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Date:   Fri, 8 Jun 2018 00:05:29 +0100
From:   Brian Starkey <brian.starkey@arm.com>
To:     Liviu Dudau <Liviu.Dudau@arm.com>
Cc:     Gustavo Padovan <gustavo@padovan.org>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Sean Paul <seanpaul@chromium.org>,
        Jonathan Corbet <corbet@lwn.net>,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        David Airlie <airlied@linux.ie>,
        Alexandru-Cosmin Gheorghe <alexandru-cosmin.gheorghe@arm.com>,
        Eric Anholt <eric@anholt.net>,
        Boris Brezillon <boris.brezillon@bootlin.com>,
        Maxime Ripard <maxime.ripard@bootlin.com>,
        Daniel Stone <daniels@collabora.com>,
        Rob Clark <robdclark@gmail.com>,
        Mihail Atanassov <mihail.atanassov@arm.com>
Subject: Re: [PATCH v9 1/3] drm: Add writeback connector type
Message-ID: <20180607230529.GA11886@e107564-lin.cambridge.arm.com>
References: <20180524164103.8378-1-Liviu.Dudau@arm.com>
 <20180524164103.8378-2-Liviu.Dudau@arm.com>
 <20180525153237.GA1139@e107564-lin.cambridge.arm.com>
 <20180607092639.GB4144@e110455-lin.cambridge.arm.com>
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On Thu, Jun 07, 2018 at 10:26:39AM +0100, Liviu Dudau wrote:
>On Fri, May 25, 2018 at 04:32:38PM +0100, Brian Starkey wrote:
>> On Thu, May 24, 2018 at 05:41:01PM +0100, Liviu Dudau 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
>> >
>> > Chances since v4:
>> > - Embed a drm_encoder inside the drm_writeback_connector to
>> >   reduce the amount of boilerplate code required from the drivers
>> >   that are using it.
>> >
>> > Changes since v5:
>> > - Added Rob Clark's atomic_commit() vfunc to connector helper
>> >   funcs, so that writeback jobs are commited from atomic helpers
>> > - Updated create_writeback_properties() signature to return an
>> >   error code rather than a boolean false for failure.
>> > - Free writeback job with the connector state rather than when
>> >   doing the cleanup_work()
>> >
>> > Changes since v7:
>> > - fix extraneous use of out_fence that is only introduced in a
>> >   subsequent patch.
>> >
>> > Changes since v8:
>> > - whitespace changes pull from subsequent patch
>> >
>> > Reviewed-by: Eric Anholt <eric@anholt.net>
>> > Signed-off-by: Brian Starkey <brian.starkey@arm.com>
>> > [rebased and fixed conflicts]
>> > Signed-off-by: Mihail Atanassov <mihail.atanassov@arm.com>
>> > [rebased and added atomic_commit() vfunc for writeback jobs]
>> > Signed-off-by: Rob Clark <robdclark@gmail.com>
>> > Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
>> > ---
>> > Documentation/gpu/drm-kms.rst            |   9 +
>> > drivers/gpu/drm/Makefile                 |   2 +-
>> > drivers/gpu/drm/drm_atomic.c             | 128 ++++++++++++
>> > drivers/gpu/drm/drm_atomic_helper.c      |  30 +++
>> > drivers/gpu/drm/drm_connector.c          |   4 +-
>> > drivers/gpu/drm/drm_writeback.c          | 255 +++++++++++++++++++++++
>> > include/drm/drm_atomic.h                 |   3 +
>> > include/drm/drm_connector.h              |  13 ++
>> > include/drm/drm_mode_config.h            |  15 ++
>> > include/drm/drm_modeset_helper_vtables.h |  11 +
>> > include/drm/drm_writeback.h              |  90 ++++++++
>> > include/uapi/drm/drm_mode.h              |   1 +
>> > 12 files changed, 559 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 1dffd1ac4cd44..809d403087f95 100644
>> > --- a/Documentation/gpu/drm-kms.rst
>> > +++ b/Documentation/gpu/drm-kms.rst
>> > @@ -373,6 +373,15 @@ 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 ef9f3dab287fd..69c13517ea3a6 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 895741e9cd7db..e32a93931c0dc 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"
>> > @@ -676,6 +677,45 @@ 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) {
>> > +		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
>> > @@ -1286,6 +1326,12 @@ 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);
>> > @@ -1367,6 +1413,9 @@ 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);
>> > @@ -1584,6 +1633,74 @@ 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
>> > @@ -1702,6 +1819,8 @@ 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);
>> > @@ -1724,6 +1843,15 @@ 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 130da5195f3b6..5fd12666f7d7a 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"
>> > @@ -1172,6 +1173,25 @@ void drm_atomic_helper_commit_modeset_disables(struct drm_device *dev,
>> > }
>> > EXPORT_SYMBOL(drm_atomic_helper_commit_modeset_disables);
>> >
>> > +static void drm_atomic_helper_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
>> > @@ -1251,6 +1271,8 @@ void drm_atomic_helper_commit_modeset_enables(struct drm_device *dev,
>> >
>> > 		drm_bridge_enable(encoder->bridge);
>> > 	}
>> > +
>> > +	drm_atomic_helper_commit_writebacks(dev, old_state);
>> > }
>> > EXPORT_SYMBOL(drm_atomic_helper_commit_modeset_enables);
>> >
>> > @@ -3660,6 +3682,9 @@ __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);
>> >
>> > @@ -3789,6 +3814,11 @@ __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);
>> > +		kfree(state->writeback_job);
>> > +	}
>>
>> I don't think this will work properly. After calling
>> drm_writeback_queue_job, the driver is meant to set
>> state->writeback_job to NULL, and then the writeback queue was meant
>> to take full ownership of the job (and free it on the
>> drm_writeback_signal_completion path).
>>
>> So, if drivers do that - by the time we get here, the driver will have
>> set the pointer to NULL, and we'll leak the job (because it's no
>> longer freed on the signal path).
>>
>> If drivers don't set it to NULL after queueing, then this can race
>> with the drm_writeback_signal_completion() path and do nasty things.
>>
>> If there's no way to free the job on the signal_completion() path,
>> then we'll probably need to refcount the writeback_job, and add a lock
>> to it to protect the job->fb.
>>
>> I was trying originally to avoid the refcount, but tbh the fact that
>> drivers had to set the pointer to NULL after calling
>> drm_writeback_queue_job is a little ugly and a little fragile, so
>> perhaps it's the best thing to do.
>
>Hi Brian,
>
>I've got back from my holiday and had another look at the code when I've
>realised that all this is probably not needed, as I don't think that
>your initial assumption that drm_framebuffer_unreference() can sleep is
>true. That means we can drop the framebuffer reference in the
>drm_writeback_signal_completion() safely and we don't need all the
>cleanup jobs.

I still think it can.

drm_framebuffer_unreference() drops a kref. The framebuffer's kref
release function is set to drm_framebuffer_free(), which calls
drm_mode_object_unregister() (which can sleep), and ->destroy, which
must call drm_framebuffer_cleanup() (which can sleep).

>
>I'm going to update the patch and also switch to
>drm_framebuffer_{get,put} functions and send the new series today.

Yeah I just noticed and was going to suggest that :-)

Cheers,
-Brian

>
>Best regards, Liviu

>
>>
>> -Brian
>>
>> > }
>> > 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 9b9ba5d5ec0cb..d031e46ebffcf 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 0000000000000..1141325b004ab
>> > --- /dev/null
>> > +++ b/drivers/gpu/drm/drm_writeback.c
>> > @@ -0,0 +1,255 @@
>> > +/*
>> > + * (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).
>> > + *  - 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 int create_writeback_properties(struct drm_device *dev)
>> > +{
>> > +	struct drm_property *prop;
>> > +
>> > +	if (!dev->mode_config.writeback_fb_id_property) {
>> > +		prop = drm_property_create_object(dev, DRM_MODE_PROP_ATOMIC,
>> > +						  "WRITEBACK_FB_ID",
>> > +						  DRM_MODE_OBJECT_FB);
>> > +		if (!prop)
>> > +			return -ENOMEM;
>> > +		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_ATOMIC |
>> > +					   DRM_MODE_PROP_IMMUTABLE,
>> > +					   "WRITEBACK_PIXEL_FORMATS", 0);
>> > +		if (!prop)
>> > +			return -ENOMEM;
>> > +		dev->mode_config.writeback_pixel_formats_property = prop;
>> > +	}
>> > +
>> > +	return 0;
>> > +}
>> > +
>> > +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)
>> > +{
>> > +	struct drm_property_blob *blob;
>> > +	struct drm_connector *connector = &wb_connector->base;
>> > +	struct drm_mode_config *config = &dev->mode_config;
>> > +	int ret = create_writeback_properties(dev);
>> > +
>> > +	if (ret != 0)
>> > +		return ret;
>> > +
>> > +	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->fb)
>> > +		drm_framebuffer_unreference(job->fb);
>> > +}
>> > +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);
>> > +}
>> > +
>> > +/**
>> > + * 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 a57a8aa90ffb7..a5e904105db3a 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 675cc3f8cf852..60c0ffb947d5e 100644
>> > --- a/include/drm/drm_connector.h
>> > +++ b/include/drm/drm_connector.h
>> > @@ -429,6 +429,19 @@ 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 33b3a96d66d0a..9ca32bf7bf183 100644
>> > --- a/include/drm/drm_mode_config.h
>> > +++ b/include/drm/drm_mode_config.h
>> > @@ -779,6 +779,21 @@ 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 35e2a3a79fc56..3b289773297c2 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:
>> > +	 *
>> > +	 * This hook is to be used by drivers implementing writeback connectors
>> > +	 * that need a point when to commit the writeback job to the hardware.
>> > +	 *
>> > +	 * 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 0000000000000..499c5c7b0a54e
>> > --- /dev/null
>> > +++ b/include/drm/drm_writeback.h
>> > @@ -0,0 +1,90 @@
>> > +/*
>> > + * (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;
>> > +};
>> > +
>> > +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 4b3a1bb58e680..3e6541f0400b8 100644
>> > --- a/include/uapi/drm/drm_mode.h
>> > +++ b/include/uapi/drm/drm_mode.h
>> > @@ -344,6 +344,7 @@ 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 {
>> >
>> > --
>> > 2.17.0
>> >
>
>-- 
>====================
>| I would like to |
>| fix the world,  |
>| but they're not |
>| giving me the   |
> \ source code!  /
>  ---------------
>    ¯\_(ツ)_/¯