drm: Add writeback connector type

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 committed 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

Changes since v9:
 - Revert the v6 changes that free the writeback job in the connector
   state cleanup and return to doing it in the cleanup_work() function

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>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/229037/

12 files changed, 541 insertions, 2 deletions

diff --git a/Documentation/gpu/drm-kms.rst b/Documentation/gpu/drm-kms.rst
index e233c2626bd0..4f6f113a7f5d 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 ef9f3dab287f..69c13517ea3a 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 9ec5c865a043..3e53d6e5c340 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"
@@ -691,6 +692,45 @@ static void drm_atomic_crtc_print_state(struct drm_printer *p,
 }
 
 /**
+ * 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
  * @plane: plane to get state object for
@@ -1321,6 +1361,12 @@ 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_put(fb);
+                return ret;
         } else if (connector->funcs->atomic_set_property) {
                 return connector->funcs->atomic_set_property(connector,
                                 state, property, val);
@@ -1407,6 +1453,9 @@ 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);
@@ -1631,6 +1680,70 @@ 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;
+
+        drm_framebuffer_assign(&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
@@ -1752,6 +1865,8 @@ 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);
@@ -1774,6 +1889,15 @@ 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 232fa11a5e31..17baf5057132 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);
 
@@ -3647,6 +3669,9 @@ __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);
 
diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
index 549b89501e01..2f9ebddd178e 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)
@@ -253,7 +254,8 @@ 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..e5b8a4b79724
--- /dev/null
+++ b/drivers/gpu/drm/drm_writeback.c
@@ -0,0 +1,245 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * (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_blob_put(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);
+
+/*
+ * @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_framebuffer_put(job->fb);
+        kfree(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 ca461b6cf71f..8254521b4583 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 c5797c24edd3..716c3a0e0e1d 100644
--- a/include/drm/drm_connector.h
+++ b/include/drm/drm_connector.h
@@ -437,6 +437,19 @@ 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 fb45839179dd..5f24329e6927 100644
--- a/include/drm/drm_mode_config.h
+++ b/include/drm/drm_mode_config.h
@@ -784,6 +784,21 @@ 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 35e2a3a79fc5..3b289773297c 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 000000000000..17cd1feecd7e
--- /dev/null
+++ b/include/drm/drm_writeback.h
@@ -0,0 +1,91 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * (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 writeback 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 971c016b368c..8d67243952f4 100644
--- a/include/uapi/drm/drm_mode.h
+++ b/include/uapi/drm/drm_mode.h
@@ -351,6 +351,7 @@ 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 {