[v4,0/3] i2c: gpio: support write-only sda

There are slave devices that understand I2C but have read-only SDA and
SCL. Examples are FD650 7-segment LED controller and its derivatives.
Typical board designs don't even have a pull-up for both pins.
Therefore add properties for not using open-drain. For write-only SCL
we have a property already, add one for write-only SDA.

v2:
- improve commit message for patch 1

v3:
- patch 2: check for adap->getsda in readbytes()
- patch 2: align warning message level for info on missing getscl/getsda
- patch 3: improve description of attribute sda_is_output_only

v4:
- patch 1: add no-pullup properties
- patch 2: handle SDA and SCL independently
- patch 2: properly handle case that SDA is NULL but SCL not
- patch 3: handle new no-pullup attributes

Heiner Kallweit (3):
  dt-bindings: i2c-gpio: Add properties for dealing with write-only SDA/SCL w/o pullup
  i2c: algo: bit: allow getsda to be NULL
  i2c: gpio: support write-only sda/scl w/o pull-up

 .../devicetree/bindings/i2c/i2c-gpio.yaml     | 16 ++++
 drivers/i2c/algos/i2c-algo-bit.c              | 77 +++++++++----------
 drivers/i2c/busses/i2c-gpio.c                 | 13 +++-
 include/linux/platform_data/i2c-gpio.h        |  9 +++
 4 files changed, 70 insertions(+), 45 deletions(-)

On Tue, Jan 17, 2023 at 09:36:05PM +0100, Heiner Kallweit wrote:
> There are slave devices that understand I2C but have read-only SDA and
> SCL. Examples are FD650 7-segment LED controller and its derivatives.
> Typical board designs don't even have a pull-up for both pins.
> Therefore add properties for not using open-drain. For write-only SCL
> we have a property already, add one for write-only SDA.
> 
> Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
> ---
> v4:
> - add no-pullup properties
> ---
>  .../devicetree/bindings/i2c/i2c-gpio.yaml        | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/i2c/i2c-gpio.yaml b/Documentation/devicetree/bindings/i2c/i2c-gpio.yaml
> index e0d76d5eb..67898cc52 100644
> --- a/Documentation/devicetree/bindings/i2c/i2c-gpio.yaml
> +++ b/Documentation/devicetree/bindings/i2c/i2c-gpio.yaml
> @@ -33,6 +33,10 @@ properties:
>        open drain.
>      maxItems: 1
>  
> +  i2c-gpio,sda-output-only:
> +    description: sda as output only
> +    type: boolean
> +
>    i2c-gpio,scl-output-only:
>      description: scl as output only
>      type: boolean
> @@ -63,6 +67,18 @@ properties:
>        GPIO line used for SCL into open drain mode, and that something is not
>        the GPIO chip. It is essentially an inconsistency flag.
>  
> +  i2c-gpio,sda-has-no-pullup:
> +    type: boolean
> +    description: sda is used in a non-compliant way and has no pull-up.
> +      Therefore disable open-drain. This property is mutually-exclusive
> +      with i2c-gpio,sda-open-drain.
> +
> +  i2c-gpio,scl-has-no-pullup:
> +    type: boolean
> +    description: scl is used in a non-compliant way and has no pull-up.
> +      Therefore disable open-drain. This property is mutually-exclusive
> +      with i2c-gpio,scl-open-drain.

The mutual-exclusion can be expressed as a schema instead:

allOf:
  - not:
      required:
        - i2c-gpio,scl-has-no-pullup
        - i2c-gpio,scl-open-drain
  - not:
      required:
        - i2c-gpio,sda-has-no-pullup
        - i2c-gpio,sda-open-drain

Using 'dependencies' with a schema would also work.

Rob

On 18.01.2023 17:32, Rob Herring wrote:
> On Tue, Jan 17, 2023 at 09:36:05PM +0100, Heiner Kallweit wrote:
>> There are slave devices that understand I2C but have read-only SDA and
>> SCL. Examples are FD650 7-segment LED controller and its derivatives.
>> Typical board designs don't even have a pull-up for both pins.
>> Therefore add properties for not using open-drain. For write-only SCL
>> we have a property already, add one for write-only SDA.
>>
>> Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
>> ---
>> v4:
>> - add no-pullup properties
>> ---
>>  .../devicetree/bindings/i2c/i2c-gpio.yaml        | 16 ++++++++++++++++
>>  1 file changed, 16 insertions(+)
>>
>> diff --git a/Documentation/devicetree/bindings/i2c/i2c-gpio.yaml b/Documentation/devicetree/bindings/i2c/i2c-gpio.yaml
>> index e0d76d5eb..67898cc52 100644
>> --- a/Documentation/devicetree/bindings/i2c/i2c-gpio.yaml
>> +++ b/Documentation/devicetree/bindings/i2c/i2c-gpio.yaml
>> @@ -33,6 +33,10 @@ properties:
>>        open drain.
>>      maxItems: 1
>>  
>> +  i2c-gpio,sda-output-only:
>> +    description: sda as output only
>> +    type: boolean
>> +
>>    i2c-gpio,scl-output-only:
>>      description: scl as output only
>>      type: boolean
>> @@ -63,6 +67,18 @@ properties:
>>        GPIO line used for SCL into open drain mode, and that something is not
>>        the GPIO chip. It is essentially an inconsistency flag.
>>  
>> +  i2c-gpio,sda-has-no-pullup:
>> +    type: boolean
>> +    description: sda is used in a non-compliant way and has no pull-up.
>> +      Therefore disable open-drain. This property is mutually-exclusive
>> +      with i2c-gpio,sda-open-drain.
>> +
>> +  i2c-gpio,scl-has-no-pullup:
>> +    type: boolean
>> +    description: scl is used in a non-compliant way and has no pull-up.
>> +      Therefore disable open-drain. This property is mutually-exclusive
>> +      with i2c-gpio,scl-open-drain.
> 
> The mutual-exclusion can be expressed as a schema instead:
> 
> allOf:
>   - not:
>       required:
>         - i2c-gpio,scl-has-no-pullup
>         - i2c-gpio,scl-open-drain
>   - not:
>       required:
>         - i2c-gpio,sda-has-no-pullup
>         - i2c-gpio,sda-open-drain
> 
> Using 'dependencies' with a schema would also work.
> 
Great, I wasn't aware of options to express mutually-exclusive properties.

I'll wait a little for other feedback regarding the series and then submit
a new version incl. the extended schema.

> Rob

Heiner