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Date:   Fri, 23 Feb 2018 16:21:05 +0000
From:   Liviu Dudau <liviu.dudau@arm.com>
To:     Sean Paul <seanpaul@chromium.org>
Cc:     Rob Clark <robdclark@gmail.com>, dri-devel@lists.freedesktop.org,
        freedreno@lists.freedesktop.org, linux-arm-msm@vger.kernel.org,
        Brian Starkey <brian.starkey@arm.com>,
        Mihail Atanassov <mihail.atanassov@arm.com>,
        Gustavo Padovan <gustavo@padovan.org>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        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: <20180223162104.GX9111@e110455-lin.cambridge.arm.com>
References: <20180223131758.18362-1-robdclark@gmail.com>
 <20180223131758.18362-2-robdclark@gmail.com>
 <20180223155935.GJ223881@art_vandelay>
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On Fri, Feb 23, 2018 at 10:59:35AM -0500, Sean Paul wrote:
> 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.

I somehow don't see that check. We check if (!writebac_job) and then all
other checks are logic ands with other conditions, so I think the check
here is still relevant.

> 
> > +		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?

Yes, changelog says that the client cap was dropped in v2 based on
feedback (as far as I remember). 

> 
> > + *  - 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. 

I can move the writeback-related properties into the connector in the
next series.


> 
> > +		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.

Agree, probably all the writeback properties should only be available if
the client selects 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.

Was not aware that it is possible. I can update the code to do it.

Thanks for the feedback and review!

Best regards,
Liviu

> 
> > +}
> > +
> > +/**
> > + * 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

-- 
====================
| I would like to |
| fix the world,  |
| but they're not |
| giving me the   |
 \ source code!  /
  ---------------
    ¯\_(ツ)_/¯