Re: [PATCH v5 1/2] drm: Add writeback connector type

[Rob Clark]

On Mon, May 15, 2017 at 1:20 PM, Liviu Dudau <Liviu.Dudau@xxxxxxx> wrote:
> From: Brian Starkey <brian.starkey@xxxxxxx>
>
> 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.

So one thought.. since rmfb is kinda horrible, I think
rmfb/drm_framebuffer_remove() probably needs to fish out writeback
connectors that are using the fb and disable them as well.

BR,
-R


> 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 inside drivers using it.
>
> Signed-off-by: Brian Starkey <brian.starkey@xxxxxxx>
> [rebased and fixed conflicts]
> Signed-off-by: Mihail Atanassov <mihail.atanassov@xxxxxxx>
> Signed-off-by: Liviu Dudau <liviu.dudau@xxxxxxx>
> ---
>  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 |   6 +
>  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_writeback.h         |  88 ++++++++++++
>  include/uapi/drm/drm_mode.h         |   1 +
>  11 files changed, 525 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 bfecd21a8cdf..7a8ef1a4fe17 100644
> --- a/Documentation/gpu/drm-kms.rst
> +++ b/Documentation/gpu/drm-kms.rst
> @@ -364,6 +364,15 @@ 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 59f0f9b696eb..99ef500ae8b2 100644
> --- a/drivers/gpu/drm/Makefile
> +++ b/drivers/gpu/drm/Makefile
> @@ -16,7 +16,7 @@ drm-y       :=        drm_auth.o drm_bufs.o drm_cache.o \
>                 drm_framebuffer.o drm_connector.o drm_blend.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_dumb_buffers.o drm_mode_config.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 f32506a7c1d6..317ae625b417 100644
> --- a/drivers/gpu/drm/drm_atomic.c
> +++ b/drivers/gpu/drm/drm_atomic.c
> @@ -31,6 +31,7 @@
>  #include <drm/drm_mode.h>
>  #include <drm/drm_plane_helper.h>
>  #include <drm/drm_print.h>
> +#include <drm/drm_writeback.h>
>  #include <linux/sync_file.h>
>
>  #include "drm_crtc_internal.h"
> @@ -654,6 +655,46 @@ 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
> @@ -1123,6 +1164,12 @@ int drm_atomic_connector_set_property(struct drm_connector *connector,
>                  */
>                 if (state->link_status != DRM_LINK_STATUS_GOOD)
>                         state->link_status = val;
> +       } else if (property == config->writeback_fb_id_property) {
> +               struct drm_framebuffer *fb = drm_framebuffer_lookup(dev, 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);
> @@ -1199,6 +1246,9 @@ drm_atomic_connector_get_property(struct drm_connector *connector,
>                 *val = state->tv.hue;
>         } else if (property == config->link_status_property) {
>                 *val = state->link_status;
> +       } 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);
> @@ -1406,6 +1456,75 @@ 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
> @@ -1556,6 +1675,8 @@ 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);
> @@ -1578,6 +1699,15 @@ int drm_atomic_check_only(struct drm_atomic_state *state)
>                 }
>         }
>
> +       for_each_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 8be9719284b0..fa2d8d7a602a 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_internal.h"
> @@ -3361,6 +3362,9 @@ __drm_atomic_helper_connector_duplicate_state(struct drm_connector *connector,
>         memcpy(state, connector->state, sizeof(*state));
>         if (state->crtc)
>                 drm_connector_get(connector);
> +
> +       /* Don't copy over a writeback job, they are used only once */
> +       state->writeback_job = NULL;
>  }
>  EXPORT_SYMBOL(__drm_atomic_helper_connector_duplicate_state);
>
> @@ -3487,6 +3491,8 @@ __drm_atomic_helper_connector_destroy_state(struct drm_connector_state *state)
>  {
>         if (state->crtc)
>                 drm_connector_put(state->connector);
> +       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 9f847615ac74..fb9ace3ef744 100644
> --- a/drivers/gpu/drm/drm_connector.c
> +++ b/drivers/gpu/drm/drm_connector.c
> @@ -86,6 +86,7 @@ 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)
> @@ -222,7 +223,8 @@ 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..48c8599e8248
> --- /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@xxxxxxx>
> + *
> + * 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 bool 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 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);
> +               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);
> +}
> +
> +/**
> + * @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 788daf756f48..183b7c60adab 100644
> --- a/include/drm/drm_atomic.h
> +++ b/include/drm/drm_atomic.h
> @@ -478,6 +478,9 @@ 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 4eeda120e46d..c97a4729c071 100644
> --- a/include/drm/drm_connector.h
> +++ b/include/drm/drm_connector.h
> @@ -326,6 +326,19 @@ struct drm_connector_state {
>         struct drm_atomic_state *state;
>
>         struct drm_tv_connector_state tv;
> +
> +       /**
> +        * @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 42981711189b..cbc1b2998085 100644
> --- a/include/drm/drm_mode_config.h
> +++ b/include/drm/drm_mode_config.h
> @@ -741,6 +741,20 @@ struct drm_mode_config {
>          */
>         struct drm_property *suggested_y_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_writeback.h b/include/drm/drm_writeback.h
> new file mode 100644
> index 000000000000..cf3a28676006
> --- /dev/null
> +++ b/include/drm/drm_writeback.h
> @@ -0,0 +1,88 @@
> +/*
> + * (C) COPYRIGHT 2016 ARM Limited. All rights reserved.
> + * Author: Brian Starkey <brian.starkey@xxxxxxx>
> + *
> + * 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 8c67fc03d53d..686ae5599c22 100644
> --- a/include/uapi/drm/drm_mode.h
> +++ b/include/uapi/drm/drm_mode.h
> @@ -269,6 +269,7 @@ 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 {
>
> --
> 2.12.2
>
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