Re: [PATCH v13 1/2] media: dt-bindings: media: i2c: Document OV02A10 bindings

[Sakari Ailus <sakari.ailus@linux.intel.com>]

Hi Tomasz,

Thanks for dressing this into words.

On Mon, Aug 31, 2020 at 01:44:27PM +0200, Tomasz Figa wrote:
> Hi Dongchun,
> 
> On Fri, Jul 10, 2020 at 12:19 PM Dongchun Zhu <dongchun.zhu@mediatek.com> wrote:
> >
> > Add YAML device tree binding for OV02A10 CMOS image sensor,
> > and the relevant MAINTAINERS entries.
> >
> > Reviewed-by: Tomasz Figa <tfiga@chromium.org>
> > Reviewed-by: Rob Herring <robh@kernel.org>
> > Signed-off-by: Dongchun Zhu <dongchun.zhu@mediatek.com>
> > ---
> >  .../bindings/media/i2c/ovti,ov02a10.yaml           | 172 +++++++++++++++++++++
> >  MAINTAINERS                                        |   7 +
> >  2 files changed, 179 insertions(+)
> >  create mode 100644 Documentation/devicetree/bindings/media/i2c/ovti,ov02a10.yaml
> >
> > diff --git a/Documentation/devicetree/bindings/media/i2c/ovti,ov02a10.yaml b/Documentation/devicetree/bindings/media/i2c/ovti,ov02a10.yaml
> > new file mode 100644
> > index 0000000..3a916cc
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/media/i2c/ovti,ov02a10.yaml
> > @@ -0,0 +1,172 @@
> > +# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
> > +# Copyright (c) 2020 MediaTek Inc.
> > +%YAML 1.2
> > +---
> > +$id: http://devicetree.org/schemas/media/i2c/ovti,ov02a10.yaml#
> > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > +
> > +title: Omnivision OV02A10 CMOS Sensor Device Tree Bindings
> > +
> > +maintainers:
> > +  - Dongchun Zhu <dongchun.zhu@mediatek.com>
> > +
> > +description: |-
> > +  The Omnivision OV02A10 is a low-cost, high performance, 1/5-inch, 2 megapixel
> > +  image sensor, which is the latest production derived from Omnivision's CMOS
> > +  image sensor technology. Ihis chip supports high frame rate speeds up to 30fps
> > +  @ 1600x1200 (UXGA) resolution transferred over a 1-lane MIPI interface. The
> > +  sensor output is available via CSI-2 serial data output.
> > +
> > +properties:
> > +  compatible:
> > +    const: ovti,ov02a10
> > +
> > +  reg:
> > +    maxItems: 1
> > +
> > +  clocks:
> > +    items:
> > +      - description: top mux camtg clock
> > +      - description: divider clock
> > +
> > +  clock-names:
> > +    items:
> > +      - const: eclk
> > +      - const: freq_mux
> 
> Why do we have two clocks here? Looking at the example suggests that
> they may be the clocks of the SoC that the integration was done with.
> However, the binding must only define the aspects of the particular
> device, i.e. this sensor.
> 
> I suppose we should only have "eclk" here and it should be described
> as "external clock for the sensor".
> 
> > +
> > +  clock-frequency:
> > +    description:
> > +      Frequency of the eclk clock in Hertz.
> 
> nit: maybe Hz?
> 
> > +
> > +  dovdd-supply:
> > +    description:
> > +      Definition of the regulator used as Digital I/O voltage supply.
> > +
> > +  avdd-supply:
> > +    description:
> > +      Definition of the regulator used as Analog voltage supply.
> > +
> > +  dvdd-supply:
> > +    description:
> > +      Definition of the regulator used as Digital core voltage supply.
> > +
> > +  powerdown-gpios:
> > +    description:
> > +      Must be the device tree identifier of the GPIO connected to the
> > +      PD_PAD pin. This pin is used to place the OV02A10 into standby mode
> > +      or shutdown mode. As the line needs to be high for the powerdown mode
> > +      to be active, it should be marked GPIO_ACTIVE_HIGH.
> > +    maxItems: 1
> > +
> > +  reset-gpios:
> > +    description:
> > +      Must be the device tree identifier of the GPIO connected to the
> > +      RST_PD pin. If specified, it will be asserted during driver probe.
> > +      As the line needs to be low for the reset to be active, it should be
> > +      marked GPIO_ACTIVE_LOW.
> > +    maxItems: 1
> > +
> > +  rotation:
> > +    description:
> > +      Definition of the sensor's placement.
> > +    allOf:
> > +      - $ref: "/schemas/types.yaml#/definitions/uint32"
> > +      - enum:
> > +          - 0    # Sensor Mounted Upright
> > +          - 180  # Sensor Mounted Upside Down
> > +        default: 0
> > +
> > +  ovti,mipi-tx-speed:
> > +    description:
> > +      Indication of MIPI transmission speed select, which is to control D-PHY
> > +      timing setting by adjusting MIPI clock voltage to improve the clock
> > +      driver capability.
> 
> The description says that the value adjusts "MIPI clock voltage".
> Should the property be renamed to "ovti,mipi-clock-voltage"?
> 
> > +    allOf:
> > +      - $ref: "/schemas/types.yaml#/definitions/uint32"
> > +      - enum:
> > +          - 0    #  20MHz -  30MHz
> > +          - 1    #  30MHz -  50MHz
> > +          - 2    #  50MHz -  75MHz
> > +          - 3    #  75MHz - 100MHz
> > +          - 4    # 100MHz - 130MHz
> > +        default: 3
> > +
> 
> I've discussed this on IRC with Sakari. It sounds like this works as
> is for us because the driver currently only supports 1 mode, always
> running the link at 390 MHz. This won't scale if one intends to add
> more modes, because DT can't be expected to be updated when the driver
> changes. The two are expected to be separate and backwards compatible.
> 
> I think we could model this in DT as an array of <link speed, clock
> voltage> pairs. Similarly to the OPP bindings [1]. An example to have
> all link speeds up to 390 MHz use the value 4:
> 
> ovti,mipi-clock-voltages = <
>               /* KHz         clock voltage unit */
>                  390000    4
> >;
> 
> if one wants to select different voltage for different link, they
> could do so as well. With the example below, the driver should
> configure "3" for link frequencies <= 150 MHz and "4" for > 150 MHz <=
> 390 MHz. Link frequencies > 390 MHz should be disallowed.
> 
> ovti,mipi-clock-voltages = <
>               /* KHz         clock voltage unit */
>                  150000    3
>                  390000    4
> >;
> 
> What do you think?

One more note on my proposal: use the index of the link frequency to access
the ovti,mipi-clock-voltages array. That should be more simple for the
driver, as the driver is expected to use only link frequencies listed in DT
bindings anyway.

-- 
Kind regards,

Sakari Ailus

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