CDF meeting @FOSDEM report

Wed Feb 6 03:11:27 PST 2013

Hi,

On 2013-02-06 00:27, Laurent Pinchart wrote:
> Hello,
> 
> We've hosted a CDF meeting at the FOSDEM on Sunday morning. Here's a summary 
> of the discussions.

Thanks for the summary. I've been on a longish leave, and just got back,
so I haven't read the recent CDF discussions on lists yet. I thought
I'll start by replying to this summary first =).

> 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.

In case you'll re-use these abbrevs in later posts, I think it would be
good to mention that DPI is a one-way bus, whereas DBI and DSI are
two-way (perhaps that's implicit with control bus, though).

> 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.

What is an encoder? Something that takes a video signal in, and lets the
CPU store the received data to memory? Isn't that a decoder?

Or do you mean something that takes a video signal in, and outputs a
video signal in another format? (transcoder?)

If the latter, I don't see them as lower priority. If we use CDF also
for SoC internal components (which I think would be great), then every
OMAP board has transcoders.

I'm not sure about the vocabulary on this area, but a normal OMAP
scenario could have a following video pipeline:

1. encoder (OMAP's DISPC, reads pixels from memory and outputs parallel RGB)
2. transcoder (OMAP's DSI, gets parallel RGB and outputs DSI)
3. transcoder (external DSI-to-LVDS chip)
4. panel (LVDS panel)

Even in the case where a panel would be connected directly to the OMAP,
there would be the internal transcoder.

> 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.

Hmm, I'm no expert on omap_vout, but it doesn't use KMS nor omapfb. It
uses omapdss directly, and thus accesses the panels.

That said, I'm fine with leaving omap_vout out from the equation.

> 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.

Isn't there also the problem that KSM doesn't support arbitrarily long
chains of display devices? That actually sounds more like
"deep-pipelining" than what you said, getting the source data from a
data stream.

> 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.

I've been thinking that a good rule of thumb would be that the device
must be somewhat usable after the parent bus is ready. So for, say,
DPI+SPI panel, when the SPI is set up the driver can send messages to
the panel, perhaps read an ID or such, even if the actual video cannot
be shown yet (presuming DPI bus is still missing).

Of course there are the funny cases, as always. Say, a DSI panel,
controlled via i2c, and the panel gets its functional clock from the DSI
bus's clock. In that case both busses need to be up and running before
the panel can do anything.

> - 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.

I think this is always the case. If a bus can be used for control and
video data, you can always use it only for video data.

> 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 

I have luckily not encountered such a device. However, many of the DSI
devices do have i2c control as an option. From the device's point of
view, both can be used at the same time, but I think usually it's saner
to just pick one and use it.

The driver for the device should support both control busses, though.
Probably 99% of the driver code is common for both cases.

> 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 ;-)).

I don't think OMAP DSS should be used as a model. It has too much legacy
crap that should be rewritten. However, it can be used as a reference to
see what kind of features are needed, as it does support both video and
command mode DSI modes, and has been used with many different kinds of
DSI panels and DSI transcoders.

As for the semaphore, sure, it can be removed, although I'm not aware of
this lockdep problem. If there's a problem it should be fixed in any case.

> - 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.

If encoder means the same as the "transcoder" term I used earlier, then
yes, I think so.

As I wrote, I'd like to model the OMAP DSS internal components with CDF.
The internal IP blocks are in no way different than external IP blocks,
they just happen to be integrated into OMAP. The same display IPs are
used with multiple different TI SoCs.

Also, the IPs vary between TI SoCs (for ex, omap2 doesn't have DSI,
omap3 has one DSI, omap4 has two DSIs), so we'll anyway need to have
some kind of dynamic system inside omapdss driver. If I can't use CDF
for that, I'll need to implement a custom one, which I believe would
resemble CDF in many ways.

I'm guessing that having multiple external transcoders is quite rare on
production hardware, but is a very useful feature with development
boards. It's not just once or twice that we've used a transcoder or two
between a SoC and a panel, because we haven't had the final panel yet.

Also, sometimes there are small simple chips in the video pipeline, that
do things like level shifting or ESD protection. In some cases these
chips just work automatically, but in some cases one needs to setup
regulators and gpios to get them up and running (for example,
http://www.ti.com/product/tpd12s015). And if that's the case, then I
believe having a CDF "transcoder" driver for the chip is the easiest
solution.

 Tomi

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