Hi all
Background
==========
With ARM adoption of flat Device Trees a need arises to move platform
device descriptions and their data from platform files to DT. This has
also to be done for media devices, e.g., video capture and output
interfaces, data processing devices, camera sensors, TV decoders and
encoders. This RFC is trying to spawn a discussion to define standard V4L
DT bindings. The first version will concentrate on the capture path,
mostly taking care of simple capture-interface - camera sensor / TV
decoder configurations. Since the author is not working intensively yet
with the Media Controller API, pad-level configuration, these topics might
be underrepresented in this RFC. I hope others, actively working in these
areas, will fill me in on them.
Overview
========
As mentioned above, typical configurations, that we'll be dealing with
consist of a DMA data capture engine, one or more data sources like camera
sensors, possibly some data processing units. Data capture and processing
engines are usually platform devices, whereas data source devices are
typically I2C slaves. Apart from defining each device we'll also describe
connections between them as well as properties of those connections.
Capture devices
==============================
These are usually platform devices, integrated into respective SoCs. There
also exist external image processing devices, but they are rare. Obvious
differences between them and integrated devices include a different bus
attribution and a need to explicitly describe the connection to the SoC.
As far as capture devices are concerned, their configuration will
typically include a few device-specific bindings, as well as standard
ones. Standard bindings will include the usual "reg," "interrupts,"
"clock-frequency" properties.
It is more complex to describe external links. We need to describe
configurations, used with various devices, attached to various pads. It is
proposed to describe such links as child nodes. Each such link will
reference a client pad, a local pad and specify the bus configuration. The
media bus can be either parallel or serial, e.g., MIPI CSI-2. It is
proposed to describe both the bus-width in the parallel case and the
number of lanes in the serial case, using the standard "bus-width"
property.
On the parallel bus common properties include signal polarities, possibly
data line shift (8 if lines 15:8 are used, 2 if 9:2, and 0 if lines 7:0),
protocol (e.g., BT.656). Additionally device-specific properties can be
defined.
A MIPI CSI-2 bus common properties would include, apart from the number of
lanes, routed to that client, the clock frequency, a channel number,
possibly CRC and ECC flags.
An sh-mobile CEU DT node could look like
ceu0@0xfe910000 = {
compatible = "renesas,sh-mobile-ceu";
reg = <0xfe910000 0xa0>;
interrupts = <0x880>;
bus-width = <16>; /* #lines routed on the board */
clock-frequency = <50000000>; /* max clock */
#address-cells = <1>;
#size-cells = <0>;
...
ov772x-1 = {
reg = <0>;
client = <&ov772x@0x21-0>;
local-pad = "parallel-sink";
remote-pad = "parallel-source";
bus-width = <8>; /* used data lines */
data-shift = <0>; /* lines 7:0 are used */
hsync-active = <1>; /* active high */
vsync-active = <1>; /* active high */
pclk-sample = <1>; /* rising */
clock-frequency = <24000000>;
};
};
Client devices
==============
Client nodes are children on their respective busses, e.g., i2c. This
placement leads to these devices being possibly probed before respective
host interfaces, which will fail due to known reasons. Therefore client
drivers have to be adapted to request a delayed probing, as long as the
respective video host hasn't probed.
Client nodes will include all the properties, usual for their busses.
Additionally they will specify properties private to this device type and
common for all V4L2 client devices - device global and per-link. I think,
we should make it possible to define client devices, that can at run-time
be connected to different sinks, even though such configurations might not
be very frequent. To achieve this we also specify link information in
child devices, similar to those in host nodes above. This also helps
uniformity and will let us implement and use a universal link-binding
parser. So, a node, that has been referenced above could look like
ov772x@0x21-0 = {
compatible = "omnivision,ov772x";
reg = <0x21>;
vdd-supply = <®ulator>;
bus-width = <10>;
#address-cells = <1>;
#size-cells = <0>;
...
ceu0-1 = {
reg = <0>;
media-parent = <&ceu0@0xfe910000>;
bus-width = <8>;
hsync-active = <1>;
vsync-active = <0>; /* who came up with an inverter here?... */
pclk-sample = <1>;
};
};
Data processors
===============
Data processing modules include resizers, codecs, rotators, serialisers,
etc. A node for an sh-mobile CSI-2 subdevice could look like
csi2@0xffc90000 = {
compatible = "renesas,sh-mobile-csi2";
reg = <0xffc90000 0x1000>;
interrupts = <0x17a0>;
bus-width = <4>;
clock-frequency = <30000000>;
...
imx074-1 = {
client = <&imx074@0x1a-0>;
local-pad = "csi2-sink";
remote-pad = "csi2-source";
bus-width = <2>;
clock-frequency = <25000000>;
csi2-crc;
csi2-ecc;
sh-csi2,phy = <0>;
};
ceu0 = {
media-parent = <&ceu0@0xfe910000>;
immutable;
};
};
The respective child binding in the CEU node could then look like
csi2-1 = {
reg = <1>;
client = <&csi2@0xffc90000>;
immutable;
};
Comments welcome.
Thanks
Guennadi
---
Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer
http://www.open-technology.de/
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