[RFC 0/6] Common Display Framework-T

[Tomasz Figa]

Hi Laurent,

On Wednesday 26 of December 2012 13:14:46 Laurent Pinchart wrote:
> Hi Vikas,
> 
> On Monday 24 December 2012 12:33:50 Vikas Sajjan wrote:
> > On Wed, Dec 19, 2012 at 6:51 PM, Laurent Pinchart wrote:
> > > On Friday 14 December 2012 16:27:26 Tomi Valkeinen wrote:
> > >> Hi,
> > >> 
> > >> I have been testing Common Display Framework on OMAP, and making
> > >> changes that I've discussed in the posts I've sent in reply to the
> > >> CDF series from Laurent. While my CDF code is rather hacky and not
> > >> at all ready, I wanted to post the code for comments and also as a
> > >> reference code to my posts.
> > >> 
> > >> So here is CDF-T (Tomi-edition =).
> > > 
> > > We've discussed your approach extensively face-to-face today so I
> > > won't
> > > review the patches in detail, but I will instead summarize our
> > > discussion to make sure we understood each other (and let other
> > > developers jump in).
> > > 
> > > For the purpose of this discussion the term "display controller
> > > driver"
> > > (or just "display controller") refer to both the low-level driver
> > > layer
> > > that communicates directly with the display controller hardware, and
> > > to
> > > the higher- level driver layer that implements and exposes the
> > > userspace API (FBDEV, KMS and/or V4L). Those layers can be
> > > implemented in multiple kernel modules (such as in the OMAP DSS
> > > case, with omapdss for the low-level layer and omapdrm, omapfb and
> > > omapvout for the API-level layer) or a single kernel module.
> > > 
> > > Control model
> > > -------------
> > > 
> > > The figure at
> > > http://www.ideasonboard.org/media/cdf/cdf-panel-control-
> > > model.png shows the CDF control model.
> > > 
> > > The panel object depicted on the figure doesn't need to be a panel
> > > in the stricter sense but could be any chain of off-SoC (both
> > > on-board or off-board) display entities. It however helps thinking
> > > about it as a panel and doesn't hurt the model.
> > > 
> > > The panel is controlled through abstract control requests. Those
> > > requests are used to retrieve panel information (such as the
> > > physical size, the supported video modes, EDID information, ...),
> > > set the panel
> > > configuration (such as the active video timings) or control the
> > > panel
> > > operation state (enabling/disabling the panel, controlling panel
> > > blanking and power management, ...). They are exposed by the panel
> > > using function pointers, and called by other kernel components in
> > > response to userspace requests (through the FBDEV, KMS or V4L2
> > > APIs) or in-kernel events (for instance hotplug notifications).
> > > 
> > > In response to the control requests the panel driver will
> > > communicate with the panel through the panel control bus (I2C, SPI,
> > > DBI, DSI, GPIO, ..., not shown on the figure) and will control the
> > > video stream it receives on its input.
> > > 
> > > The panel is connected at the hardware level to a video source
> > > (shown as a green hashed rectangle) that provides it with a video
> > > stream. The video stream flows from the video source to the panel
> > > and is directly controlled by its source, as shown by the green
> > > arrow from the display controller to the video stream. The video
> > > source exposes stream control operations as function pointers that
> > > are used by the panel to control the video stream, as shown by the
> > > green arrow from the panel to the video source.
> > > 
> > > The figure at
> > > http://www.ideasonboard.org/media/cdf/cdf-panel-control-
> > > model-2.png shows the call flow across entities when the panel is a
> > > pipeline made of more than a single entity. In this case the SoC (on
> > > the left of the dashed line) outputs a video stream on a DSI bus
> > > connected to a DSI to LVDS transmitter. The output of the DSI to
> > > LVDS transmitter is connected to an LVDS panel (or, more
> > > accurately, an LVDS panel module made of an LVDS panel controller
> > > and a panel).
> > > 
> > > The transmitter and panel module are seen by the display controller
> > > and
> > > userspace API implementations as a single entity that exposes
> > > control
> > > request operations and controls its input video stream. When a
> > > control
> > > request is performed (outermost green arrow) the DSI to LVDS
> > > transmitter will propagate it to the panel, possibly mangling the
> > > input parameters or the response. For panel operation state control
> > > requests the last entity in the pipeline will likely want to
> > > control the video stream it receives on its input. The video stream
> > > control calls will be propagated from right to left as shown by the
> > > red arrows.
> > > 
> > > Every entity in the call stack can communicate with its hardware
> > > device
> > > through the corresponding control bus, and/or control the video
> > > stream it receives on its input.
> > > 
> > > This model allows filtering out modes and timings supported by the
> > > panel but unsupported by the transmitter and mangling the modes and
> > > timings according to the transmitter limitations. It has no
> > > complexity drawback for simple devices, as the corresponding
> > > drivers can just forward the calls directly. Similar use cases
> > > could exist for other control operations than mode and information
> > > retrieval.
> > > 
> > > Discovery
> > > ---------
> > > 
> > > Before being able to issue control requests, panel devices need to
> > > be
> > > discovered and associated with the connected display controller(s).
> > > 
> > > Panels and display controllers are cross-dependent. There is no way
> > > around that, as the display controller needs a reference to the
> > > panel to call control requests in response to userspace API, and
> > > the panel needs a reference to the display controller to call video
> > > stream control functions (in addition to requiring generic
> > > resources such as clocks, GPIOs or even regulators that could be
> > > provided by the display controller).
> > > 
> > > As we can't probe the display controller and the panel together, a
> > > probe order needs to be defined. The decision was to consider video
> > > sources as resources and defer panel probing until all required
> > > resources (video stream source, clocks, GPIOs, regulators and more)
> > > are available. Display controller probing must succeed without the
> > > panel being available. This mimicks the hotpluggable monitor model
> > > (VGA, HDMI, DP) that doesn't prevent display controllers from being
> > > successfully probed without a connected monitor.
> > > 
> > > Our design goal is to handle panel discovery in a similar (if not
> > > identical) way as HDMI/DP hotplug in order to implement a single
> > > display discovery method in display controller drivers. This might
> > > not be achievable, in which case we'll reconsider the design
> > > requirement.
> > > 
> > > When the display controller driver probes the device it will
> > > register the video source(s) at the output of the display
> > > controller with the CDF core. Those sources will be identified by
> > > the display controller dev_name() and a source integer index. A new
> > > structure, likely called
> > > display_entity_port, will be used to represent a source or sink
> > > video port on a display entity.
> > > 
> > > Panel drivers will handle video sources as resources. They will
> > > retrieve at probe time the video source the panel is connected to
> > > using a phandle or a source name (depending on whether the platform
> > > uses DT). If the source isn't available the probe function will
> > > return -EPROBE_DEFER.
> > > 
> > > In addition to the video stream control operations mentioned above,
> > > ports will also expose a connect/disconnect operation use to notify
> > > them of connection/disconnection events. After retrieving the
> > > connected video source panel drivers call the connect/disconnect
> > > operation on the video source to notify it that the panel is
> > > available.
> > > 
> > > When the panel is a pipeline made of more than a single entity,
> > > entities are probed in video source to video sink order.
> > > Out-of-order probe will result in probe deferral as explained above
> > > due to the video source not being available, resulting in the
> > > source to sink probe order. Entities should not call the connect
> > > operation of their video source at probe time in that case, but
> > > only when their own connect operation for the video source(s) they
> > > provide to the next entity is called by the next entity. Connect
> > > operations will thus be called in sink to source order starting at
> > > the entity at the end of the pipeline and going all the way back to
> > > the display controller.
> > > 
> > > This notification system is a hotplug mechanism that replaces the
> > > display entity notifier system from my previous RFC. Alan Cox
> > > rightly objected to the notification system, arguing that such
> > > system-wide notifications were used by FBDEV and very subject to
> > > abuse. I agree with his argument, this new mechanism should result
> > > in a cleaner implementation as video sources will only be notified
> > > of connect/disconnect events for the entity they're connected to.
> > > 
> > > DBI/DSI busses
> > > --------------
> > > 
> > > My RFC introduced a DBI bus using the Linux device and bus model.
> > > Its
> > > purpose was multifold:
> > > 
> > > - Support (un)registration, matching and binding of devices and
> > > drivers.
> > > 
> > > - Provide power management (suspend/resume) services through the
> > > standard Linux PM bus/device model, to make sure that DBI devices
> > > will be suspended/resumed after/before their DBI bus controller.
> > > 
> > > - Provide bus services to access the connected devices. For DBI that
> > > took the form of command read and data read/write functions.
> > > 
> > > A DSI bus implementation using the same model was also planned.
> > > 
> > > Tomi's patches removed the DBI bus and replaced DBI devices with
> > > platform devices, moving the bus services implementation to the
> > > video source. DBI and DSI busses are always either pure video or
> > > video + control busses (although controlling a DPI panel through
> > > DSI is conceivable, nobody in his right mind, not even a hardware
> > > engineer, would likely implement that), so there will always be a
> > > video source to provide the DBI/DSI control operations.
> > > 
> > > (Un)registration, matching and binding of devices and drivers is
> > > provided by the platform device bus. Bus services to access
> > > connected devices are provided by the video source, wrapper
> > > functions will be used to handle serialization and locking, and
> > > possibly to offer higher level services (such as DCS for instance).
> > > 
> > > One drawback of using the platform bus is that PM relationships
> > > between
> > > the bus master and slaves will not be taken into account during
> > > suspend/resume. However, a similar issue exists for DPI panels, and
> > > PM
> > > relationships at the video bus level for DBI and DSI are not handled
> > > by
> > > the DBI/DSI busses either. As we need a generic solution to handle
> > > those (likely through early suspend and late resume), the same
> > > solution can be used to handle DBI and DSI control bus PM
> > > relationships without requiring a Linux DBI or DSI bus.
> > > 
> > > Even though I still like the idea of DBI and DSI busses, I agree
> > > with Tomi that they're not strictly needed and I will drop them.
> > > 
> > > Entity model
> > > ------------
> > > 
> > > Tomi's proposal split the display entities into video sources
> > > (struct
> > > video_source) and display entities (struct display_entity). To make
> > > generic pipeline operations easier, we agreed to merge the video
> > > source
> > > and the display entity back. struct display_entity thus models a
> > > display entity that has any number of sink and/or source ports,
> > > modeled as struct display_entity_port instances.
> > 
> > Looking at Tomi's patchset, he has considered panel as "display
> > entity"
> > and  MIPI DSI as "video source entity". So if we are planning to merge
> > it back how should we treat panel and MIPI DSI. i mean should we
> > consider both panel  and MIPI DSI has 2 different display entities.
> > i.e, during the probe of each of these drivers, should we register a
> > display entity with CDF.
> 
> Both the DSI encoder and the DSI panel would be modeled as display
> entities. The DSI encoder would have a source port that models its DSI
> video source, and the DSI panel would have a sink port.
> 
> > > Video stream operations will be exposed by the display entity as
> > > function pointers and will take a port reference as argument (this
> > > could take the form of struct display_entity * and port index, or
> > > struct
> > > display_entity_port *). The DVI and DSI operations model proposed by
> > > Tomi in this patch series will be kept.
> > 
> > so you mean you will be adding these "ops" as part of "struct display
> > entity" rather than  video source ops,
> 
> That's correct.
> 
> > static const struct dsi_video_source_ops dsi_dsi_ops = {
> > 
> > 	.update = dsi_bus_update,
> > 	.dcs_write = dsi_bus_dcs_write,
> > 	.dcs_read = dsi_bus_dcs_read,
> > 	.configure_pins = dsi_bus_configure_pins,
> > 	.set_clocks = dsi_bus_set_clocks,
> > 	.enable = dsi_bus_enable,
> > 	.disable = dsi_bus_disable,
> > 	.set_size = dsi_bus_set_size,
> > 	.set_operation_mode = dsi_bus_set_operation_mode,
> > 	.set_pixel_format = dsi_bus_set_pixel_format,
> > 	.enable_hs = dsi_bus_enable_hs,
> > 
> > };
> > 
> > if you can post CDF v3 patches early, it will give us more clarity
> > w.r.t to discussions you and Tomi had.
> 
> I'm working on that.

May I ask you to add me on CC in v3?

This would allow me to track the development, without missing anything 
like it happened with the discussion about bus-less design.

Best regards,
-- 
Tomasz Figa
Samsung Poland R&D Center
SW Solution Development, Linux Platform