[RFC] media DT bindings
Sylwester Nawrocki
sylvester.nawrocki at gmail.com
Thu Jul 19 03:00:15 EST 2012
Hi Guennadi,
On 07/16/2012 01:41 PM, Guennadi Liakhovetski wrote:
[...]
>> On 07/11/2012 04:27 PM, 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.
>>
>> We've done some work on device tree support for SoC camera interface
>> with driver based on the Media Controller API, I've posted some RFC
>> patches a few weeks ago:
>> http://www.mail-archive.com/devicetree-discuss@lists.ozlabs.org/msg14769.html
>> But unfortunately didn't receive any comments,
>
> You have now ;-)
:)
>> perhaps because the actual
>> bindings were not abstracted enough from a specific hardware support.
>> An updated version of these patch set can be found here:
>> https://github.com/snawrocki/linux/commits/camera-of-2
>>
>> Of course we shouldn't be forgetting that underlying bindings need to
>> be the same, regardless of the drivers are based on soc_camera, Media
>> Controller/subdev pad-level frameworks, or something else.
>
> Of course, the more properties are common - the better. I also made sure
> not to introduce any soc-camera specific properties. But since I mostly
> work with those systems, I am not fully aware of requirements, imposed by
> other hardware types, so, I hope others will contribute to this work:-)
>
>> Anything linux specific in the bindings would be inappropriate.
>
> Not sure what you mean here - which Linux specific bindings and why they
> wouldn't be appropriate? Don't think our bindings would be used by any
> other OS kernels.
The bindings should be OS agnostic, so they can be used by other operating
systems and bootloaders. That's one of fundamental FDT assumptions, AFAIU.
It is outlined in this presentation (slide 22):
http://events.linuxfoundation.org/images/stories/pdf/lf_elc12_abraham.pdf
>>> 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 at 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>;
This property might be redundant, we already have the "client" phandle
pointing to "ov772x at 0x21-0", which has all interesting properties inside
it. Other than there is probably no reasonable usage for it under
"ceu0 at 0xfe910000" node ?
>>> client =<&ov772x at 0x21-0>;
>>> local-pad = "parallel-sink";
>>> remote-pad = "parallel-source";
>>
>> I'm not sure I like that. Is it really needed when we already have
>> the child/parent properties around ?
>
> I think it is. Both the host and the client can have multiple pads (e.g.,
> parallel / serial). These properties specify which pads are used and make
> the translation between DT data and our subdev / pad APIs simpler.
OK, sorry, but isn't it all about just specifying what sort of data bus
is used ? :-)
>>> bus-width =<8>; /* used data lines */
>>> data-shift =<0>; /* lines 7:0 are used */
>>> hsync-active =<1>; /* active high */
>>> vsync-active =<1>; /* active high */
>>
>> In the end I took a bit different approach, similar to how the interrupt
>> flag bindings are defined:
>> https://github.com/snawrocki/linux/commit/c17a61a07008eeb8faea0205f7cc440545641adb
>>
>> However using a separate boolean for each signal, as you proposed, might
>> not be that much of string parsing, and it would have hurt less if this
>> would have been done is some common helper modules.
>
> Personally, I think, I would prefer to have 1 property per flag. Whether
> we use an integer as above or a boolean as in this patch:
>
> http://thread.gmane.org/gmane.linux.drivers.devicetree/17495
>
> can be discussed.
I don't have strong preference, but I would vote for
vsync/hsync/field-active-low; and pclk-sample-falling; boolean keys.
>>> 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.
>>
>> I doubt this is going to be sufficient. Video bridge drivers may require
>> all their sub-devices (e.g. I2C client devices) to be registered, in order
>> to complete their probing. This was discussed in the past:
>> http://www.mail-archive.com/devicetree-discuss@lists.ozlabs.org/msg06595.html
>
> How about this:
>
> - if sensors get probed before the host, they request deferred probing;
> - when the host gets probed, it checks, which clients it should be
> getting, enables respective clocks, registers a notifier on respective
> busses (&i2c_bus_type for I2C) and returns success
> - when after this sensors are re-probed, the notifier gets called and the
> host can complete its initialisation
This could work, the sensor nodes would need to contain a reference to
their respective media parent nodes. And of course a lot of existing
code would have to be modified to support that.
I'd like just to point one detail here, as sensor subdev drivers control
their voltage regulators and RESET/STANDBY (gpio) signals, they should
also be able to control the master clock. In order to ensure proper power
up/down sequences. It is a bad practice to enable clocks before voltage
supplies are switched on and we shouldn't have that as a general
assumption at the kernel frameworks.
One possible solution would be to have host/bridge drivers to register
a clkdev entry for I2C client device, so it can acquire the clock through
just clk_get(). We would have to ensure the clock is not tried to be
accessed before it is registered by a bridge. This would require to add
clock handling code to all sensor/encoder subdev drivers though..
Or maybe we could add some capability flags, so for sensors that don't
control the clock themselves the hosts would enable/disable it when
needed.
>> So we ended up with defining an aggregate DT node that happens to be
>> associated with the camera media device driver.
>>
>> Here is an example of how it might look like:
>> https://github.com/snawrocki/linux/commit/eae639132681df6c068dc869bb8973f8d9d3efa1
>
> Yeah, it can be done if absolutely needed, but I wouldn't make it
> mandatory. On simpler sistems 1 host and 1 sensor nodes should suffice.
>
>>> 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,
>>
>> Sounds good.
>>
>>> 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
>>
>> It seems dubious to me to push media links' description into DT. Links can
>> be configurable, and I don't think it's a rare situation. I'm inclined to
>> code the interconnections within a composite device driver.
>
> Shouldn't a list of all possible links be provided by the platform? If a
> certain pad can participate in several links, all of them should be
> specified and at run-time we just switch between them?
Again, this could be entirely coded in drivers with minimal needed
information retrieved from DT. Or we could have all possible link
templates in DT which would then have to be parsed/translated to
respective user API calls. I'd like to see a real example of such
link parsing at some driver.
>>> ov772x at 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 at 0xfe910000>;
>>> bus-width =<8>;
>>> hsync-active =<1>;
>>> vsync-active =<0>; /* who came up with an inverter here?... */
>>> pclk-sample =<1>;
>>> };
Don't we need to also specify what sort of video bus (serial/parallel)
is used in such a client node ? How the sensor sub-device driver would
determine its bus type ?
>>
>> Are these mostly supposed to be properties specific to a sub-device and
>> used by a host ?
>
> These parameters specify the subdevice configuration and are also used by
> the subdevice to configure its signal polarities.
>
>> If so, how about adding them under the host or the aggregate node grouped
>> into a sub-device specific child node ?
>
> We need 2 copies of them - for both pads, that's exactly the reason for
> the above inverter "joke."
Yeah, I see. But why we need a "ceu0-1" node under "ov772x at 0x21-0" node
above ? A driver associated with ceu0 at 0xfe910000 node could still access
properties at "ov772x at 0x21-0" node through its "client" phandle.
I can't see an advantage of having those properties grouped into a media
parent specific child node under the sensor node. It's not needed by the
sensor subdev driver, is it ?
>>> };
>>>
>>> Data processors
>>> ===============
>>>
>>> Data processing modules include resizers, codecs, rotators, serialisers,
>>> etc. A node for an sh-mobile CSI-2 subdevice could look like
>>>
>>> csi2 at 0xffc90000 = {
>>> compatible = "renesas,sh-mobile-csi2";
>>> reg =<0xffc90000 0x1000>;
>>> interrupts =<0x17a0>;
>>> bus-width =<4>;
>>> clock-frequency =<30000000>;
>>> ...
>>> imx074-1 = {
>>> client =<&imx074 at 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 at 0xfe910000>;
>>> immutable;
>>
>>> };
The child nodes don't seem to have standard names. How would the csi2
driver determine what is the purpose of each of them ?
>>> };
>>>
>>> The respective child binding in the CEU node could then look like
>>>
>>> csi2-1 = {
>>> reg =<1>;
>>> client =<&csi2 at 0xffc90000>;
>>> immutable;
>>> };
>>
>> It look a bit complex, and could more complex when we run into a system
>> supporting more complex interconnections. I would prefer some sort of
>> more flat structure...
>
> Ideas welcome:-)
:) It just looked to me like a lot of parsing code for all these constructs.
And would such design be flexible enough to support pipelines like:
[sensor] -> [MIPI-CSI2 receiver (frontend)] -> [ISP frontend (+ DMA)] ->
[ISP (...)] -> [scaler/rotator + DMA] ->
i.e. containing more than 3 entities ?
--
Thanks,
Sylwester
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