Re: [PATCH v5 2/6] dt-bindings: auxdisplay: Add Titan Micro Electronics TM1628

[Heiner Kallweit <hkallweit1@xxxxxxxxx>]

On 20.04.2022 01:04, Robin Murphy wrote:
> On 2022-03-30 06:54, Heiner Kallweit wrote:
>> On 23.03.2022 21:33, Heiner Kallweit wrote:
>>> On 21.03.2022 09:34, Krzysztof Kozlowski wrote:
>>>> On 18/03/2022 21:50, Robin Murphy wrote:
>>>>> On 2022-02-25 21:13, Heiner Kallweit wrote:
>>>>>> Add a YAML schema binding for TM1628 auxdisplay
>>>>>> (7/11-segment LED) controller.
>>>>>>
>>>>>> This patch is partially based on previous work from
>>>>>> Andreas Färber <afaerber@xxxxxxx>.
>>>>>>
>>>>>> Co-developed-by: Andreas Färber <afaerber@xxxxxxx>
>>>>>> Signed-off-by: Andreas Färber <afaerber@xxxxxxx>
>>>>>> Signed-off-by: Heiner Kallweit <hkallweit1@xxxxxxxxx>
>>>>>> ---
>>>>>> v5:
>>>>>> - add vendor prefix to driver-specific properties
>>>>>> ---
>>>>>>    .../bindings/auxdisplay/titanmec,tm1628.yaml  | 92 +++++++++++++++++++
>>>>>>    1 file changed, 92 insertions(+)
>>>>>>    create mode 100644 Documentation/devicetree/bindings/auxdisplay/titanmec,tm1628.yaml
>>>>>>
>>>>>> diff --git a/Documentation/devicetree/bindings/auxdisplay/titanmec,tm1628.yaml b/Documentation/devicetree/bindings/auxdisplay/titanmec,tm1628.yaml
>>>>>> new file mode 100644
>>>>>> index 000000000..2a1ef692c
>>>>>> --- /dev/null
>>>>>> +++ b/Documentation/devicetree/bindings/auxdisplay/titanmec,tm1628.yaml
>>>>>> @@ -0,0 +1,92 @@
>>>>>> +# SPDX-License-Identifier: (GPL-2.0-or-later OR BSD-2-Clause)
>>>>>> +%YAML 1.2
>>>>>> +---
>>>>>> +$id: http://devicetree.org/schemas/auxdisplay/titanmec,tm1628.yaml#
>>>>>> +$schema: http://devicetree.org/meta-schemas/core.yaml#
>>>>>> +
>>>>>> +title: Titan Micro Electronics TM1628 LED controller
>>>>>> +
>>>>>> +maintainers:
>>>>>> +  - Andreas Färber <afaerber@xxxxxxx>
>>>>>> +  - Heiner Kallweit <hkallweit1@xxxxxxxxx>
>>>>>> +
>>>>>> +allOf:
>>>>>> +  - $ref: /schemas/spi/spi-peripheral-props.yaml#
>>>>>> +
>>>>>> +properties:
>>>>>> +  compatible:
>>>>>> +    const: titanmec,tm1628
>>>>>> +
>>>>>> +  reg:
>>>>>> +    maxItems: 1
>>>>>> +
>>>>>> +  titanmec,grid:
>>>>>> +    description:
>>>>>> +      Mapping of display digit position to grid number.
>>>>>> +      This implicitly defines the display size.
>>>>>> +    $ref: /schemas/types.yaml#/definitions/uint8-array
>>>>>> +    minItems: 1
>>>>>> +    maxItems: 7
>>>>>> +
>>>>>> +  titanmec,segment-mapping:
>>>>>> +    description:
>>>>>> +      Mapping of 7 segment display segments A-G to bit numbers 1-12.
>>>>>> +    $ref: /schemas/types.yaml#/definitions/uint8-array
>>>>>> +    minItems: 7
>>>>>> +    maxItems: 7
>>>>>> +
>>>>>> +  "#address-cells":
>>>>>> +    const: 2
>>>>>> +
>>>>>> +  "#size-cells":
>>>>>> +    const: 0
>>>>>> +
>>>>>> +required:
>>>>>> +  - compatible
>>>>>> +  - reg
>>>>>
>>>>> Would it be fair to say that "spi-lsb-first" and "spi-3wire" are also
>>>>> required? The chips aren't configurable so won't exactly be usable any
>>>>> other way. Furthermore I believe the transmission format actually works
>>>>> out equivalent to SPI mode 3, so should warrant "spi-cpha" and
>>>>> "spi-cpol" as well.
>>>>>
>>>>>> +
>>>>>> +patternProperties:
>>>>>> +  "^.*@[1-7],([1-9]|1[0-6])$":
>>>>>> +    type: object
>>>>>> +    $ref: /schemas/leds/common.yaml#
>>>>>> +    unevaluatedProperties: false
>>>>>> +    description: |
>>>>>> +      Properties for a single LED.
>>>>>> +
>>>>>> +    properties:
>>>>>> +      reg:
>>>>>> +        description: |
>>>>>> +          1-based grid number, followed by 1-based segment bit number.
>>>>>> +        maxItems: 1
>>>>>> +
>>>>>> +    required:
>>>>>> +      - reg
>>>>>
>>>>> I'm concerned that this leaves us no room to support the additional
>>>>> keypad functionality in future. Having now double-checked a datasheet,
>>>>> the inputs are also a two-dimensional mux (sharing the segment lines),
>>>>> so the device effectively has two distinct but numerically-overlapping
>>>>> child address spaces - one addressed by (grid,segment) and the other by
>>>>> (segment,key).
>>>>>
>>>>> Rob, Krysztof, any thoughts on the best DT idiom to leave accommodation
>>>>> for that? I'm thinking either require an intermediate node to contain
>>>>> each notional address space, or perhaps add another leading address cell
>>>>> to select between them? I don't believe any of these things have further
>>>>> functionality beyond that.
>>>>
>>>> I think intermediate nodes - leds, keys - are more appropriate, because
>>>> it is self-describing. Additional address space number would require
>>>> decoding this "0" or "1" into LED/key. For complex devices - like PMICs
>>>> with regulators, RTC and clocks - we already have such patterns.
>>>>
>>> Then it's just the question who can implement such an intermediate node
>>> based on what has been done so far.
>>>
>> As it is now it seems we end up with empty hands again and have to wait
>> further two years for the next one to make an attempt.
>> That's a pity because for most users the relevant use cases are supported.
> 
> Or, y'know, we could just reach a productive conclusion rather than doom-and-gloom catastrophising. I apologise for not having much time for non-work-related kernel hacking at the moment, but it didn't seem particularly urgent to follow up on this in the middle of a merge window anyway. In the course of helpfully being left to address my own review feedback, I did eventually get round to implementing the intermediate "leds" node[1] last weekend, but having now stumbled across the matrix-keymap helpers and common "linux,keymap" property, I'm personally inclined to think that that's even cleaner than a "keys" node with children that we'd have to write more parsing code for, and thus may well make the whole intermediate node notion moot anyway. If only anyone had pointed it out sooner...
> 
Thanks for following up on the recent discussion. Good to see that the parrot
is not dead but was just resting. My reaction was based on a number of discussions 
I witnessed that ended in nowhere. One example that comes to my mind is
LED subsystem support for network devices with hardware-triggered LEDs.

> Thanks,
> Robin.

Heiner 

> [1] https://gitlab.arm.com/linux-arm/linux-rm/-/commits/tm1628