On Wed July 11 2012 16:27:52 Guennadi Liakhovetski wrote:
> 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.
One thing that is missing, but that is quite important is that the information
from ENUMINPUT/ENUMOUTPUT needs to be part of the device tree as well, since
that is generally completely board specific. See for example how the davinci
vpif_capture.c and vpif_display.c do that now using platform data. This would
be solved much more elegantly using the device tree.
This tends not to feature much when dealing with sensors, but any video receiver
or transmitter will need this.
Regards,
Hans
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