linux-kernel - Re: [PATCH 1/3] dt-bindings: leds: Add bindings for the TLC5925 controller

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Date:   Mon, 23 May 2022 17:39:43 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Jean-Jacques Hiblot <jjhiblot@...phandler.com>,
        Pavel Machek <pavel@....cz>, Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>
Cc:     linux-leds@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] dt-bindings: leds: Add bindings for the TLC5925
 controller

On 23/05/2022 17:37, Jean-Jacques Hiblot wrote:
> 
> On 23/05/2022 17:30, Krzysztof Kozlowski wrote:
>> On 23/05/2022 17:16, Jean-Jacques Hiblot wrote:
>>> On 23/05/2022 12:14, Krzysztof Kozlowski wrote:
>>>> On 23/05/2022 10:49, Jean-Jacques Hiblot wrote:
>>>>> Add bindings documentation for the TLC5925 LED controller.
>>>>>
>>>>> Signed-off-by: Jean-Jacques Hiblot <jjhiblot@...phandler.com>
>>>> Thank you for your patch. There is something to discuss/improve.
>>>>
>>>>> ---
>>>>> devicetree@...r.kernel.org
>>>>>    .../bindings/leds/leds-tlc5925.yaml           | 100 ++++++++++++++++++
>>>>>    1 file changed, 100 insertions(+)
>>>>>    create mode 100644 Documentation/devicetree/bindings/leds/leds-tlc5925.yaml
>>>>>
>>>>> diff --git a/Documentation/devicetree/bindings/leds/leds-tlc5925.yaml b/Documentation/devicetree/bindings/leds/leds-tlc5925.yaml
>>>>> new file mode 100644
>>>>> index 000000000000..156db599d5a1
>>>>> --- /dev/null
>>>>> +++ b/Documentation/devicetree/bindings/leds/leds-tlc5925.yaml
>>>> Filename: vendor,device
>>>> so "ti,tlc5925-leds.yaml" for example.
>>>>
>>>>
>>>>
>>>>> @@ -0,0 +1,100 @@
>>>>> +# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
>>>>> +%YAML 1.2
>>>>> +---
>>>>> +$id: http://devicetree.org/schemas/leds/leds-tlc5925.yaml#
>>>>> +$schema: http://devicetree.org/meta-schemas/core.yaml#
>>>>> +
>>>>> +title: LEDs connected to TI TLC5925 controller
>>>>> +
>>>>> +maintainers:
>>>>> +  - Jean-Jacques Hiblot <jjhiblot@...phandler.com>
>>>>> +
>>>>> +description: |
>>>>> +  The TLC5925 is a low-power 16-channel constant-current LED sink driver.
>>>>> +  It is controlled through a SPI interface.
>>>>> +  It is built around a shift register and latches which convert serial
>>>>> +  input data into a parallel output. Several TLC5925 can be chained to
>>>>> +  control more than 16 LEDs with a single chip-select.
>>>>> +  The brightness level cannot be controlled, each LED is either on or off.
>>>>> +
>>>>> +  Each LED is represented as a sub-node of the ti,tlc5925 device.
>>>>> +
>>>>> +properties:
>>>>> +  compatible:
>>>>> +    const: ti,tlc5925
>>>>> +
>>>>> +  shift_register_length:
>>>>> +    maxItems: 1
>>>> No...
>>>> 1. Did you test your bindings with dt_binding_check? This fails
>>>> obviously... please, do not send untested bindings.
>>>>
>>>> 2. vendor prefix, no underscores, proper type, maxItems look wrong here
>>>>
>>>>> +    description: |
>>>>> +      The length of the shift register. If several TLC5925 are chained,
>>>>> +      shift_register_length should be set to 16 times the number of TLC5925.
>>>>> +      The value must be a multiple of 8.
>>>>> +
>>>>> +  "#address-cells":
>>>>> +    const: 1
>>>>> +
>>>>> +  "#size-cells":
>>>>> +    const: 0
>>>>> +
>>>>> +  output-enable-b-gpios:
>>>>> +    description: |
>>>>> +      GPIO pins to enable/disable the parallel output. They describe the GPIOs
>>>>> +      connected to the OE/ pin of the TLC5925s.
>>>> maxItems
>>> There is no limitation in the driver itself. The actual number of items
>>> here really depends on the number of chips and how they are wired.
>> So you could daisy chain 4 billion of devices? Because by not using any
>> limit you claim that 4 billion is doable?
> 
> You could chain 1000 devices or more and have 16000 leds. It would be a 
> bit tedious to describe them all in the DTS though.
> 
> We can impose a limit but it will be arbitrary. Is this how it is 
> usually treated ?

1000 is still less than billion, although above that number it probably
does not make sense to have any limit.

Best regards,
Krzysztof