CDF meeting @FOSDEM report

[Laurent Pinchart]

Hello,

We've hosted a CDF meeting at the FOSDEM on Sunday morning. Here's a summary 
of the discussions.

I would like to start with a big thank to UrLab, the ULB university hacker 
space, for providing us with a meeting room.

The meeting would of course not have been successful without the wide range of 
participants, so I also want to thank all the people who woke up on Sunday 
morning to attend the meeting :-)

(The CC list is pretty long, please let me know - by private e-mail in order 
not to spam the list - if you would like not to receive future CDF-related e-
mails directly)

0. Abbreviations
----------------

DBI - Display Bus Interface, a parallel video control and data bus that 
transmits data using parallel data, read/write, chip select and address 
signals, similarly to 8051-style microcontroller parallel busses. This is a 
mixed video control and data bus.

DPI - Display Pixel Interface, a parallel video data bus that transmits data 
using parallel data, h/v sync and clock signals. This is a video data bus 
only.

DSI - Display Serial Interface, a serial video control and data bus that 
transmits data using one or more differential serial lines. This is a mixed 
video control and data bus.

DT - Device Tree, a representation of a hardware system as a tree of physical 
devices with associated properties.

SFI - Simple Firmware Interface, a lightweight method for firmware to export 
static tables to the operating system. Those tables can contain display device 
topology information.

VBT - Video BIOS Table, a block of data residing in the video BIOS that can 
contain display device topology information.

1. Goals
--------

The meeting started with a brief discussion about the CDF goals.

Tomi Valkeinin and Tomasz Figa have sent RFC patches to show their views of 
what CDF could/should be. Many others have provided very valuable feedback. 
Given the early development stage propositions were sometimes contradictory, 
and focused on different areas of interest. We have thus started the meeting 
with a discussion about what CDF should try to achieve, and what it shouldn't.

CDF has two main purposes. The original goal was to support display panels in 
a platform- and subsystem-independent way. While mostly useful for embedded 
systems, the emergence of platforms such as Intel Medfield and ARM-based PCs 
that blends the embedded and PC worlds makes panel support useful for the PC 
world as well. 

The second purpose is to provide a cross-subsystem interface to support video 
encoders. The idea originally came from a generalisation of the original RFC 
that supported panels only. While encoder support is considered as lower 
priority than display panel support by developers focussed on display 
controller driver (Intel, Renesas, ST Ericsson, TI), companies that produce 
video encoders (Analog Devices, and likely others) don't share that point of 
view and would like to provide a single encoder driver that can be used in 
both KMS and V4L2 drivers.

Both display panels and encoders are thus the target of a lot of attention, 
depending on the audience. As long as none of them is forgotten in CDF, the 
overall agreement was that focussing on panels first is acceptable. Care shall 
be taken in that case to avoid any architecture that would make encoders 
support difficult or impossible.

2. Subsystems
-------------

Display panels are used in conjunction with FBDEV and KMS drivers. There was 
to the audience knowledge no V4L2 driver that needs to explicitly handle 
display panels. Even though at least one V4L2 output drivers (omap_vout) can 
output video to a display panel, it does so in conjunction with the KMS and/or 
FBDEV APIs that handle panel configuration. Panels are thus not exposed to 
V4L2 drivers.

Encoders, on the other hand, are widely used in the V4L2 subsystem. Many V4L2 
devices output video in either analog (Composite, S-Video, VGA) or digital 
(DVI, HDMI) way.

Display panel drivers don't need to be shared with the V4L2 subsystem. 
Furthermore, as the general opinion during the meeting was that the FBDEV 
subsystem should be considered as legacy and deprecate in the future, 
restricting panel support to KMS hasn't been considered by anyone as an issue. 
KMS will thus be the main target of display panel support in CDF, and FBDEV 
will be supported if that doesn't bring any drawback from an architecture 
point of view.

Encoder drivers need to be shared with the V4L2 subsystem. Similarly to panel 
drivers, excluding FBDEV support from CDF isn't considered as an issue.

3. KMS Extensions
-----------------

The usefulness of V4L2 for output devices was questioned, and the possibility 
of using KMS for complex video devices usually associated with V4L2 was 
raised. The TI DaVinci 8xxx family is an example of chips that could benefit 
from KMS support.

The KMS API is lacking support for deep-pipelining ("framebuffers" that are 
sourced from a data stream instead of a memory buffer) today. Extending the 
KMS API with deep-pipelining support was considered as a sensible goal that 
would mostly require the creation of a new KMS source object. Exposing the 
topology of the whole device would then be handled by the Media Controller 
API.

Given that no evidence of this KMS extension being ready in a reasonable time 
frame exists, sharing encoder drivers with the V4L2 subsystem hasn't been 
seriously questioned.

4. Discovery and Initialization
-------------------------------

As CDF will split support for complete display devices across different 
drivers, the question of physical devices discovery and initialization caused 
concern among the audience.

Topology and connectivity information can come from a wide variety of sources. 
Embedded platforms typically provide that information in platform data 
supplied by board code or through the device tree. PC platforms usually store 
the information in the firmware exposed through ACPI, SFI, VBT or other 
interfaces. Pluggable devices (PCI being the most common case) can also store 
the information on an on-board non-volatile memory or hardcode it in drivers.

When using the device tree display entity information are bundled with the 
display entity device DT node. The associated driver shall thus extract itself 
information from the DT node. In all other cases the display entity driver 
shall not parse data from the information source directly, but shall instead 
received a platform data structure filled with data parsed by the display 
controller driver. In the most complex case a machine driver, similar to ASoC 
machine drivers, might be needed, in which case platform data could be 
provided by that machine driver.

Display entity drivers are encouraged to internally fill a platform data 
structure from their DT node to reuse the same code path for both platform 
data- and DT-based initialization.

5. Bus Model
------------

Display panels are connected to a video bus that transmits video data and 
optionally to a control bus. Those two busses can be separate physical 
interfaces or combined into a single physical interface.

The Linux device model represents the system as a tree of devices (not to be 
confused by the device tree, abreviated as DT). The tree is organized around 
control busses, with every device being a child of its control bus master. For 
instance an I2C device will be a child of its I2C controller device, which can 
itself be a child of its parent PCI device.

Display panels will be represented as Linux devices. They will have a single 
parent from the Linux device model point of view, but will be potentially 
connected to multiple physical busses. CDF thus needs to define what bus to 
select as the Linux parent bus.

In theory any physical bus that the device is attached to can be selected as 
the parent bus. However, selecting a video data bus would depart from the 
traditional Linux device model that uses control busses only. This caused 
concern among several people who argued that not presenting the device to the 
kernel as attached to its control bus would bring issues in embedded system. 
Unlike on PC systems where the control bus master is usually the same physical 
device as the data bus master, embedded systems are made of a potentially 
complex assembly of completely unrelated devices. Not representing an I2C-
controlled panel as a child of its I2C master in DT was thus frown upon, even 
though no clear agreement was reached on the subject.

Panels can be divided in three categories based on their bus model.

- No control bus

Many panels don't offer any control interface. They are usually referred to as 
'dumb panels' as they directly display the data received on their video bus 
without any configurable option. Panels in this category often use DPI is 
their video bus, but other options such as DSI (using the DSI video mode only) 
are possible.

Panels with no control bus can be represented in the device model as platform 
devices, or as being attached to their video bus. In the later case we would 
need Linux busses for pure video data interfaces such as DPI or VGA. Nobody 
was particularly enthousiastic about this idea. Dumb panels will thus likely 
be represented as platform devices.

- Separate video and control busses

The typical case is a panel connected to an I2C or SPI bus that receives data 
through a DPI video interface or DSI video mode interface.

Using a mixed control and video bus (such as DSI and DBI) for control only 
with a different bus for video data is possible in theory but very unlikely in 
practice (although the creativity of hardware developers should never be 
underestimated).

Display panels that use a control bus supported by the Linux kernel should 
likely be represented as children of their control bus master. Other options 
are possible as mentioned above but were received without enthousiasm by most 
embedded kernel developers.

When the control bus isn't supported by the kernel, a new bus type can be 
developed, or the panel can be represented as a platform device. The right 
option will likely very depending on the control bus.

- Combined video and control busses

When the two busses are combined in a single physical bus the panel device 
will obviously be represented as a child of that single physical bus. 

In such cases the control bus could expose video bus control methods. This 
would remove the need for a video source as proposed by Tomi Valkeinen in his 
CDF model. However, if the bus can be used for video data transfer in 
combination with a different control bus, a video source corresponding to the 
data bus will be needed.

No decision has been taken on whether to use a video source in addition to the 
control bus in the combined busses case. Experimentation will be needed, and 
the right solution might depend on the bus type.

- Multiple control busses

One panel was mentioned as being connected to a DSI bus and an I2C bus. The 
DSI bus is used for both control and video, and the I2C bus for control only. 
configuring the panel requires sending commands through both DSI and I2C. The 
opinion on such panels was a large *sigh* followed by a "this should be 
handled by the device core, let's ask Greg KH".

6. Miscellaneous
----------------

- If the OMAP3 DSS driver is used as a model for the DSI support 
implementation, Daniel Vetter requested the DSI bus lock semaphore to be 
killed as it prevents lockdep from working correctly (reference needed ;-)).

- Do we need to support chaining several encoders ? We can come up with 
several theoretical use cases, some of them probably exist in real hardware, 
but the details are still a bit fuzzy.

7. Actions
----------

I will post a third CDF RFC that will target DPI panels only, with support for 
a concept similar to the video source as proposed by Tomi Valkeinen. Marcus 
Lorentzon will then try to implement support for DSI panel using a single bus 
without a video source. Rob Clark will test DPI support with the OMAP DSS 
driver on a OMAP3 platform.

-- 
Regards,

Laurent Pinchart