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Date:   Fri, 23 Feb 2018 10:59:35 -0500
From:   Sean Paul <seanpaul@chromium.org>
To:     Rob Clark <robdclark@gmail.com>
Cc:     dri-devel@lists.freedesktop.org, freedreno@lists.freedesktop.org,
        linux-arm-msm@vger.kernel.org,
        Brian Starkey <brian.starkey@arm.com>,
        Liviu Dudau <liviu.dudau@arm.com>,
        Mihail Atanassov <mihail.atanassov@arm.com>,
        Gustavo Padovan <gustavo@padovan.org>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Sean Paul <seanpaul@chromium.org>,
        David Airlie <airlied@linux.ie>,
        Jonathan Corbet <corbet@lwn.net>, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC 1/4] drm: Add writeback connector type
Message-ID: <20180223155935.GJ223881@art_vandelay>
References: <20180223131758.18362-1-robdclark@gmail.com>
 <20180223131758.18362-2-robdclark@gmail.com>
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On Fri, Feb 23, 2018 at 08:17:51AM -0500, Rob Clark wrote:
> From: Brian Starkey <brian.starkey@arm.com>
> 
> Writeback connectors represent writeback engines which can write the
> CRTC output to a memory framebuffer. Add a writeback connector type and
> related support functions.
> 
> Drivers should initialize a writeback connector with
> drm_writeback_connector_init() which takes care of setting up all the
> writeback-specific details on top of the normal functionality of
> drm_connector_init().
> 
> Writeback connectors have a WRITEBACK_FB_ID property, used to set the
> output framebuffer, and a WRITEBACK_PIXEL_FORMATS blob used to expose the
> supported writeback formats to userspace.
> 
> When a framebuffer is attached to a writeback connector with the
> WRITEBACK_FB_ID property, it is used only once (for the commit in which
> it was included), and userspace can never read back the value of
> WRITEBACK_FB_ID. WRITEBACK_FB_ID can only be set if the connector is
> attached to a CRTC.
> 
> Changes since v1:
>  - Added drm_writeback.c + documentation
>  - Added helper to initialize writeback connector in one go
>  - Added core checks
>  - Squashed into a single commit
>  - Dropped the client cap
>  - Writeback framebuffers are no longer persistent
> 
> Changes since v2:
>  Daniel Vetter:
>  - Subclass drm_connector to drm_writeback_connector
>  - Relax check to allow CRTC to be set without an FB
>  - Add some writeback_ prefixes
>  - Drop PIXEL_FORMATS_SIZE property, as it was unnecessary
>  Gustavo Padovan:
>  - Add drm_writeback_job to handle writeback signalling centrally
> 
> Changes since v3:
>  - Rebased
>  - Rename PIXEL_FORMATS -> WRITEBACK_PIXEL_FORMATS
> 
> Changes since v4:
>  - Added atomic_commit() vfunc to connector helper funcs, so that
>    writeback jobs are committed from atomic helpers
> 
> Signed-off-by: Brian Starkey <brian.starkey@arm.com>
> [rebased and fixed conflicts]
> Signed-off-by: Mihail Atanassov <mihail.atanassov@arm.com>
> Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
> [rebased and added atomic_commit() vfunc for writeback jobs]
> Signed-off-by: Rob Clark <robdclark@gmail.com>
> ---
>  Documentation/gpu/drm-kms.rst            |   9 ++
>  drivers/gpu/drm/Makefile                 |   2 +-
>  drivers/gpu/drm/drm_atomic.c             | 130 ++++++++++++++++
>  drivers/gpu/drm/drm_atomic_helper.c      |  30 ++++
>  drivers/gpu/drm/drm_connector.c          |   4 +-
>  drivers/gpu/drm/drm_writeback.c          | 257 +++++++++++++++++++++++++++++++
>  include/drm/drm_atomic.h                 |   3 +
>  include/drm/drm_connector.h              |  13 ++
>  include/drm/drm_mode_config.h            |  14 ++
>  include/drm/drm_modeset_helper_vtables.h |  11 ++
>  include/drm/drm_writeback.h              |  89 +++++++++++
>  include/uapi/drm/drm_mode.h              |   1 +
>  12 files changed, 561 insertions(+), 2 deletions(-)
>  create mode 100644 drivers/gpu/drm/drm_writeback.c
>  create mode 100644 include/drm/drm_writeback.h
> 
> diff --git a/Documentation/gpu/drm-kms.rst b/Documentation/gpu/drm-kms.rst
> index 2dcf5b42015d..e7590dd2f71e 100644
> --- a/Documentation/gpu/drm-kms.rst
> +++ b/Documentation/gpu/drm-kms.rst
> @@ -370,6 +370,15 @@ Connector Functions Reference
>  .. kernel-doc:: drivers/gpu/drm/drm_connector.c
>     :export:
>  
> +Writeback Connectors
> +--------------------
> +
> +.. kernel-doc:: drivers/gpu/drm/drm_writeback.c
> +  :doc: overview
> +
> +.. kernel-doc:: drivers/gpu/drm/drm_writeback.c
> +  :export:
> +
>  Encoder Abstraction
>  ===================
>  
> diff --git a/drivers/gpu/drm/Makefile b/drivers/gpu/drm/Makefile
> index 50093ff4479b..3d708959b224 100644
> --- a/drivers/gpu/drm/Makefile
> +++ b/drivers/gpu/drm/Makefile
> @@ -18,7 +18,7 @@ drm-y       :=	drm_auth.o drm_bufs.o drm_cache.o \
>  		drm_encoder.o drm_mode_object.o drm_property.o \
>  		drm_plane.o drm_color_mgmt.o drm_print.o \
>  		drm_dumb_buffers.o drm_mode_config.o drm_vblank.o \
> -		drm_syncobj.o drm_lease.o
> +		drm_syncobj.o drm_lease.o drm_writeback.o
>  
>  drm-$(CONFIG_DRM_LIB_RANDOM) += lib/drm_random.o
>  drm-$(CONFIG_DRM_VM) += drm_vm.o
> diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
> index 46733d534587..019f131fe8be 100644
> --- a/drivers/gpu/drm/drm_atomic.c
> +++ b/drivers/gpu/drm/drm_atomic.c
> @@ -30,6 +30,7 @@
>  #include <drm/drm_atomic.h>
>  #include <drm/drm_mode.h>
>  #include <drm/drm_print.h>
> +#include <drm/drm_writeback.h>
>  #include <linux/sync_file.h>
>  
>  #include "drm_crtc_internal.h"
> @@ -638,6 +639,46 @@ static void drm_atomic_crtc_print_state(struct drm_printer *p,
>  		crtc->funcs->atomic_print_state(p, state);
>  }
>  
> +/**
> + * drm_atomic_connector_check - check connector state
> + * @connector: connector to check
> + * @state: connector state to check
> + *
> + * Provides core sanity checks for connector state.
> + *
> + * RETURNS:
> + * Zero on success, error code on failure
> + */
> +static int drm_atomic_connector_check(struct drm_connector *connector,
> +		struct drm_connector_state *state)
> +{
> +	struct drm_crtc_state *crtc_state;
> +	struct drm_writeback_job *writeback_job = state->writeback_job;
> +
> +	if ((connector->connector_type != DRM_MODE_CONNECTOR_WRITEBACK) ||
> +	    !writeback_job)
> +		return 0;
> +
> +	if (writeback_job->fb && !state->crtc) {
> +		DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] framebuffer without CRTC\n",
> +				 connector->base.id, connector->name);
> +		return -EINVAL;
> +	}
> +
> +	if (state->crtc)
> +		crtc_state = drm_atomic_get_existing_crtc_state(state->state,
> +								state->crtc);
> +
> +	if (writeback_job->fb && !crtc_state->active) {

Checking for writeback_job->fb is redundant here and above since it's checked
first thing as early exit criteria.

> +		DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] has framebuffer, but [CRTC:%d] is off\n",
> +				 connector->base.id, connector->name,
> +				 state->crtc->base.id);
> +		return -EINVAL;
> +	}
> +
> +	return 0;
> +}
> +
>  /**
>   * drm_atomic_get_plane_state - get plane state
>   * @state: global atomic state object
> @@ -1230,6 +1271,12 @@ static int drm_atomic_connector_set_property(struct drm_connector *connector,
>  			return -EINVAL;
>  		}
>  		state->content_protection = val;
> +	} else if (property == config->writeback_fb_id_property) {
> +		struct drm_framebuffer *fb = drm_framebuffer_lookup(dev, NULL, val);
> +		int ret = drm_atomic_set_writeback_fb_for_connector(state, fb);
> +		if (fb)
> +			drm_framebuffer_unreference(fb);
> +		return ret;
>  	} else if (connector->funcs->atomic_set_property) {
>  		return connector->funcs->atomic_set_property(connector,
>  				state, property, val);
> @@ -1311,6 +1358,9 @@ drm_atomic_connector_get_property(struct drm_connector *connector,
>  		*val = state->scaling_mode;
>  	} else if (property == connector->content_protection_property) {
>  		*val = state->content_protection;
> +	} else if (property == config->writeback_fb_id_property) {
> +		/* Writeback framebuffer is one-shot, write and forget */
> +		*val = 0;
>  	} else if (connector->funcs->atomic_get_property) {
>  		return connector->funcs->atomic_get_property(connector,
>  				state, property, val);
> @@ -1518,6 +1568,75 @@ drm_atomic_set_crtc_for_connector(struct drm_connector_state *conn_state,
>  }
>  EXPORT_SYMBOL(drm_atomic_set_crtc_for_connector);
>  
> +/*
> + * drm_atomic_get_writeback_job - return or allocate a writeback job
> + * @conn_state: Connector state to get the job for
> + *
> + * Writeback jobs have a different lifetime to the atomic state they are
> + * associated with. This convenience function takes care of allocating a job
> + * if there isn't yet one associated with the connector state, otherwise
> + * it just returns the existing job.
> + *
> + * Returns: The writeback job for the given connector state
> + */
> +static struct drm_writeback_job *
> +drm_atomic_get_writeback_job(struct drm_connector_state *conn_state)
> +{
> +	WARN_ON(conn_state->connector->connector_type !=
> +		DRM_MODE_CONNECTOR_WRITEBACK);
> +
> +	if (!conn_state->writeback_job)
> +		conn_state->writeback_job =
> +			kzalloc(sizeof(*conn_state->writeback_job), GFP_KERNEL);
> +
> +	return conn_state->writeback_job;
> +}
> +
> +/**
> + * drm_atomic_set_writeback_fb_for_connector - set writeback framebuffer
> + * @conn_state: atomic state object for the connector
> + * @fb: fb to use for the connector
> + *
> + * This is used to set the framebuffer for a writeback connector, which outputs
> + * to a buffer instead of an actual physical connector.
> + * Changing the assigned framebuffer requires us to grab a reference to the new
> + * fb and drop the reference to the old fb, if there is one. This function
> + * takes care of all these details besides updating the pointer in the
> + * state object itself.
> + *
> + * Note: The only way conn_state can already have an fb set is if the commit
> + * sets the property more than once.
> + *
> + * See also: drm_writeback_connector_init()
> + *
> + * Returns: 0 on success
> + */
> +int drm_atomic_set_writeback_fb_for_connector(
> +		struct drm_connector_state *conn_state,
> +		struct drm_framebuffer *fb)
> +{
> +	struct drm_writeback_job *job =
> +		drm_atomic_get_writeback_job(conn_state);
> +	if (!job)
> +		return -ENOMEM;
> +
> +	if (job->fb)
> +		drm_framebuffer_unreference(job->fb);
> +	if (fb)
> +		drm_framebuffer_reference(fb);
> +	job->fb = fb;
> +
> +	if (fb)
> +		DRM_DEBUG_ATOMIC("Set [FB:%d] for connector state %p\n",
> +				 fb->base.id, conn_state);
> +	else
> +		DRM_DEBUG_ATOMIC("Set [NOFB] for connector state %p\n",
> +				 conn_state);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL(drm_atomic_set_writeback_fb_for_connector);
> +
>  /**
>   * drm_atomic_add_affected_connectors - add connectors for crtc
>   * @state: atomic state
> @@ -1636,6 +1755,8 @@ int drm_atomic_check_only(struct drm_atomic_state *state)
>  	struct drm_plane_state *plane_state;
>  	struct drm_crtc *crtc;
>  	struct drm_crtc_state *crtc_state;
> +	struct drm_connector *conn;
> +	struct drm_connector_state *conn_state;
>  	int i, ret = 0;
>  
>  	DRM_DEBUG_ATOMIC("checking %p\n", state);
> @@ -1658,6 +1779,15 @@ int drm_atomic_check_only(struct drm_atomic_state *state)
>  		}
>  	}
>  
> +	for_each_new_connector_in_state(state, conn, conn_state, i) {
> +		ret = drm_atomic_connector_check(conn, conn_state);
> +		if (ret) {
> +			DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] atomic core check failed\n",
> +					 conn->base.id, conn->name);
> +			return ret;
> +		}
> +	}
> +
>  	if (config->funcs->atomic_check)
>  		ret = config->funcs->atomic_check(state->dev, state);
>  
> diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> index ae3cbfe9e01c..12b910755d84 100644
> --- a/drivers/gpu/drm/drm_atomic_helper.c
> +++ b/drivers/gpu/drm/drm_atomic_helper.c
> @@ -30,6 +30,7 @@
>  #include <drm/drm_plane_helper.h>
>  #include <drm/drm_crtc_helper.h>
>  #include <drm/drm_atomic_helper.h>
> +#include <drm/drm_writeback.h>
>  #include <linux/dma-fence.h>
>  
>  #include "drm_crtc_helper_internal.h"
> @@ -1159,6 +1160,27 @@ void drm_atomic_helper_commit_modeset_disables(struct drm_device *dev,
>  }
>  EXPORT_SYMBOL(drm_atomic_helper_commit_modeset_disables);
>  
> +static void commit_writebacks(struct drm_device *dev, struct drm_atomic_state *old_state)
> +{
> +	struct drm_connector *connector;
> +	struct drm_connector_state *new_conn_state;
> +	int i;
> +
> +	for_each_new_connector_in_state(old_state, connector, new_conn_state, i) {
> +		const struct drm_connector_helper_funcs *funcs;
> +
> +		funcs = connector->helper_private;
> +
> +		if (new_conn_state->writeback_job &&
> +		    new_conn_state->writeback_job->fb) {
> +			WARN_ON(connector->connector_type !=
> +				DRM_MODE_CONNECTOR_WRITEBACK);
> +			funcs->atomic_commit(connector,
> +					     new_conn_state->writeback_job);
> +		}
> +	}
> +}
> +
>  /**
>   * drm_atomic_helper_commit_modeset_enables - modeset commit to enable outputs
>   * @dev: DRM device
> @@ -1238,6 +1260,8 @@ void drm_atomic_helper_commit_modeset_enables(struct drm_device *dev,
>  
>  		drm_bridge_enable(encoder->bridge);
>  	}
> +
> +	commit_writebacks(dev, old_state);
>  }
>  EXPORT_SYMBOL(drm_atomic_helper_commit_modeset_enables);
>  
> @@ -3627,6 +3651,9 @@ __drm_atomic_helper_connector_duplicate_state(struct drm_connector *connector,
>  	if (state->crtc)
>  		drm_connector_get(connector);
>  	state->commit = NULL;
> +
> +	/* Don't copy over a writeback job, they are used only once */
> +	state->writeback_job = NULL;
>  }
>  EXPORT_SYMBOL(__drm_atomic_helper_connector_duplicate_state);
>  
> @@ -3756,6 +3783,9 @@ __drm_atomic_helper_connector_destroy_state(struct drm_connector_state *state)
>  
>  	if (state->commit)
>  		drm_crtc_commit_put(state->commit);
> +
> +	if (state->writeback_job)
> +		drm_writeback_cleanup_job(state->writeback_job);
>  }
>  EXPORT_SYMBOL(__drm_atomic_helper_connector_destroy_state);
>  
> diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
> index 16b9c3810af2..add47f06ae70 100644
> --- a/drivers/gpu/drm/drm_connector.c
> +++ b/drivers/gpu/drm/drm_connector.c
> @@ -87,6 +87,7 @@ static struct drm_conn_prop_enum_list drm_connector_enum_list[] = {
>  	{ DRM_MODE_CONNECTOR_VIRTUAL, "Virtual" },
>  	{ DRM_MODE_CONNECTOR_DSI, "DSI" },
>  	{ DRM_MODE_CONNECTOR_DPI, "DPI" },
> +	{ DRM_MODE_CONNECTOR_WRITEBACK, "Writeback" },
>  };
>  
>  void drm_connector_ida_init(void)
> @@ -249,7 +250,8 @@ int drm_connector_init(struct drm_device *dev,
>  	config->num_connector++;
>  	spin_unlock_irq(&config->connector_list_lock);
>  
> -	if (connector_type != DRM_MODE_CONNECTOR_VIRTUAL)
> +	if (connector_type != DRM_MODE_CONNECTOR_VIRTUAL &&
> +	    connector_type != DRM_MODE_CONNECTOR_WRITEBACK)
>  		drm_object_attach_property(&connector->base,
>  					      config->edid_property,
>  					      0);
> diff --git a/drivers/gpu/drm/drm_writeback.c b/drivers/gpu/drm/drm_writeback.c
> new file mode 100644
> index 000000000000..da61f929cbc3
> --- /dev/null
> +++ b/drivers/gpu/drm/drm_writeback.c
> @@ -0,0 +1,257 @@
> +/*
> + * (C) COPYRIGHT 2016 ARM Limited. All rights reserved.
> + * Author: Brian Starkey <brian.starkey@arm.com>
> + *
> + * This program is free software and is provided to you under the terms of the
> + * GNU General Public License version 2 as published by the Free Software
> + * Foundation, and any use by you of this program is subject to the terms
> + * of such GNU licence.
> + */
> +
> +#include <drm/drm_crtc.h>
> +#include <drm/drm_modeset_helper_vtables.h>
> +#include <drm/drm_property.h>
> +#include <drm/drm_writeback.h>
> +#include <drm/drmP.h>
> +
> +/**
> + * DOC: overview
> + *
> + * Writeback connectors are used to expose hardware which can write the output
> + * from a CRTC to a memory buffer. They are used and act similarly to other
> + * types of connectors, with some important differences:
> + *  - Writeback connectors don't provide a way to output visually to the user.
> + *  - Writeback connectors should always report as "disconnected" (so that
> + *    clients which don't understand them will ignore them).

Have we considered hiding writeback behind a client cap instead?

> + *  - Writeback connectors don't have EDID.
> + *
> + * A framebuffer may only be attached to a writeback connector when the
> + * connector is attached to a CRTC. The WRITEBACK_FB_ID property which sets the
> + * framebuffer applies only to a single commit (see below). A framebuffer may
> + * not be attached while the CRTC is off.
> + *
> + * Writeback connectors have some additional properties, which userspace
> + * can use to query and control them:
> + *
> + *  "WRITEBACK_FB_ID":
> + *	Write-only object property storing a DRM_MODE_OBJECT_FB: it stores the
> + *	framebuffer to be written by the writeback connector. This property is
> + *	similar to the FB_ID property on planes, but will always read as zero
> + *	and is not preserved across commits.
> + *	Userspace must set this property to an output buffer every time it
> + *	wishes the buffer to get filled.
> + *
> + *  "WRITEBACK_PIXEL_FORMATS":
> + *	Immutable blob property to store the supported pixel formats table. The
> + *	data is an array of u32 DRM_FORMAT_* fourcc values.
> + *	Userspace can use this blob to find out what pixel formats are supported
> + *	by the connector's writeback engine.
> + */
> +
> +static bool create_writeback_properties(struct drm_device *dev)
> +{
> +	struct drm_property *prop;
> +
> +	if (!dev->mode_config.writeback_fb_id_property) {

Somewhat an aside, but we should probably come up with more formal rules on
where to stick properties. We have some in drm_connector, which I interpret as
"connector-only properties", but we also have a bunch in mode_config.

For optional properties which will generally only apply to one connector on a
system, I'd be inclined to put it in connector. 

> +		prop = drm_property_create_object(dev, DRM_MODE_PROP_ATOMIC,
> +						  "WRITEBACK_FB_ID",
> +						  DRM_MODE_OBJECT_FB);
> +		if (!prop)
> +			return false;
> +		dev->mode_config.writeback_fb_id_property = prop;
> +	}
> +
> +	if (!dev->mode_config.writeback_pixel_formats_property) {
> +		prop = drm_property_create(dev, DRM_MODE_PROP_BLOB | DRM_MODE_PROP_IMMUTABLE,
> +					   "WRITEBACK_PIXEL_FORMATS", 0);

Does it make sense to expose the formats if u/s doesn't support atomic and can't
use writeback? Perhaps this should also be DRM_MODE_PROP_ATOMIC.


> +		if (!prop)
> +			return false;
> +		dev->mode_config.writeback_pixel_formats_property = prop;
> +	}
> +
> +	return true;
> +}
> +
> +static const struct drm_encoder_funcs drm_writeback_encoder_funcs = {
> +	.destroy = drm_encoder_cleanup,
> +};
> +
> +/**
> + * drm_writeback_connector_init - Initialize a writeback connector and its properties
> + * @dev: DRM device
> + * @wb_connector: Writeback connector to initialize
> + * @con_funcs: Connector funcs vtable
> + * @enc_helper_funcs: Encoder helper funcs vtable to be used by the internal encoder
> + * @formats: Array of supported pixel formats for the writeback engine
> + * @n_formats: Length of the formats array
> + *
> + * This function creates the writeback-connector-specific properties if they
> + * have not been already created, initializes the connector as
> + * type DRM_MODE_CONNECTOR_WRITEBACK, and correctly initializes the property
> + * values. It will also create an internal encoder associated with the
> + * drm_writeback_connector and set it to use the @enc_helper_funcs vtable for
> + * the encoder helper.
> + *
> + * Drivers should always use this function instead of drm_connector_init() to
> + * set up writeback connectors.
> + *
> + * Returns: 0 on success, or a negative error code
> + */
> +int drm_writeback_connector_init(struct drm_device *dev,
> +				 struct drm_writeback_connector *wb_connector,
> +				 const struct drm_connector_funcs *con_funcs,
> +				 const struct drm_encoder_helper_funcs *enc_helper_funcs,
> +				 const u32 *formats, int n_formats)
> +{
> +	int ret;
> +	struct drm_property_blob *blob;
> +	struct drm_connector *connector = &wb_connector->base;
> +	struct drm_mode_config *config = &dev->mode_config;
> +
> +	if (!create_writeback_properties(dev))
> +		return -EINVAL;
> +
> +	blob = drm_property_create_blob(dev, n_formats * sizeof(*formats),
> +					formats);
> +	if (IS_ERR(blob))
> +		return PTR_ERR(blob);
> +
> +	drm_encoder_helper_add(&wb_connector->encoder, enc_helper_funcs);
> +	ret = drm_encoder_init(dev, &wb_connector->encoder,
> +			       &drm_writeback_encoder_funcs,
> +			       DRM_MODE_ENCODER_VIRTUAL, NULL);
> +	if (ret)
> +		goto fail;
> +
> +	connector->interlace_allowed = 0;
> +
> +	ret = drm_connector_init(dev, connector, con_funcs,
> +				 DRM_MODE_CONNECTOR_WRITEBACK);
> +	if (ret)
> +		goto connector_fail;
> +
> +	ret = drm_mode_connector_attach_encoder(connector,
> +						&wb_connector->encoder);
> +	if (ret)
> +		goto attach_fail;
> +
> +	INIT_LIST_HEAD(&wb_connector->job_queue);
> +	spin_lock_init(&wb_connector->job_lock);
> +
> +	drm_object_attach_property(&connector->base,
> +				   config->writeback_fb_id_property, 0);
> +
> +	drm_object_attach_property(&connector->base,
> +				   config->writeback_pixel_formats_property,
> +				   blob->base.id);
> +	wb_connector->pixel_formats_blob_ptr = blob;
> +
> +	return 0;
> +
> +attach_fail:
> +	drm_connector_cleanup(connector);
> +connector_fail:
> +	drm_encoder_cleanup(&wb_connector->encoder);
> +fail:
> +	drm_property_unreference_blob(blob);
> +	return ret;
> +}
> +EXPORT_SYMBOL(drm_writeback_connector_init);
> +
> +/**
> + * drm_writeback_queue_job - Queue a writeback job for later signalling
> + * @wb_connector: The writeback connector to queue a job on
> + * @job: The job to queue
> + *
> + * This function adds a job to the job_queue for a writeback connector. It
> + * should be considered to take ownership of the writeback job, and so any other
> + * references to the job must be cleared after calling this function.
> + *
> + * Drivers must ensure that for a given writeback connector, jobs are queued in
> + * exactly the same order as they will be completed by the hardware (and
> + * signaled via drm_writeback_signal_completion).
> + *
> + * For every call to drm_writeback_queue_job() there must be exactly one call to
> + * drm_writeback_signal_completion()
> + *
> + * See also: drm_writeback_signal_completion()
> + */
> +void drm_writeback_queue_job(struct drm_writeback_connector *wb_connector,
> +			     struct drm_writeback_job *job)
> +{
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&wb_connector->job_lock, flags);
> +	list_add_tail(&job->list_entry, &wb_connector->job_queue);
> +	spin_unlock_irqrestore(&wb_connector->job_lock, flags);
> +}
> +EXPORT_SYMBOL(drm_writeback_queue_job);
> +
> +/**
> + * drm_writeback_cleanup_job - Cleanup and free a writeback job
> + * @job: The writeback job to free
> + *
> + * Drops any references held by the writeback job, and frees the structure.
> + */
> +void drm_writeback_cleanup_job(struct drm_writeback_job *job)
> +{
> +	if (!job)
> +		return;
> +
> +	if (job->fb)
> +		drm_framebuffer_unreference(job->fb);
> +	kfree(job);
> +}
> +EXPORT_SYMBOL(drm_writeback_cleanup_job);
> +
> +/*
> + * @cleanup_work: deferred cleanup of a writeback job
> + *
> + * The job cannot be cleaned up directly in drm_writeback_signal_completion,
> + * because it may be called in interrupt context. Dropping the framebuffer
> + * reference can sleep, and so the cleanup is deferred to a workqueue.
> + */
> +static void cleanup_work(struct work_struct *work)
> +{
> +	struct drm_writeback_job *job = container_of(work,
> +						     struct drm_writeback_job,
> +						     cleanup_work);
> +	drm_writeback_cleanup_job(job);

TIL that you can free work while using it.

> +}
> +
> +/**
> + * drm_writeback_signal_completion - Signal the completion of a writeback job
> + * @wb_connector: The writeback connector whose job is complete
> + *
> + * Drivers should call this to signal the completion of a previously queued
> + * writeback job. It should be called as soon as possible after the hardware
> + * has finished writing, and may be called from interrupt context.
> + * It is the driver's responsibility to ensure that for a given connector, the
> + * hardware completes writeback jobs in the same order as they are queued.
> + *
> + * Unless the driver is holding its own reference to the framebuffer, it must
> + * not be accessed after calling this function.
> + *
> + * See also: drm_writeback_queue_job()
> + */
> +void
> +drm_writeback_signal_completion(struct drm_writeback_connector *wb_connector)
> +{
> +	unsigned long flags;
> +	struct drm_writeback_job *job;
> +
> +	spin_lock_irqsave(&wb_connector->job_lock, flags);
> +	job = list_first_entry_or_null(&wb_connector->job_queue,
> +				       struct drm_writeback_job,
> +				       list_entry);
> +	if (job)
> +		list_del(&job->list_entry);
> +	spin_unlock_irqrestore(&wb_connector->job_lock, flags);
> +
> +	if (WARN_ON(!job))
> +		return;
> +
> +	INIT_WORK(&job->cleanup_work, cleanup_work);
> +	queue_work(system_long_wq, &job->cleanup_work);
> +}
> +EXPORT_SYMBOL(drm_writeback_signal_completion);
> diff --git a/include/drm/drm_atomic.h b/include/drm/drm_atomic.h
> index cf13842a6dbd..d7b0263cc5cf 100644
> --- a/include/drm/drm_atomic.h
> +++ b/include/drm/drm_atomic.h
> @@ -594,6 +594,9 @@ void drm_atomic_set_fence_for_plane(struct drm_plane_state *plane_state,
>  int __must_check
>  drm_atomic_set_crtc_for_connector(struct drm_connector_state *conn_state,
>  				  struct drm_crtc *crtc);
> +int drm_atomic_set_writeback_fb_for_connector(
> +		struct drm_connector_state *conn_state,
> +		struct drm_framebuffer *fb);
>  int __must_check
>  drm_atomic_add_affected_connectors(struct drm_atomic_state *state,
>  				   struct drm_crtc *crtc);
> diff --git a/include/drm/drm_connector.h b/include/drm/drm_connector.h
> index 758a176e7b57..8701ebcc68b3 100644
> --- a/include/drm/drm_connector.h
> +++ b/include/drm/drm_connector.h
> @@ -425,6 +425,19 @@ struct drm_connector_state {
>  	 * protection. This is most commonly used for HDCP.
>  	 */
>  	unsigned int content_protection;
> +
> +	/**
> +	 * @writeback_job: Writeback job for writeback connectors
> +	 *
> +	 * Holds the framebuffer for a writeback connector. As the writeback
> +	 * completion may be asynchronous to the normal commit cycle, the
> +	 * writeback job lifetime is managed separately from the normal atomic
> +	 * state by this object.
> +	 *
> +	 * See also: drm_writeback_queue_job() and
> +	 * drm_writeback_signal_completion()
> +	 */
> +	struct drm_writeback_job *writeback_job;
>  };
>  
>  /**
> diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
> index 7569f22ffef6..c012e1148ec0 100644
> --- a/include/drm/drm_mode_config.h
> +++ b/include/drm/drm_mode_config.h
> @@ -779,6 +779,20 @@ struct drm_mode_config {
>  	 */
>  	struct drm_property *panel_orientation_property;
>  
> +	/**
> +	 * @writeback_fb_id_property: Property for writeback connectors, storing
> +	 * the ID of the output framebuffer.
> +	 * See also: drm_writeback_connector_init()
> +	 */
> +	struct drm_property *writeback_fb_id_property;
> +	/**
> +	 * @writeback_pixel_formats_property: Property for writeback connectors,
> +	 * storing an array of the supported pixel formats for the writeback
> +	 * engine (read-only).
> +	 * See also: drm_writeback_connector_init()
> +	 */
> +	struct drm_property *writeback_pixel_formats_property;
> +
>  	/* dumb ioctl parameters */
>  	uint32_t preferred_depth, prefer_shadow;
>  
> diff --git a/include/drm/drm_modeset_helper_vtables.h b/include/drm/drm_modeset_helper_vtables.h
> index 3e76ca805b0f..97d3a810bc85 100644
> --- a/include/drm/drm_modeset_helper_vtables.h
> +++ b/include/drm/drm_modeset_helper_vtables.h
> @@ -974,6 +974,17 @@ struct drm_connector_helper_funcs {
>  	 */
>  	int (*atomic_check)(struct drm_connector *connector,
>  			    struct drm_connector_state *state);
> +
> +	/**
> +	 * @atomic_commit:
> +	 *
> +	 * For write-back connectors, this is the point to commit the write-
> +	 * back job to hw.
> +	 *
> +	 * This callback is used by the atomic modeset helpers.
> +	 */
> +	void (*atomic_commit)(struct drm_connector *connector,
> +			      struct drm_writeback_job *writeback_job);
>  };
>  
>  /**
> diff --git a/include/drm/drm_writeback.h b/include/drm/drm_writeback.h
> new file mode 100644
> index 000000000000..0bb95fd4907d
> --- /dev/null
> +++ b/include/drm/drm_writeback.h
> @@ -0,0 +1,89 @@
> +/*
> + * (C) COPYRIGHT 2016 ARM Limited. All rights reserved.
> + * Author: Brian Starkey <brian.starkey@arm.com>
> + *
> + * This program is free software and is provided to you under the terms of the
> + * GNU General Public License version 2 as published by the Free Software
> + * Foundation, and any use by you of this program is subject to the terms
> + * of such GNU licence.
> + */
> +
> +#ifndef __DRM_WRITEBACK_H__
> +#define __DRM_WRITEBACK_H__
> +#include <drm/drm_connector.h>
> +#include <drm/drm_encoder.h>
> +#include <linux/workqueue.h>
> +
> +struct drm_writeback_connector {
> +	struct drm_connector base;
> +
> +	/**
> +	 * @encoder: Internal encoder used by the connector to fulfill
> +	 * the DRM framework requirements. The users of the
> +	 * @drm_writeback_connector control the behaviour of the @encoder
> +	 * by passing the @enc_funcs parameter to drm_writeback_connector_init()
> +	 * function.
> +	 */
> +	struct drm_encoder encoder;
> +
> +	/**
> +	 * @pixel_formats_blob_ptr:
> +	 *
> +	 * DRM blob property data for the pixel formats list on writeback
> +	 * connectors
> +	 * See also drm_writeback_connector_init()
> +	 */
> +	struct drm_property_blob *pixel_formats_blob_ptr;
> +
> +	/** @job_lock: Protects job_queue */
> +	spinlock_t job_lock;
> +
> +	/**
> +	 * @job_queue:
> +	 *
> +	 * Holds a list of a connector's writeback jobs; the last item is the
> +	 * most recent. The first item may be either waiting for the hardware
> +	 * to begin writing, or currently being written.
> +	 *
> +	 * See also: drm_writeback_queue_job() and
> +	 * drm_writeback_signal_completion()
> +	 */
> +	struct list_head job_queue;
> +};
> +#define to_wb_connector(x) container_of(x, struct drm_writeback_connector, base)
> +
> +struct drm_writeback_job {
> +	/**
> +	 * @cleanup_work:
> +	 *
> +	 * Used to allow drm_writeback_signal_completion to defer dropping the
> +	 * framebuffer reference to a workqueue.
> +	 */
> +	struct work_struct cleanup_work;
> +	/**
> +	 * @list_entry:
> +	 *
> +	 * List item for the connector's @job_queue
> +	 */
> +	struct list_head list_entry;
> +	/**
> +	 * @fb:
> +	 *
> +	 * Framebuffer to be written to by the writeback connector. Do not set
> +	 * directly, use drm_atomic_set_writeback_fb_for_connector()
> +	 */
> +	struct drm_framebuffer *fb;
> +};
> +
> +int drm_writeback_connector_init(struct drm_device *dev,
> +				 struct drm_writeback_connector *wb_connector,
> +				 const struct drm_connector_funcs *con_funcs,
> +				 const struct drm_encoder_helper_funcs *enc_helper_funcs,
> +				 const u32 *formats, int n_formats);
> +
> +void drm_writeback_queue_job(struct drm_writeback_connector *wb_connector,
> +			     struct drm_writeback_job *job);
> +
> +void drm_writeback_cleanup_job(struct drm_writeback_job *job);
> +void drm_writeback_signal_completion(struct drm_writeback_connector *wb_connector);
> +#endif
> diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
> index 2c575794fb52..7b47e184e95e 100644
> --- a/include/uapi/drm/drm_mode.h
> +++ b/include/uapi/drm/drm_mode.h
> @@ -338,6 +338,7 @@ enum drm_mode_subconnector {
>  #define DRM_MODE_CONNECTOR_VIRTUAL      15
>  #define DRM_MODE_CONNECTOR_DSI		16
>  #define DRM_MODE_CONNECTOR_DPI		17
> +#define DRM_MODE_CONNECTOR_WRITEBACK	18
>  
>  struct drm_mode_get_connector {
>  
> -- 
> 2.14.3
> 

-- 
Sean Paul, Software Engineer, Google / Chromium OS