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

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

On Tue, Sep 01, 2020 at 10:35:37PM +0800, Dongchun Zhu wrote:
> Hi Tomasz, Sakari,
> 
> Thanks for digging into the imperfections.
> 
> On Mon, 2020-08-31 at 19:06 +0300, Sakari Ailus wrote:
> > 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".
> > > 
> 
> I cannot agree with you more, it would be fixed in next release.
> 
> > > > +
> > > > +  clock-frequency:
> > > > +    description:
> > > > +      Frequency of the eclk clock in Hertz.
> > > 
> > > nit: maybe Hz?
> > > 
> 
> Both shall be okay, let's use Hz instead, which seems to be more simple.
> 
> > > > +
> > > > +  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"?
> > > 
> 
> Sure, mipi-clock-voltage seems more clear, perfect naming :-)
> 
> > > > +    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?
> > 
> 
> So now we define the MIPI clock voltage (unit) as one function of link
> frequency?
> 
> For current sensor mode (1600x1200, 390MHz link frequency), we then
> shall always use the value 4 to configure the MIPI transmission speed
> selection (P1:0xA1). Am I correct in thinking that?
> 
> > 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.
> > 
> 
> Good idea! That's exactly what I mean.
> Just submitted one draft change to handle the new property in DT.
> @Sakari @Tomasz please help to review it.
> https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/2387980/1/drivers/media/i2c/ov02a10.c#883

Please send it to the list.

-- 
Sakari Ailus