hz.
> + *
> + ******************************************************************************/

Since you've typed this all up, pls convert it to kernel-doc and pull it
into a xen-front.rst driver section in Documentation/gpu/ There's a few
examples for i915 and vc4 already.

> +
>  struct xen_drm_front_ops {
>  	int (*mode_set)(struct xen_drm_front_drm_pipeline *pipeline,
>  			uint32_t x, uint32_t y, uint32_t width, uint32_t height,
> diff --git a/drivers/gpu/drm/xen/xen_drm_front_drv.c b/drivers/gpu/drm/xen/xen_drm_front_drv.c
> index e8862d26ba27..35e7e9cda9d1 100644
> --- a/drivers/gpu/drm/xen/xen_drm_front_drv.c
> +++ b/drivers/gpu/drm/xen/xen_drm_front_drv.c
> @@ -23,12 +23,58 @@
>  #include "xen_drm_front.h"
>  #include "xen_drm_front_cfg.h"
>  #include "xen_drm_front_drv.h"
> +#include "xen_drm_front_gem.h"
>  #include "xen_drm_front_kms.h"
>  
>  static int dumb_create(struct drm_file *filp,
>  		struct drm_device *dev, struct drm_mode_create_dumb *args)
>  {
> -	return -EINVAL;
> +	struct xen_drm_front_drm_info *drm_info = dev->dev_private;
> +	struct drm_gem_object *obj;
> +	int ret;
> +
> +	ret = drm_info->gem_ops->dumb_create(filp, dev, args);
> +	if (ret)
> +		goto fail;
> +
> +	obj = drm_gem_object_lookup(filp, args->handle);
> +	if (!obj) {
> +		ret = -ENOENT;
> +		goto fail_destroy;
> +	}
> +
> +	drm_gem_object_unreference_unlocked(obj);
> +
> +	/*
> +	 * In case of CONFIG_DRM_XEN_FRONTEND_CMA gem_obj is constructed
> +	 * via DRM CMA helpers and doesn't have ->pages allocated
> +	 * (xendrm_gem_get_pages will return NULL), but instead can provide
> +	 * sg table
> +	 */

My recommendation is to use an sg table for everything if you deal with
mixed objects (CMA, special blocks 1:1 mapped from host, normal pages).
That avoids the constant get_pages vs. get_sgt differences. For examples
see how e.g. i915 handles the various gem object backends.

> +	if (drm_info->gem_ops->get_pages(obj))
> +		ret = drm_info->front_ops->dbuf_create_from_pages(
> +				drm_info->front_info,
> +				xen_drm_front_dbuf_to_cookie(obj),
> +				args->width, args->height, args->bpp,
> +				args->size,
> +				drm_info->gem_ops->get_pages(obj));
> +	else
> +		ret = drm_info->front_ops->dbuf_create_from_sgt(
> +				drm_info->front_info,
> +				xen_drm_front_dbuf_to_cookie(obj),
> +				args->width, args->height, args->bpp,
> +				args->size,
> +				drm_info->gem_ops->prime_get_sg_table(obj));
> +	if (ret)
> +		goto fail_destroy;
> +
> +	return 0;
> +
> +fail_destroy:
> +	drm_gem_dumb_destroy(filp, dev, args->handle);
> +fail:
> +	DRM_ERROR("Failed to create dumb buffer: %d\n", ret);
> +	return ret;
>  }
>  
>  static void free_object(struct drm_gem_object *obj)
> @@ -37,6 +83,7 @@ static void free_object(struct drm_gem_object *obj)
>  
>  	drm_info->front_ops->dbuf_destroy(drm_info->front_info,
>  			xen_drm_front_dbuf_to_cookie(obj));
> +	drm_info->gem_ops->free_object_unlocked(obj);
>  }
>  
>  static void on_frame_done(struct platform_device *pdev,
> @@ -60,32 +107,52 @@ static void lastclose(struct drm_device *dev)
>  
>  static int gem_mmap(struct file *filp, struct vm_area_struct *vma)
>  {
> -	return -EINVAL;
> +	struct drm_file *file_priv = filp->private_data;
> +	struct drm_device *dev = file_priv->minor->dev;
> +	struct xen_drm_front_drm_info *drm_info = dev->dev_private;
> +
> +	return drm_info->gem_ops->mmap(filp, vma);

Uh, so 1 midlayer for the kms stuff and another midlayer for the gem
stuff. That's way too much indirection.

>  }
>  
>  static struct sg_table *prime_get_sg_table(struct drm_gem_object *obj)
>  {
> -	return NULL;
> +	struct xen_drm_front_drm_info *drm_info;
> +
> +	drm_info = obj->dev->dev_private;
> +	return drm_info->gem_ops->prime_get_sg_table(obj);
>  }
>  
>  static struct drm_gem_object *prime_import_sg_table(struct drm_device *dev,
>  		struct dma_buf_attachment *attach, struct sg_table *sgt)
>  {
> -	return NULL;
> +	struct xen_drm_front_drm_info *drm_info;
> +
> +	drm_info = dev->dev_private;
> +	return drm_info->gem_ops->prime_import_sg_table(dev, attach, sgt);
>  }
>  
>  static void *prime_vmap(struct drm_gem_object *obj)
>  {
> -	return NULL;
> +	struct xen_drm_front_drm_info *drm_info;
> +
> +	drm_info = obj->dev->dev_private;
> +	return drm_info->gem_ops->prime_vmap(obj);
>  }
>  
>  static void prime_vunmap(struct drm_gem_object *obj, void *vaddr)
>  {
> +	struct xen_drm_front_drm_info *drm_info;
> +
> +	drm_info = obj->dev->dev_private;
> +	drm_info->gem_ops->prime_vunmap(obj, vaddr);
>  }
>  
>  static int prime_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma)
>  {
> -	return -EINVAL;
> +	struct xen_drm_front_drm_info *drm_info;
> +
> +	drm_info = obj->dev->dev_private;
> +	return drm_info->gem_ops->prime_mmap(obj, vma);
>  }
>  
>  static const struct file_operations xendrm_fops = {
> @@ -147,6 +214,7 @@ int xen_drm_front_drv_probe(struct platform_device *pdev,
>  
>  	drm_info->front_ops = front_ops;
>  	drm_info->front_ops->on_frame_done = on_frame_done;
> +	drm_info->gem_ops = xen_drm_front_gem_get_ops();
>  	drm_info->front_info = cfg->front_info;
>  
>  	dev = drm_dev_alloc(&xen_drm_driver, &pdev->dev);
> diff --git a/drivers/gpu/drm/xen/xen_drm_front_drv.h b/drivers/gpu/drm/xen/xen_drm_front_drv.h
> index 563318b19f34..34228eb86255 100644
> --- a/drivers/gpu/drm/xen/xen_drm_front_drv.h
> +++ b/drivers/gpu/drm/xen/xen_drm_front_drv.h
> @@ -43,6 +43,7 @@ struct xen_drm_front_drm_pipeline {
>  struct xen_drm_front_drm_info {
>  	struct xen_drm_front_info *front_info;
>  	struct xen_drm_front_ops *front_ops;
> +	const struct xen_drm_front_gem_ops *gem_ops;
>  	struct drm_device *drm_dev;
>  	struct xen_drm_front_cfg *cfg;
>  
> diff --git a/drivers/gpu/drm/xen/xen_drm_front_gem.c b/drivers/gpu/drm/xen/xen_drm_front_gem.c
> new file mode 100644
> index 000000000000..367e08f6a9ef
> --- /dev/null
> +++ b/drivers/gpu/drm/xen/xen_drm_front_gem.c
> @@ -0,0 +1,360 @@
> +/*
> + *  Xen para-virtual DRM device
> + *
> + *   This program is free software; you can redistribute it and/or modify
> + *   it under the terms of the GNU General Public License as published by
> + *   the Free Software Foundation; either version 2 of the License, or
> + *   (at your option) any later version.
> + *
> + *   This program is distributed in the hope that it will be useful,
> + *   but WITHOUT ANY WARRANTY; without even the implied warranty of
> + *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> + *   GNU General Public License for more details.
> + *
> + * Copyright (C) 2016-2018 EPAM Systems Inc.
> + *
> + * Author: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
> + */
> +
> +#include "xen_drm_front_gem.h"
> +
> +#include <drm/drmP.h>
> +#include <drm/drm_crtc_helper.h>
> +#include <drm/drm_fb_helper.h>
> +#include <drm/drm_gem.h>
> +
> +#include <linux/dma-buf.h>
> +#include <linux/scatterlist.h>
> +#include <linux/shmem_fs.h>
> +
> +#include <xen/balloon.h>
> +
> +#include "xen_drm_front.h"
> +#include "xen_drm_front_drv.h"
> +#include "xen_drm_front_shbuf.h"
> +
> +struct xen_gem_object {
> +	struct drm_gem_object base;
> +
> +	size_t num_pages;
> +	struct page **pages;
> +
> +	/* set for buffers allocated by the backend */
> +	bool be_alloc;
> +
> +	/* this is for imported PRIME buffer */
> +	struct sg_table *sgt_imported;
> +};
> +
> +static inline struct xen_gem_object *to_xen_gem_obj(
> +		struct drm_gem_object *gem_obj)
> +{
> +	return container_of(gem_obj, struct xen_gem_object, base);
> +}
> +
> +static int gem_alloc_pages_array(struct xen_gem_object *xen_obj,
> +		size_t buf_size)
> +{
> +	xen_obj->num_pages = DIV_ROUND_UP(buf_size, PAGE_SIZE);
> +	xen_obj->pages = kvmalloc_array(xen_obj->num_pages,
> +			sizeof(struct page *), GFP_KERNEL);
> +	return xen_obj->pages == NULL ? -ENOMEM : 0;
> +}
> +
> +static void gem_free_pages_array(struct xen_gem_object *xen_obj)
> +{
> +	kvfree(xen_obj->pages);
> +	xen_obj->pages = NULL;
> +}
> +
> +static struct xen_gem_object *gem_create_obj(struct drm_device *dev,
> +	size_t size)
> +{
> +	struct xen_gem_object *xen_obj;
> +	int ret;
> +
> +	xen_obj = kzalloc(sizeof(*xen_obj), GFP_KERNEL);
> +	if (!xen_obj)
> +		return ERR_PTR(-ENOMEM);
> +
> +	ret = drm_gem_object_init(dev, &xen_obj->base, size);
> +	if (ret < 0) {
> +		kfree(xen_obj);
> +		return ERR_PTR(ret);
> +	}
> +
> +	return xen_obj;
> +}
> +
> +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;
> +	int ret;
> +
> +	size = round_up(size, PAGE_SIZE);
> +	xen_obj = gem_create_obj(dev, size);
> +	if (IS_ERR_OR_NULL(xen_obj))
> +		return xen_obj;
> +
> +	if (drm_info->cfg->be_alloc) {
> +		/*
> +		 * backend will allocate space for this buffer, so
> +		 * only allocate array of pointers to pages
> +		 */
> +		xen_obj->be_alloc = true;
> +		ret = gem_alloc_pages_array(xen_obj, size);
> +		if (ret < 0) {
> +			gem_free_pages_array(xen_obj);
> +			goto fail;
> +		}
> +
> +		ret = alloc_xenballooned_pages(xen_obj->num_pages,
> +				xen_obj->pages);
> +		if (ret < 0) {
> +			DRM_ERROR("Cannot allocate %zu ballooned pages: %d\n",
> +					xen_obj->num_pages, ret);
> +			goto fail;
> +		}
> +
> +		return xen_obj;
> +	}
> +	/*
> +	 * need to allocate backing pages now, so we can share those
> +	 * with the backend
> +	 */
> +	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)) {
> +		ret = PTR_ERR(xen_obj->pages);
> +		xen_obj->pages = NULL;
> +		goto fail;
> +	}
> +
> +	return xen_obj;
> +
> +fail:
> +	DRM_ERROR("Failed to allocate buffer with size %zu\n", size);
> +	return ERR_PTR(ret);
> +}
> +
> +static struct xen_gem_object *gem_create_with_handle(struct drm_file *filp,
> +		struct drm_device *dev, size_t size, uint32_t *handle)
> +{
> +	struct xen_gem_object *xen_obj;
> +	struct drm_gem_object *gem_obj;
> +	int ret;
> +
> +	xen_obj = gem_create(dev, size);
> +	if (IS_ERR_OR_NULL(xen_obj))
> +		return xen_obj;
> +
> +	gem_obj = &xen_obj->base;
> +	ret = drm_gem_handle_create(filp, gem_obj, handle);
> +	/* handle holds the reference */
> +	drm_gem_object_unreference_unlocked(gem_obj);
> +	if (ret < 0)
> +		return ERR_PTR(ret);
> +
> +	return xen_obj;
> +}
> +
> +static int gem_dumb_create(struct drm_file *filp, struct drm_device *dev,
> +		struct drm_mode_create_dumb *args)
> +{
> +	struct xen_gem_object *xen_obj;
> +
> +	args->pitch = DIV_ROUND_UP(args->width * args->bpp, 8);
> +	args->size = args->pitch * args->height;
> +
> +	xen_obj = gem_create_with_handle(filp, dev, args->size, &args->handle);
> +	if (IS_ERR_OR_NULL(xen_obj))
> +		return xen_obj == NULL ? -ENOMEM : PTR_ERR(xen_obj);
> +
> +	return 0;
> +}
> +
> +static void gem_free_object(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);
> +		gem_free_pages_array(xen_obj);
> +	} else {
> +		if (xen_obj->pages) {
> +			if (xen_obj->be_alloc) {
> +				free_xenballooned_pages(xen_obj->num_pages,
> +						xen_obj->pages);
> +				gem_free_pages_array(xen_obj);
> +			} else
> +				drm_gem_put_pages(&xen_obj->base,
> +						xen_obj->pages, true, false);
> +		}
> +	}
> +	drm_gem_object_release(gem_obj);
> +	kfree(xen_obj);
> +}
> +
> +static struct page **gem_get_pages(struct drm_gem_object *gem_obj)
> +{
> +	struct xen_gem_object *xen_obj = to_xen_gem_obj(gem_obj);
> +
> +	return xen_obj->pages;
> +}
> +
> +static struct sg_table *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 NULL;
> +
> +	return drm_prime_pages_to_sg(xen_obj->pages, xen_obj->num_pages);
> +}
> +
> +static struct drm_gem_object *gem_import_sg_table(struct drm_device *dev,
> +		struct dma_buf_attachment *attach, struct sg_table *sgt)
> +{
> +	struct xen_drm_front_drm_info *drm_info = dev->dev_private;
> +	struct xen_gem_object *xen_obj;
> +	size_t size;
> +	int ret;
> +
> +	size = attach->dmabuf->size;
> +	xen_obj = gem_create_obj(dev, size);
> +	if (IS_ERR_OR_NULL(xen_obj))
> +		return ERR_CAST(xen_obj);
> +
> +	ret = gem_alloc_pages_array(xen_obj, size);
> +	if (ret < 0)
> +		return ERR_PTR(ret);
> +
> +	xen_obj->sgt_imported = sgt;
> +
> +	ret = drm_prime_sg_to_page_addr_arrays(sgt, xen_obj->pages,
> +			NULL, xen_obj->num_pages);
> +	if (ret < 0)
> +		return ERR_PTR(ret);
> +
> +	/*
> +	 * N.B. Although we have an API to create display buffer from sgt
> +	 * we use pages API, because we still need those for GEM handling,
> +	 * e.g. for mapping etc.
> +	 */
> +	ret = drm_info->front_ops->dbuf_create_from_pages(
> +			drm_info->front_info,
> +			xen_drm_front_dbuf_to_cookie(&xen_obj->base),
> +			0, 0, 0, size, xen_obj->pages);
> +	if (ret < 0)
> +		return ERR_PTR(ret);
> +
> +	DRM_DEBUG("Imported buffer of size %zu with nents %u\n",
> +		size, sgt->nents);
> +
> +	return &xen_obj->base;
> +}
> +
> +static int gem_mmap_obj(struct xen_gem_object *xen_obj,
> +		struct vm_area_struct *vma)
> +{
> +	unsigned long addr = vma->vm_start;
> +	int i;
> +
> +	/*
> +	 * clear the VM_PFNMAP flag that was set by drm_gem_mmap(), and set the
> +	 * vm_pgoff (used as a fake buffer offset by DRM) to 0 as we want to map
> +	 * the whole buffer.
> +	 */
> +	vma->vm_flags &= ~VM_PFNMAP;
> +	vma->vm_flags |= VM_MIXEDMAP;
> +	vma->vm_pgoff = 0;
> +	vma->vm_page_prot = pgprot_writecombine(vm_get_page_prot(vma->vm_flags));
> +
> +	/*
> +	 * vm_operations_struct.fault handler will be called if CPU access
> +	 * to VM is here. For GPUs this isn't the case, because CPU
> +	 * doesn't touch the memory. Insert pages now, so both CPU and GPU are
> +	 * happy.
> +	 * FIXME: as we insert all the pages now then no .fault handler must
> +	 * be called, so don't provide one
> +	 */
> +	for (i = 0; i < xen_obj->num_pages; i++) {
> +		int ret;
> +
> +		ret = vm_insert_page(vma, addr, xen_obj->pages[i]);
> +		if (ret < 0) {
> +			DRM_ERROR("Failed to insert pages into vma: %d\n", ret);
> +			return ret;
> +		}
> +
> +		addr += PAGE_SIZE;
> +	}
> +	return 0;
> +}
> +
> +static int gem_mmap(struct file *filp, struct vm_area_struct *vma)
> +{
> +	struct xen_gem_object *xen_obj;
> +	struct drm_gem_object *gem_obj;
> +	int ret;
> +
> +	ret = drm_gem_mmap(filp, vma);
> +	if (ret < 0)
> +		return ret;
> +
> +	gem_obj = vma->vm_private_data;
> +	xen_obj = to_xen_gem_obj(gem_obj);
> +	return gem_mmap_obj(xen_obj, vma);
> +}
> +
> +static void *gem_prime_vmap(struct drm_gem_object *gem_obj)
> +{
> +	struct xen_gem_object *xen_obj = to_xen_gem_obj(gem_obj);
> +
> +	if (!xen_obj->pages)
> +		return NULL;
> +
> +	return vmap(xen_obj->pages, xen_obj->num_pages,
> +			VM_MAP, pgprot_writecombine(PAGE_KERNEL));
> +}
> +
> +static void gem_prime_vunmap(struct drm_gem_object *gem_obj, void *vaddr)
> +{
> +	vunmap(vaddr);
> +}
> +
> +static int gem_prime_mmap(struct drm_gem_object *gem_obj,
> +	struct vm_area_struct *vma)
> +{
> +	struct xen_gem_object *xen_obj;
> +	int ret;
> +
> +	ret = drm_gem_mmap_obj(gem_obj, gem_obj->size, vma);
> +	if (ret < 0)
> +		return ret;
> +
> +	xen_obj = to_xen_gem_obj(gem_obj);
> +	return gem_mmap_obj(xen_obj, vma);
> +}
> +
> +static const struct xen_drm_front_gem_ops xen_drm_gem_ops = {
> +	.free_object_unlocked  = gem_free_object,
> +	.prime_get_sg_table    = gem_get_sg_table,
> +	.prime_import_sg_table = gem_import_sg_table,
> +
> +	.prime_vmap            = gem_prime_vmap,
> +	.prime_vunmap          = gem_prime_vunmap,
> +	.prime_mmap            = gem_prime_mmap,
> +
> +	.dumb_create           = gem_dumb_create,
> +
> +	.mmap                  = gem_mmap,
> +
> +	.get_pages             = gem_get_pages,
> +};
> +
> +const struct xen_drm_front_gem_ops *xen_drm_front_gem_get_ops(void)
> +{
> +	return &xen_drm_gem_ops;
> +}
> diff --git a/drivers/gpu/drm/xen/xen_drm_front_gem.h b/drivers/gpu/drm/xen/xen_drm_front_gem.h
> new file mode 100644
> index 000000000000..d1e1711cc3fc
> --- /dev/null
> +++ b/drivers/gpu/drm/xen/xen_drm_front_gem.h
> @@ -0,0 +1,46 @@
> +/*
> + *  Xen para-virtual DRM device
> + *
> + *   This program is free software; you can redistribute it and/or modify
> + *   it under the terms of the GNU General Public License as published by
> + *   the Free Software Foundation; either version 2 of the License, or
> + *   (at your option) any later version.
> + *
> + *   This program is distributed in the hope that it will be useful,
> + *   but WITHOUT ANY WARRANTY; without even the implied warranty of
> + *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> + *   GNU General Public License for more details.
> + *
> + * Copyright (C) 2016-2018 EPAM Systems Inc.
> + *
> + * Author: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
> + */
> +
> +#ifndef __XEN_DRM_FRONT_GEM_H
> +#define __XEN_DRM_FRONT_GEM_H
> +
> +#include <drm/drmP.h>
> +
> +struct xen_drm_front_gem_ops {
> +	void (*free_object_unlocked)(struct drm_gem_object *obj);
> +
> +	struct sg_table *(*prime_get_sg_table)(struct drm_gem_object *obj);
> +	struct drm_gem_object *(*prime_import_sg_table)(struct drm_device *dev,
> +			struct dma_buf_attachment *attach,
> +			struct sg_table *sgt);
> +	void *(*prime_vmap)(struct drm_gem_object *obj);
> +	void (*prime_vunmap)(struct drm_gem_object *obj, void *vaddr);
> +	int (*prime_mmap)(struct drm_gem_object *obj,
> +			struct vm_area_struct *vma);
> +
> +	int (*dumb_create)(struct drm_file *file_priv, struct drm_device *dev,
> +			struct drm_mode_create_dumb *args);
> +
> +	int (*mmap)(struct file *filp, struct vm_area_struct *vma);
> +
> +	struct page **(*get_pages)(struct drm_gem_object *obj);
> +};
> +
> +const struct xen_drm_front_gem_ops *xen_drm_front_gem_get_ops(void);
> +
> +#endif /* __XEN_DRM_FRONT_GEM_H */
> diff --git a/drivers/gpu/drm/xen/xen_drm_front_gem_cma.c b/drivers/gpu/drm/xen/xen_drm_front_gem_cma.c
> new file mode 100644
> index 000000000000..5ffcbfa652d5
> --- /dev/null
> +++ b/drivers/gpu/drm/xen/xen_drm_front_gem_cma.c
> @@ -0,0 +1,93 @@
> +/*
> + *  Xen para-virtual DRM device
> + *
> + *   This program is free software; you can redistribute it and/or modify
> + *   it under the terms of the GNU General Public License as published by
> + *   the Free Software Foundation; either version 2 of the License, or
> + *   (at your option) any later version.
> + *
> + *   This program is distributed in the hope that it will be useful,
> + *   but WITHOUT ANY WARRANTY; without even the implied warranty of
> + *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> + *   GNU General Public License for more details.
> + *
> + * Copyright (C) 2016-2018 EPAM Systems Inc.
> + *
> + * Author: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
> + */
> +
> +#include <drm/drmP.h>
> +#include <drm/drm_gem.h>
> +#include <drm/drm_fb_cma_helper.h>
> +#include <drm/drm_gem_cma_helper.h>
> +
> +#include "xen_drm_front.h"
> +#include "xen_drm_front_drv.h"
> +#include "xen_drm_front_gem.h"
> +
> +static struct drm_gem_object *gem_import_sg_table(struct drm_device *dev,
> +		struct dma_buf_attachment *attach, struct sg_table *sgt)
> +{
> +	struct xen_drm_front_drm_info *drm_info = dev->dev_private;
> +	struct drm_gem_object *gem_obj;
> +	struct drm_gem_cma_object *cma_obj;
> +	int ret;
> +
> +	gem_obj = drm_gem_cma_prime_import_sg_table(dev, attach, sgt);
> +	if (IS_ERR_OR_NULL(gem_obj))
> +		return gem_obj;
> +
> +	cma_obj = to_drm_gem_cma_obj(gem_obj);
> +
> +	ret = drm_info->front_ops->dbuf_create_from_sgt(
> +			drm_info->front_info,
> +			xen_drm_front_dbuf_to_cookie(gem_obj),
> +			0, 0, 0, gem_obj->size,
> +			drm_gem_cma_prime_get_sg_table(gem_obj));
> +	if (ret < 0)
> +		return ERR_PTR(ret);
> +
> +	DRM_DEBUG("Imported CMA buffer of size %zu\n", gem_obj->size);
> +
> +	return gem_obj;
> +}
> +
> +static int gem_dumb_create(struct drm_file *filp, struct drm_device *dev,
> +	struct drm_mode_create_dumb *args)
> +{
> +	struct xen_drm_front_drm_info *drm_info = dev->dev_private;
> +
> +	if (drm_info->cfg->be_alloc) {
> +		/* This use-case is not yet supported and probably won't be */
> +		DRM_ERROR("Backend allocated buffers and CMA helpers are not supported at the same time\n");
> +		return -EINVAL;
> +	}
> +
> +	return drm_gem_cma_dumb_create(filp, dev, args);
> +}
> +
> +static struct page **gem_get_pages(struct drm_gem_object *gem_obj)
> +{
> +	return NULL;
> +}
> +
> +static const struct xen_drm_front_gem_ops xen_drm_front_gem_cma_ops = {
> +	.free_object_unlocked  = drm_gem_cma_free_object,
> +	.prime_get_sg_table    = drm_gem_cma_prime_get_sg_table,
> +	.prime_import_sg_table = gem_import_sg_table,
> +
> +	.prime_vmap            = drm_gem_cma_prime_vmap,
> +	.prime_vunmap          = drm_gem_cma_prime_vunmap,
> +	.prime_mmap            = drm_gem_cma_prime_mmap,
> +
> +	.dumb_create           = gem_dumb_create,
> +
> +	.mmap                  = drm_gem_cma_mmap,
> +
> +	.get_pages             = gem_get_pages,
> +};

Again quite a midlayer you have here. Please inline this to avoid
confusion for other people (since it looks like you only have 1
implementation).

> +
> +const struct xen_drm_front_gem_ops *xen_drm_front_gem_get_ops(void)
> +{
> +	return &xen_drm_front_gem_cma_ops;
> +}
> -- 
> 2.7.4
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch