Re: [PATCH 13/15] dt-bindings: pinctrl: add support for Ambarella

[Li Chen]

Hi Linus,

Sorry for my late reply.

 ---- On Mon, 23 Jan 2023 20:32:28 +0800  Linus Walleij  wrote --- 
 > Hi Li,
 > 
 > thanks for your patch!
 > 
 > It's nice to see Ambarella working with the kernel community.
 > 
 > On Mon, Jan 23, 2023 at 8:41 AM Li Chen lchen@xxxxxxxxxxxxx> wrote:
 > 
 > > +properties:
 > > +  compatible:
 > > +    items:
 > > +      - const: ambarella,pinctrl
 > 
 > I bet there will be more instances of pin controllers from Ambarella, so I would
 > use this only as a fallback, so the for should likely be:
 > 
 > compatible = "ambarella,-pinctrl", "ambarella,pinctrl";
 > 
 > we need to establish this already otherwise "ambarella,pinctrl" just becomes
 > the "weird name of the first ambarella SoC supported by standard DT bindings".
 
There is only single "ambarella,pinctrl" in Ambarella downstream kernels, and we use soc_device_attribute->data
and soc_device_attribute->soc_id/family to get correct SoC-specific information like reg offset and etc.

Krzysztof has taught me that this way is wrong and
SoC is required in compatible: https://www.spinics.net/lists/arm-kernel/msg1043145.html

So I will update this property to "ambarella,s6lm-pinctrl" in the new version.

 > > +  amb,pull-regmap:
 > > +    $ref: /schemas/types.yaml#/definitions/phandle-array
 > > +    items:
 > > +      maxItems: 1
 > > +
 > > +  amb,ds-regmap:
 > > +    items:
 > > +      maxItems: 1
 > 
 > Interesting that these registers are elsewhere, but I bet there is an
 > engineering
 > explanation for this :)
 > 
 > > +    properties:
 > > +      amb,pinmux-ids:
 > > +        description: |
 > > +          an integer array. Each integer in the array specifies a pin
 > > +          with given mux function, with pin id and mux packed as:
 > > +          mux << 12 | pin id
 > > +          Here mux means function of this pin, and pin id is identical to gpio id. For
 > > +          the SoC supporting IOMUX, like S2L, the maximal value of mux is 5. However,
 > > +          for the SoC not supporting IOMUX, like A5S, S2, the third or fourth function
 > > +          is selected by other "virtual pins" setting. Here the "virtual pins" means
 > > +          there is no real hardware pins mapping to the corresponding register address.
 > > +          So the registers for the "virtual pins" can be used for the selection of 3rd
 > > +          or 4th function for other real pins.
 > 
 > I think you can use the standard bindings for this if you insist on
 > using the "magic
 > numbers" scheme.
 > 
 > (I prefer function names and group names as strings, but I gave up on trying
 > to convince the world to use this because people have so strong opions about
 > it.)
 > 
 > From Documentation/devicetree/bindings/pinctrl/pinmux-node.yaml:
 > 
 >   pinmux:
 >     description:
 >       The list of numeric pin ids and their mux settings that properties in the
 >       node apply to (either this, "pins" or "groups" have to be specified)
 >     $ref: /schemas/types.yaml#/definitions/uint32-array
 > 

Well noted, I will switch to pinmux in v2.

 > > +      amb,pinconf-ids:
 > > +        description: |
 > > +          an integer array. Each integer in the array specifies a pin
 > > +          with given configuration, with pin id and config packed as:
 > > +            config << 16 | pin id
 > > +          Here config is used to configure pull up/down and drive strength of the pin,
 > > +          and it's orgnization is:
 > > +          bit1~0: 00: pull down, 01: pull up, 1x: clear pull up/down
 > > +          bit2:   reserved
 > > +          bit3:   0: leave pull up/down as default value, 1: config pull up/down
 > > +          bit5~4: drive strength value, 0: 2mA, 1: 4mA, 2: 8mA, 3: 12mA
 > > +          bit6:   reserved
 > > +          bit7:   0: leave drive strength as default value, 1: config drive strength
 > 
 > I would be very happy if I could convince you to use the standard (string)
 > bindings for this.
 > And from Documentation/devicetree/bindings/pinctrl/pincfg-node.yaml
 > 
 > For each config node this means using strings such as
 > bias-high-impedance; etc in the device tree pin config node.
 > 
 > Following that scheme just makes life so much easier for maintainers and
 > reviewers: very few people reviewing or debugging the system will think
 > it is easy to read a magic number and then (in their head) mask out the
 > bits to see that "OK this is drive strength 8mA" and then have energy and
 > memory enough in their head to remember that "wait a minute, that is supposed
 > to be 12mA in this design", leading to long review and development
 > cycles.
 > 
 > By using:
 > 
 > drive-push-pull;
 > drive-strength = ;
 > 
 > you make the cognitive load on the people reading the device tree much
 > lower and easier to write, maintain and debug for humans.
 > 
 > The tendency to encode this info in terse bitfields appear to be done for either
 > of these reasons:
 > 
 > - Footprint / memory usage
 > - Adopt the users to the way the machine thinks instead of the other way around
 > - "We always did it this way"
 > 
 > Neither is a very good argument on a new 64bit platform.

Thanks for your detailed explanation. I totally agree with you and I also really hate
magic number haha.

I will convert it to standard binding after convincing my manager.

Regards,
Li