[RFC 0/5] Generic panel framework

Laurent Pinchart laurent.pinchart at ideasonboard.com
Wed Oct 31 06:28:10 PDT 2012

Hi Inki,

On Saturday 20 October 2012 22:10:17 Inki Dae wrote:
> Hi Laurent. sorry for being late.

No worries.

> 2012/8/17 Laurent Pinchart <laurent.pinchart at ideasonboard.com>:
> > Hi everybody,
> > 
> > While working on DT bindings for the Renesas Mobile SoC display controller
> > (a.k.a. LCDC) I quickly realized that display panel implementation based
> > on board code callbacks would need to be replaced by a driver-based panel
> > framework.
> > 
> > Several driver-based panel support solution already exist in the kernel.
> > 
> > - The LCD device class is implemented in drivers/video/backlight/lcd.c and
> >   exposes a kernel API in include/linux/lcd.h. That API is tied to the
> >   FBDEV API for historical reason, uses board code callback for reset and
> >   power management, and doesn't include support for standard features
> >   available in today's "smart panels".
> > 
> > - OMAP2+ based systems use custom panel drivers available in
> > 
> >   drivers/video/omap2/displays. Those drivers are based on OMAP DSS
> >   (display controller) specific APIs.
> > 
> > - Similarly, Exynos based systems use custom panel drivers available in
> > 
> >   drivers/video/exynos. Only a single driver (s6e8ax0) is currently
> >   available. That driver is based on Exynos display controller specific
> >   APIs and on the LCD device class API.
> > 
> > I've brought up the issue with Tomi Valkeinen (OMAP DSS maintainer) and
> > Marcus Lorentzon (working on panel support for ST/Linaro), and we agreed
> > that a generic panel framework for display devices is needed. These
> > patches implement a first proof of concept.
> > 
> > One of the main reasons for creating a new panel framework instead of
> > adding missing features to the LCD framework is to avoid being tied to
> > the FBDEV framework. Panels will be used by DRM drivers as well, and
> > their API should thus be subsystem-agnostic. Note that the panel
> > framework used the fb_videomode structure in its API, this will be
> > replaced by a common video mode structure shared across subsystems
> > (there's only so many hours per day).
> > 
> > Panels, as used in these patches, are defined as physical devices
> > combining a matrix of pixels and a controller capable of driving that
> > matrix.
> > 
> > Panel physical devices are registered as children of the control bus the
> > panel controller is connected to (depending on the panel type, we can
> > find platform devices for dummy panels with no control bus, or I2C, SPI,
> > DBI, DSI, ... devices). The generic panel framework matches registered
> > panel devices with panel drivers and call the panel drivers probe method,
> > as done by other device classes in the kernel. The driver probe() method
> > is responsible for instantiating a struct panel instance and registering
> > it with the generic panel framework.
> > 
> > Display drivers are panel consumers. They register a panel notifier with
> > the framework, which then calls the notifier when a matching panel is
> > registered. The reason for this asynchronous mode of operation, compared
> > to how drivers acquire regulator or clock resources, is that the panel
> > can use resources provided by the display driver. 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 registered and also defer the panel device probe
> > until the display is registered. As most display drivers need to handle
> > output devices hotplug (HDMI monitors for instance), handling panel
> > through a notification system seemed to be the easiest solution.
> > 
> > Note that this brings a different issue after registration, as display and
> > panel drivers would take a reference to each other. Those circular
> > references would make driver unloading impossible. I haven't found a good
> > solution for that problem yet (hence the RFC state of those patches), and
> > I would appreciate your input here. This might also be a hint that the
> > framework design is wrong to start with. I guess I can't get everything
> > right on the first try ;-)
> > 
> > Getting hold of the panel is the most complex part. Once done, display
> > drivers can call abstract operations provided by panel drivers to control
> > the panel operation. These patches implement three of those operations
> > (enable, start transfer and get modes). More operations will be needed,
> > and those three operations will likely get modified during review. Most
> > of the panels on devices I own are 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.
> > 
> > In addition to the generic panel core, I've implemented MIPI DBI (Display
> > Bus Interface, a parallel bus for panels that supports read/write
> > transfers of commands and data) bus support, as well as three panel
> > drivers (dummy panels with no control bus, and Renesas R61505- and
> > R61517-based panels, both using DBI as their control bus). Only the dummy
> > panel driver has been tested as I lack hardware for the two other
> > drivers.
> > 
> > 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. I personally already feel that we might need something even
> > more generic to support other kinds of external devices connected to
> > display controllers, such as external DSI to HDMI converters for instance.
> > Some kind of video entity exposing abstract operations like the panels do
> > would make sense, in which case panels would "inherit" from that video
> > entity.
> > 
> > Speaking of conferences, I will attend the KS/LPC in San Diego in a bit
> > more than a week, and would be happy to discuss this topic face to face
> > there.
> >
> > Laurent Pinchart (5):
> >   video: Add generic display panel core
> >   video: panel: Add dummy panel support
> >   video: panel: Add MIPI DBI bus support
> >   video: panel: Add R61505 panel support
> >   video: panel: Add R61517 panel support
> how about using 'buses' directory instead of 'panel' and adding
> 'panel' under that like below?
> video/buess: display bus frameworks such as MIPI-DBI/DSI and eDP are placed.
> video/buess/panel: panel drivers based on display bus-based drivers are
> placed.
> I think MIPI-DBI(Display Bus Interface)/DSI(Display Serial Interface)
> and eDP are the bus interfaces for display controllers such as
> DISC(OMAP SoC) and FIMC(Exynos SoC).

After discussing the generic panel framework at Linaro Connect, we came to the 
conclusion that "panel" is too limiting a name. I will send an RFC v2 titled 
"common display framework", with a drivers/video/display/ directory. I'm 
unsure whether panels should go to drivers/video/panels/ or 


Laurent Pinchart

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