[PATCH/RFC v3 00/19] Common Display Framework

[Rob Clark]

On Fri, Aug 9, 2013 at 1:14 PM, Laurent Pinchart
<laurent.pinchart+renesas at ideasonboard.com> wrote:
> Hi everybody,
>
> Here's the third RFC of the Common Display Framework.
>
> I won't repeat all the background information from the versions one and two
> here, you can read it at http://lwn.net/Articles/512363/ and
> http://lwn.net/Articles/526965/.
>
> This RFC isn't final. Given the high interest in CDF and the urgent tasks that
> kept delaying the next version of the patch set, I've decided to release v3
> before completing all parts of the implementation. Known missing items are
>
> - documentation: kerneldoc and this cover letter should provide basic
>   information, more extensive documentation will likely make it to v4.
>
> - pipeline configuration and control: generic code to configure and control
>   display pipelines (in a nutshell, translating high-level mode setting and
>   DPMS calls to low-level entity operations) is missing. Video and stream
>   control operations have been carried over from v2, but will need to be
>   revised for v4.
>
> - DSI support: I still have no DSI hardware I can easily test the code on.

tbh, I kinda think that DSI and bridge chips are the things we should
focus on first.. that seems what is most important for current and new
hardware.  Back-filling to handle older/simpler panels can be done
later.

>
> Special thanks go to
>
> - Renesas for inviting me to LinuxCon Japan 2013 where I had the opportunity
>   to validate the CDF v3 concepts with Alexandre Courbot (NVidia) and Tomasz
>   Figa (Samsung).
>
> - Tomi Valkeinen (TI) for taking the time to deeply brainstorm v3 with me.
>
> - Linaro for inviting me to Linaro Connect Europe 2013, the discussions we had
>   there greatly helped moving CDF forward.
>
> - And of course all the developers who showed interest in CDF and spent time
>   sharing ideas, reviewing patches and testing code.
>
> I have to confess I was a bit lost and discouraged after all the CDF-related
> meetings during which we have discussed how to move from v2 to v3. With every
> meeting I was hoping to run the implementation through use cases of various
> interesting parties and narrow down the scope of the huge fuzzy beast that CDF
> was. With every meeting the scope actually broadened, with no clear path at
> sight anywhere.
>
> Earlier this year I was about to drop one of the requirements on which I had
> based CDF v2: sharing drivers between DRM/KMS and V4L2. With only two HDMI
> transmitters as use cases for that feature (with only out-of-tree drivers so
> far), I just thought the involved completely wasn't worth it and that I should
> implement CDF v3 as a DRM/KMS-only helper framework. However, a seemingly
> unrelated discussion with Xilinx developers showed me that hybrid SoC-FPGA
> platforms such as the Xilinx Zynq 7000 have a larger library of IP cores that
> can be used in camera capture pipelines and in display pipelines. The two use
> cases suddenly became tens or even hundreds of use cases that I couldn't
> ignore anymore.

Hmm, I'm still not really convinced ;-)

Or maybe, put another way, I still think we should still optimize for
the common case.  I mean I'm sure you *can* design hw that has an
LVDS->DP bridge in the capture path, and if you had something like an
FPGA where that was cheap to do maybe you even would (for fun).  But
if in the real world, there are only one or two cases of actual hw
using the same bridge in a capture pipeline which is normally used in
display pipelines, then duplicating some small bit of code for that
abnormal case if it makes things easier for the common case, seems
like a reasonable trade-off to me.

I mean, take a DSI panel driver, for example.. anything but a trivial
panel driver, you are going to want to expose some custom properties
on the connector for controlling non-standard features.  So you end up
with both cdf_foo for the common part, and drm_foo for gluing in the
properties.  And now these two parts which otherwise would be one, end
up having to stay in sync merged through different trees, etc.  It
seems a lot like making my life more difficult for a fairly
hypothetical gain ;-)

Or, take an hdmi or DP bridge.  To use any of the common
infrastructure/helpers in drm, you end up implementing a generic
interface, where both the producer and consumer is inside drm.  Which
just seems a bit pointless and extra hoops to jump through.
Especially when we discover that we need to extend/enhance the common
interface outside of drm to make it work.  (I'm thinking of
display_entity_control_ops::get_modes() here, but I'm sure there are
more examples.)  And I think we'll run into similar issues with
display_entity_control_ops::set_state(), since the on<->off sequencing
can get hairy when the upstream/downstream entity is fed a clk by the
downstream/upstream entity.  And similarly, I think we'll go through a
few revisions of DSI panel/bus params before we have everything that
everyone needs.

(Don't get me wrong.. if we were starting from scratch, I'd tend to
think that stuff downstream of the crtc should share common framework
w/ v4l.. but we are pretty far from starting from scratch, and how to
get from where we are today to future is just as important as the end
result.)

BR,
-R

> CDF v3 is thus userspace API agnostic. It isn't tied to DRM/KMS or V4L2 and
> can be used by any kernel subsystem, potentially including FBDEV (although I
> won't personally wrote FBDEV support code, as I've already advocated for FBDEV
> to be deprecated).
>
> The code you are about to read is based on the concept of display entities
> introduced in v2. Diagrams related to the explanations below are available at
> http://ideasonboard.org/media/cdf/20130709-lce-cdf.pdf.
>
>
> Display Entities
> ----------------
>
> A display entity abstracts any hardware block that sources, processes or sinks
> display-related video streams. It offers an abstract API, implemented by display
> entity drivers, that is used by master drivers (such as the main display driver)
> to query, configure and control display pipelines.
>
> Display entities are connected to at least one video data bus, and optionally
> to a control bus. The video data busses carry display-related video data out
> of sources (such as a CRTC in a display controller) to sinks (such as a panel
> or a monitor), optionally going through transmitters, encoders, decoders,
> bridges or other similar devices. A CRTC or a panel will usually be connected
> to a single data bus, while an encoder or a transmitter will be connected to
> two data busses.
>
> The simple linear display pipelines we find in most embedded platforms at the
> moment are expected to grow more complex with time. CDF needs to accomodate
> those needs from the start to be, if not future-proof, at least present-proof
> at the time it will get merged in to mainline. For this reason display
> entities have data ports through which video streams flow in or out, with link
> objects representing the connections between those ports. A typical entity in
> a linear display pipeline will have one (for video source and video sink
> entities such as CRTCs or panels) or two ports (for video processing entities
> such as encoders), but more ports are allowed, and entities can be linked in
> complex non-linear pipelines.
>
> Readers might think that this model if extremely similar to the media
> controller graph model. They would be right, and given my background this is
> most probably not a coincidence. The CDF v3 implementation uses the in-kernel
> media controller framework to model the graph of display entities, with the
> display entity data structure inheriting from the media entity structure. The
> display pipeline graph topology will be automatically exposed to userspace
> through the media controller API as an added bonus. However, ussage of the
> media controller userspace API in applications is *not* mandatory, and the
> current CDF implementation doesn't use the media controller link setup
> userspace API to configure the display pipelines.
>
> While some display entities don't require any configuration (DPI panels are a
> good example), many of them are connected to a control bus accessible to the
> CPU. Control requests can be sent on a dedicated control bus (such as I2C or
> SPI) or multiplexed on a mixed control and data bus (such as DBI or DSI). To
> support both options the CDF display entity model separates the control and
> data busses in different APIs.
>
> Display entities are abstract object that must be implemented by a real
> device. The device sits on its control bus and is registered with the Linux
> device core and matched with his driver using the control bus specific API.
> The CDF doesn't create a display entity class or bus, display entity drivers
> thus standard Linux kernel drivers using existing busses. A DBI bus is added
> as part of this patch set, but strictly speaking this isn't part of CDF.
>
> When a display entity driver probes a device it must create an instance of the
> display_entity structure, initialize it and add it to the CDF core entities
> pool. The display entity exposes abstract operations through function
> pointers, and the entity driver must implement those operations. Those
> operations can act on either the whole entity or on a given port, depending on
> the operation. They are divided in two groups, control operations and video
> operations.
>
>
> Control Operations
> ------------------
>
> Control operations are called by upper-level drivers, usually in response to a
> request originating from userspace. They query or control the display entity
> state and operation. Currently defined control operations are
>
> - get_size(), to retrieve the entity physical size (applicable to panels only)
> - get_modes(), to retrieve the video modes supported at an entity port
> - get_params(), to retrieve the data bus parameters at an entity port
>
> - set_state(), to control the state of the entity (off, standby or on)
> - update(), to trigger a display update (for entities that implement manual
>   update, such as manual-update panels that store frames in their internal
>   frame buffer)
>
> The last two operations have been carried from v2 and will be reworked.
>
>
> Pipeline Control
> ----------------
>
> The figure on page 4 shows the control model for a linear pipeline. This
> differs significantly from CDF v2 where calls where forwarded from entity to
> entity using a Russian dolls model. v3 removes the need of neighbour awareness
> from entity drivers, simplifying the entity drivers. The complexity of pipeline
> configuration is moved to a central location called a pipeline controller
> instead of being spread out to all drivers.
>
> Pipeline controllers provide library functions that display drivers can use to
> control a pipeline. Several controllers can be implemented to accomodate the
> needs of various pipeline topologies and complexities, and display drivers can
> even implement their own pipeline control algorithm if needed. I'm working on a
> linear pipeline controller for the next version of the patch set.
>
> If pipeline controllers are responsible for propagating a pipeline configuration
> on all entity ports in the pipeline, entity drivers are responsible for
> propagating the configuration inside entities, from sink (input) to source
> (output) ports as illustrated on page 5. The rationale behind this is that
> knowledge of the entity internals is located in the entity driver, while
> knowledge of the pipeline belongs to the pipeline controller. The controller
> will thus configure the pipeline by performing the following steps:
>
> - applying a configuration on sink ports of an entity
> - read the configuration that has been propagated by the entity driver on its
>   source ports
> - optionally, modify the source port configuration (to configure custom timings,
>   scaling or other parameters, if supported by the entity)
> - propagate the source port configuration to the sink ports of the next entities
>   in the pipeline and start over
>
> Beside modifying the active configuration, the entities API will allow trying
> configurations without applying them to the hardware. As configuration of a port
> possibly depend on the configurations of the other ports, trying a configuration
> must be done at the entity level instead of the port level. The implementation
> will be based on the concept of configuration store objects that will store the
> configuration of all ports for a given entity. Each entity will have a single
> active configuration store, and test configuration stores will be created
> dynamically to try a configuration on an entity. The get and set operations
> implemented by the entity will receive a configuration store pointer, and active
> and test code paths in entity drivers will be identical, except for applying the
> configuration to the hardware for the active code path.
>
>
> Video Operations
> ----------------
>
> Video operations control the video stream state on entity ports. The only
> currently defined video operation is
>
> - set_stream(), to start (in continuous or single-shot mode) the video stream
>   on an entity port
>
> The call model for video operations differ from the control operations model
> described above. The set_stream() operation is called directly by downstream
> entities on upstream entities (from a video data bus point of view).
> Terminating entities in a pipeline (such as panels) will usually call the
> set_stream() operation in their set_state() handler, and intermediate entities
> will forward the set_stream() call upstream.
>
>
> Integration
> -----------
>
> The figure on page 8 describes how a panel driver, implemented using CDF as a
> display entity, interacts with the other components in the system. The use case
> is a simple pipeline made of a display controller and a panel.
>
> The display controller driver receives control request from userspace through
> DRM (or FBDEV) API calls. It processes the request and calls the panel driver
> through the CDF control operations API. The panel driver will then issue
> requests on its control bus (several possible control busses are shown on the
> figure, panel drivers typically use one of them only) and call video operations
> of the display controller on its left side to control the video stream.
>
>
> Registration and Notification
> -----------------------------
>
> Due to possibly complex dependencies between entities we can't guarantee that
> all entities part of the display pipeline will have been successfully probed
> when the master display controller driver is probed. For instance a panel can
> be a child of the DBI or DSI bus controlled by the display device, or use a
> clock provided by that device. We can't defer the display device probe until
> the panel is probed and also defer the panel device probe until the display
> device is probed. For this reason we need a notification system that allows
> entities to register themselves with the CDF core, and display controller
> drivers to get notified when entities they need are available.
>
> The notification system has been completely redesigned in v3. This version is
> based on the V4L2 asynchronous probing notification code, with large parts of
> the code shamelessly copied. This is an interim solution to let me play with
> the notification code as needed by CDF. I'm not a fan of code duplication, and
> will work on merging the CDF and V4L2 implementations in a later stage when
> CDF will reach a mature enough state.
>
> CDF manages a pool of entities and a list of notifiers. Notifiers are
> registered by master display drivers with an array of entities match
> descriptors. When an entity is added to the CDF entities pool, all notifiers
> are searched for a match. If a match is found, the corresponding notifier is
> called to notify the master display driver.
>
> The two currently supported match methods are platform match, which uses
> device names, and DT match, which uses DT node pointers. More match method
> might be added later if needed. Two helper functions exist to build a notifier
> from a list of platform device names (in the non-DT case) or a DT
> representation of the display pipeline topology.
>
> Once all required entities have been successfully found, the master display
> driver is responsible for creating media controller links between all entities
> in the pipeline. Two helper functions are also available to automate that
> process, one for the non-DT case and one for the DT case. Once again some
> DT-related code has been copied from the V4L2 DT code, I will work on merging
> both in a future version.
>
> Note that notification brings a different issue after registration, as display
> controller and display entity drivers would take a reference to each other.
> Those circular references would make driver unloading impossible. One possible
> solution to this problem would be to simulate an unplug event for the display
> entity, to force the display driver to release the dislay entities it uses. We
> would need a userspace API for that though. Better solutions would of course
> be welcome.
>
>
> Device Tree Bindings
> --------------------
>
> CDF entities device tree bindings are not documented yet. They describe both
> the graph topology and entity-specific information. The graph description uses
> the V4L2 DT bindings (which are actually not V4L2-specific) specified at
> Documentation/devicetree/bindings/media/video-interfaces.txt. Entity-specific
> information will be described in individual DT bindings documentation. The DPI
> panel driver uses the display timing bindings documented in
> Documentation/devicetree/bindings/video/display-timing.txt.
>
>
>
>
> Please note that most of the display entities on devices I own are just dumb
> panels with no control bus, and are thus not the best candidates to design a
> framework that needs to take complex panels' needs into account. This is why I
> hope to see you using the CDF with your display device and tell me what needs to
> be modified/improved/redesigned.
>
> This patch set is split as follows:
>
> - The first patch fixes a Kconfig namespace issue with the OMAP DSS panels. It
>   could be applied already independently of this series.
> - Patches 02/19 to 07/19 add the CDF core, including the notification system
>   and the graph and OF helpers.
> - Patch 08/19 adds a MIPI DBI bus. This isn't part of CDF strictly speaking,
>   but is needed for the DBI panel drivers.
> - Patches 09/19 to 13/19 add panel drivers, a VGA DAC driver and a VGA
>   connector driver.
> - Patches 14/19 to 18/19 add CDF-compliant reference board code and DT for the
>   Renesas Marzen and Lager boards.
> - Patch 19/19 port the Renesas R-Car Display Unit driver to CDF.
>
> The patches are available in my git tree at
>
>     git://linuxtv.org/pinchartl/fbdev.git cdf/v3
>     http://git.linuxtv.org/pinchartl/fbdev.git/shortlog/refs/heads/cdf/v3
>
> For convenience I've included modifications to the Renesas R-Car Display Unit
> driver to use the CDF. You can read the code to see how the driver uses CDF to
> interface panels. Please note that the rcar-du-drm implementation is still
> work in progress, its set_stream operation implementation doesn't enable and
> disable the video stream yet as it should.
>
> As already mentioned in v2, I will appreciate all reviews, comments,
> criticisms, ideas, remarks, ... If you can find a clever way to solve the
> cyclic references issue described above I'll buy you a beer at the next
> conference we will both attend. If you think the proposed solution is too
> complex, or too simple, I'm all ears, but I'll have more arguments this time
> than I had with v2 :-)
>
> Laurent Pinchart (19):
>   OMAPDSS: panels: Rename Kconfig options to OMAP2_DISPLAY_*
>   video: Add Common Display Framework core
>   video: display: Add video and stream control operations
>   video: display: Add display entity notifier
>   video: display: Graph helpers
>   video: display: OF support
>   video: display: Add pixel coding definitions
>   video: display: Add MIPI DBI bus support
>   video: panel: Add DPI panel support
>   video: panel: Add R61505 panel support
>   video: panel: Add R61517 panel support
>   video: display: Add VGA Digital to Analog Converter support
>   video: display: Add VGA connector support
>   ARM: shmobile: r8a7790: Add DU clocks for DT
>   ARM: shmobile: r8a7790: Add DU device node to device tree
>   ARM: shmobile: marzen: Port DU platform data to CDF
>   ARM: shmobile: lager: Port DU platform data to CDF
>   ARM: shmobile: lager-reference: Add display device nodes to device
>     tree
>   drm/rcar-du: Port to the Common Display Framework
>
>  arch/arm/boot/dts/r8a7790-lager-reference.dts |  92 ++++
>  arch/arm/boot/dts/r8a7790.dtsi                |  33 ++
>  arch/arm/mach-shmobile/board-lager.c          |  76 ++-
>  arch/arm/mach-shmobile/board-marzen.c         |  77 ++-
>  arch/arm/mach-shmobile/clock-r8a7790.c        |   5 +
>  drivers/gpu/drm/rcar-du/Kconfig               |   3 +-
>  drivers/gpu/drm/rcar-du/Makefile              |   7 +-
>  drivers/gpu/drm/rcar-du/rcar_du_connector.c   | 164 ++++++
>  drivers/gpu/drm/rcar-du/rcar_du_connector.h   |  36 ++
>  drivers/gpu/drm/rcar-du/rcar_du_crtc.h        |   2 +-
>  drivers/gpu/drm/rcar-du/rcar_du_drv.c         | 279 ++++++++--
>  drivers/gpu/drm/rcar-du/rcar_du_drv.h         |  28 +-
>  drivers/gpu/drm/rcar-du/rcar_du_encoder.c     |  87 ++-
>  drivers/gpu/drm/rcar-du/rcar_du_encoder.h     |  22 +-
>  drivers/gpu/drm/rcar-du/rcar_du_kms.c         | 116 +++-
>  drivers/gpu/drm/rcar-du/rcar_du_lvdscon.c     | 131 -----
>  drivers/gpu/drm/rcar-du/rcar_du_lvdscon.h     |  25 -
>  drivers/gpu/drm/rcar-du/rcar_du_vgacon.c      |  96 ----
>  drivers/gpu/drm/rcar-du/rcar_du_vgacon.h      |  23 -
>  drivers/video/Kconfig                         |   1 +
>  drivers/video/Makefile                        |   1 +
>  drivers/video/display/Kconfig                 |  62 +++
>  drivers/video/display/Makefile                |   9 +
>  drivers/video/display/con-vga.c               | 148 +++++
>  drivers/video/display/display-core.c          | 759 ++++++++++++++++++++++++++
>  drivers/video/display/display-notifier.c      | 542 ++++++++++++++++++
>  drivers/video/display/mipi-dbi-bus.c          | 234 ++++++++
>  drivers/video/display/panel-dpi.c             | 207 +++++++
>  drivers/video/display/panel-r61505.c          | 567 +++++++++++++++++++
>  drivers/video/display/panel-r61517.c          | 460 ++++++++++++++++
>  drivers/video/display/vga-dac.c               | 152 ++++++
>  drivers/video/omap2/displays-new/Kconfig      |  24 +-
>  drivers/video/omap2/displays-new/Makefile     |  24 +-
>  include/linux/platform_data/rcar-du.h         |  55 +-
>  include/video/display.h                       | 398 ++++++++++++++
>  include/video/mipi-dbi-bus.h                  | 125 +++++
>  include/video/panel-dpi.h                     |  24 +
>  include/video/panel-r61505.h                  |  27 +
>  include/video/panel-r61517.h                  |  28 +
>  39 files changed, 4615 insertions(+), 534 deletions(-)
>  create mode 100644 drivers/gpu/drm/rcar-du/rcar_du_connector.c
>  create mode 100644 drivers/gpu/drm/rcar-du/rcar_du_connector.h
>  delete mode 100644 drivers/gpu/drm/rcar-du/rcar_du_lvdscon.c
>  delete mode 100644 drivers/gpu/drm/rcar-du/rcar_du_lvdscon.h
>  delete mode 100644 drivers/gpu/drm/rcar-du/rcar_du_vgacon.c
>  delete mode 100644 drivers/gpu/drm/rcar-du/rcar_du_vgacon.h
>  create mode 100644 drivers/video/display/Kconfig
>  create mode 100644 drivers/video/display/Makefile
>  create mode 100644 drivers/video/display/con-vga.c
>  create mode 100644 drivers/video/display/display-core.c
>  create mode 100644 drivers/video/display/display-notifier.c
>  create mode 100644 drivers/video/display/mipi-dbi-bus.c
>  create mode 100644 drivers/video/display/panel-dpi.c
>  create mode 100644 drivers/video/display/panel-r61505.c
>  create mode 100644 drivers/video/display/panel-r61517.c
>  create mode 100644 drivers/video/display/vga-dac.c
>  create mode 100644 include/video/display.h
>  create mode 100644 include/video/mipi-dbi-bus.h
>  create mode 100644 include/video/panel-dpi.h
>  create mode 100644 include/video/panel-r61505.h
>  create mode 100644 include/video/panel-r61517.h
>
> --
> Regards,
>
> Laurent Pinchart
>