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From:   Rob Herring <robh@kernel.org>
Date:   Tue, 20 Aug 2019 10:44:49 -0500
Message-ID: <CAL_JsqKDkixeDJJVxbzWebD6nqMHyk6QqDGSKrQho0THjLdmKQ@mail.gmail.com>
Subject: Re: [RFC,v2 3/6] media: dt-bindings: add DS90UB954-Q1 video deserializer
To:     Luca Ceresoli <luca@lucaceresoli.net>
Cc:     Linux Media Mailing List <linux-media@vger.kernel.org>,
        Linux I2C <linux-i2c@vger.kernel.org>,
        devicetree@vger.kernel.org,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Wolfram Sang <wsa@the-dreams.de>,
        Sakari Ailus <sakari.ailus@linux.intel.com>,
        Hans Verkuil <hverkuil-cisco@xs4all.nl>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        Kieran Bingham <kieran.bingham@ideasonboard.com>,
        Jacopo Mondi <jacopo@jmondi.org>,
        Vladimir Zapolskiy <vz@mleia.com>,
        Peter Rosin <peda@axentia.se>
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On Mon, Aug 19, 2019 at 5:41 PM Luca Ceresoli <luca@lucaceresoli.net> wrote:
>
> Hi Rob,
>
> thanks for your review.
>
> On 13/08/19 17:44, Rob Herring wrote:
> > On Tue, Jul 23, 2019 at 10:37:20PM +0200, Luca Ceresoli wrote:
> >> Describe the Texas Instruments DS90UB954-Q1, a 2-input video deserializer
> >> with I2C Address Translator and remote GPIOs.
> >>
> >> Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
> >>
> >> ---
> >>
> >> Changes RFCv1 -> RFCv2:
> >>
> >>  - add explicit aliases for the FPD-link RX ports (optional)
> >>  - add proper remote GPIO description
> >> ---
> >>  .../bindings/media/i2c/ti,ds90ub954-q1.txt    | 194 ++++++++++++++++++
> >>  1 file changed, 194 insertions(+)
> >>  create mode 100644 Documentation/devicetree/bindings/media/i2c/ti,ds90ub954-q1.txt
> >>
> >> diff --git a/Documentation/devicetree/bindings/media/i2c/ti,ds90ub954-q1.txt b/Documentation/devicetree/bindings/media/i2c/ti,ds90ub954-q1.txt
> >> new file mode 100644
> >> index 000000000000..73ce21ecc3b6
> >> --- /dev/null
> >> +++ b/Documentation/devicetree/bindings/media/i2c/ti,ds90ub954-q1.txt
> >> @@ -0,0 +1,194 @@
> >> +Texas Instruments DS90UB954-Q1 dual video deserializer
> >> +======================================================
> >> +
> >> +The TI DS90UB954-Q1 is a MIPI CSI-2 video deserializer that forwards video
> >> +streams from up to two FPD-Link 3 connections to a MIPI CSI-2 output. It
> >> +also allows access to remote I2C and GPIO.
> >> +
> >> +Required properties:
> >> +
> >> + - compatible: must be "ti,ds90ub954-q1"
> >> +
> >> + - reg: main I2C slave address; optionally aliases for RX port registers
> >> +   and remote serializers. The main address is mandatory and must be the
> >> +   first, others are optional and fall back to defaults if not
> >> +   specified. See "reg-names".
> >> +
> >> + - reset-gpios: chip reset GPIO, active low (connected to PDB pin of the chip)
> >> + - i2c-alias-pool: list of I2C addresses that are known to be available on the
> >> +                   "local" (SoC-to-deser) I2C bus; they will be picked at
> >> +               runtime and used as aliases to reach remove I2C chips
> >
> > s/remove/remote/
>
> Will fix.
>
> > Needs a vendor prefix.
>
> The ultimate goal here is to define a standard property that all chips
> able to to I2C forwarding (video serdes or, potentially, other chips)
> can use. That's why the GMSL (Maxim deser) developers are in Cc: they
> are also facing a similar need.
>
> However I'm OK to change this to "ti,i2c-alias-pool" just in case there
> are reasons to not use a common name [yet]. However, following this
> argument, shouldn't a prefix be needed also for other nonstandard
> strings, such as "i2c-atr" below?

Okay, no vendor prefix is fine.

'i2c-atr' is a node name, not a property so no vendor prefix.

>
> >> + - gpio-controller
> >> + - #gpio-cells: must be 3: FPD-Link 3 RX port number, remote gpio number, flags
> >
> > We're pretty standardized on 2 cells for GPIO. Perhaps combine the port
> > and gpio number to 1 cell.
>
> Oh dear. I dislike implementing software that does not model the
> physical reality. I know it will bite me back sooner or later. Here we
> _really_ have N physically separate GPIO controllers, and the number of
> GPIOs they have depends on the model of the chip that is connected remotely.
>
> This is how things look in the case of 2 ports:
>
>  <-- base board -->         <------- remote camera module 1 ----->
>                             .---------------------.
>  .-----.    .------.        |         SER 1       |
>  | CPU |----|port 1|========|----------.          |
>  `-----'    |      |  FPD   | GPIO ctl |          |
>             |      | Link 3 `---------------------'
>             |      | cable        ||||
>             | DES  |         remote GPIO pins
>             |      |
>             |      |        <------- remote camera module 2 ----->
>             |      |        .---------------------.
>             |port 2|        |         SER 2       |
>             |      |========|----------.          |
>             `------'  FPD   | GPIO ctl |          |
>                      Link 3 `---------------------'
>                      cable    ||||||||
>                              remote GPIO pins
>
> Perhaps we should have N separate gpiochips, one per port?

Yes, seems like it.

> Under which
> node? Not under "ports" ("We don't allow other nodes within graph
> nodes"), and "i2c-atr" does not seem like a suitable name. Under a new
> "remote-gpiochips" node?
>
> >> +Required subnodes:
> >> +
> >> + - ports: A ports node with one port child node per device input and output
> >> +          port, in accordance with the video interface bindings defined in
> >> +          Documentation/devicetree/bindings/media/video-interfaces.txt. The
> >> +          port nodes are numbered as follows:
> >> +
> >> +          Port Description
> >> +          ------------------------------------
> >> +          0    Input from FPD-Link 3 RX port 0
> >> +          1    Input from FPD-Link 3 RX port 1
> >> +          2    CSI-2 output
> >> +
> >> +          Each port must have a "remote-chip" subnode that defines the remote
> >> +      chip (serializer) with at least a "compatible" property
> >
> > We don't allow other nodes within graph nodes. I'm not really clear what
> > you are trying to do here.
>
> Each of the deser ports (2 ports in this chip) creates a physical
> point-to-point "bus". It's called "FPD-Link 3" in the TI chips, "GMSL"
> in the Maxim chips. One "remote chip" serializer can be connected to
> each bus. The remote chip has, at least, a model (e.g. DS90UB953) and
> some properties. So I need a place where model and properties can be
> described. The port node looked like a good place, but as you point out
> it is not.
>
> Adding to the above discussion about 3 gpio-cells, I can think of a
> different, tentative DT layout:
>
> deser: deser@3d {
>         compatible = "ti,ds90ub954-q1";
>         reg-names = "main", "rxport0", "rxport1", "ser0", "ser1";
>         reg       = <0x3d>,  <0x40>,    <0x41>,   <0x44>, <0x45>;
>         clocks = <&clk_25M>;
>         interrupt-parent = <&gic>;
>         interrupts = <3 1 IRQ_TYPE_LEVEL_HIGH>;
>         reset-gpios = <&gpio_ctl 4 GPIO_ACTIVE_LOW>;
>
>         i2c-alias-pool = /bits/ 16 <0x4a 0x4b 0x4c 0x4d 0x4e 0x4f>;
>
>         ports {
>                 #address-cells = <1>;
>                 #size-cells = <0>;
>
>                 port@0 {
>                         reg = <0>;
>                         endpoint {
>                                 remote-endpoint = <&sensor_0_out>;
>                         };
>                 };
>
>                 port@1 {
>                         reg = <1>;
>                         endpoint {
>                                 remote-endpoint = <&sensor_1_out>;
>                         };
>                 };
>
>                 port@2 {
>                         reg = <2>;
>                         endpoint {
>                                 data-lanes = <1 2 3 4>;
>                                 /* Actually a REFCLK multiplier */
>                                 data-rate = <1600000000>;
>                                 remote-endpoint = <&csirx_0_in>;
>                         };
>                 };
>         };
>
>         remote-chips {

I don't have a better suggestion for the location of this.

>                 #address-cells = <1>;
>                 #size-cells = <0>;
>
>                 remote-chip@0 {
>                         reg = <0>;
>                         chip {
>                                 compatible = "ti,ds90ub953-q1";

Seems like this should be in the parent? It's the ds90ub953
implementing the I2C bus, GPIO controller, etc.?

>                                 gpio-functions = <...>;
>                         };
>
>                         remote_i2c0: i2c@0 {

What does '0' mean here? At a given level, you can only have 1 number
space of addresses.

>                                 reg = <0>;
>                                 #address-cells = <1>;
>                                 #size-cells = <0>;
>                         };
>
>                         gpio-controller;
>                         /* 2 cells for _each_ gpiochip */
>                         #gpio-cells = <2>; /* remote gpio num, flags */
>                 };
>
>                 remote-chip@1 {
>                         reg = <1>;
>                         /* similar to remote chip 0 */
>                 };
>         };
> };
>
> Here there are two subnodes:
>
>  - "remote-chips" with a subnode per remote port, each describing
>    the remote chip model and properties, remote i2c bus, gpio controller
>    and others (other chips have I2S, SPI, UART...)
>
>  - "ports" with only the standard v4l2 port properties, no subnodes
>    under the endpoint nodes
>
> Does this look better from the device tree point of view?

Yes.

> >> +        - #address-cells = <1>;
> >> +        - #size-cells = <0>;
> >> +
> >> +        Optional properties for "i2c-atr" child bus nodes:
> >> +        - Other properties specific to the remote hardware
> >
> > Such as?
>
> "clock-frequency" at least. The remote chip is an I2C master, thus any
> property that applies to an I2C master might apply as well.
>
> "clock-frequency" is only half-implemented in these RFC patches:
>
>  * in patch 5, function ds90_rxport_add_serializer() passes the REFCLK
>    value of the deser to the remote chip (serializer): this value is
>    the physical reference clock the remote chip receives, all of its
>    timings are based on it
>
>  * the remote chip, given refclk, computes the register values that best
>    approximate "clock-frequency" [not implemented in this version, would
>    be in patch 6]
>
> I plan to implement and document the whole clock-frequency feature for
> the next patch iteration. But I'd love to receive comments about how
> reflck is passed from the deser to the serializer via platform_data.

Okay. Please try to make bindings as complete as possible even if
there's not yet driver support.

> >> +            i2c-atr {
> >> +                    #address-cells = <1>;
> >> +                    #size-cells = <0>;
> >> +
> >> +                    remote_i2c0: i2c@0 {
> >> +                            reg = <0>;
> >> +                            #address-cells = <1>;
> >> +                            #size-cells = <0>;
> >
> > Presumably, there are child I2C devices here. Please show that in the
> > example.
>
> They are shown below to avoid excessive nesting, look for "&remote_i2c0"
> (1).

I'd rather have longish lines than have things split up.

>
> >> +&ds90ub954_fpd3_in0 {
> >> +    remote-chip {
> >> +            compatible = "ti,ds90ub953-q1";
> >> +            gpio-functions = <DS90_GPIO_FUNC_OUTPUT_REMOTE
> >
> > Not documented.
>
> That's because this node describes the remote chip. The remote chip is a
> ti,ds90ub953-q1, its bindings are defined in patch 4. It's similar to
> showing some child I2C devices under the I2C master, as you suggested
> just above, except it's not an I2C bus but an FPD-Link 3 "bus".

I think I realized this after sending this...

>
> Is it OK if I replace the whole gpio-functions node with the comment
> "/* properties specific to the ti,ds90ub953-q1 chip */"?

Not necessary. gpio-functions does need a vendor prefix though.

Rob