Re: [[PATCH v2] 1/9] dt-bindings: pwm: Document Synopsys DesignWare snps,pwm

[Ben Dooks <ben.dooks@sifive.com>]

On 27/07/2022 13:02, Krzysztof Kozlowski wrote:
> On 27/07/2022 12:32, Ben Dooks wrote:
>> On 26/07/2022 12:05, Krzysztof Kozlowski wrote:
>>> On 26/07/2022 12:12, Ben Dooks wrote:
>>>> On 26/07/2022 11:05, Krzysztof Kozlowski wrote:
>>>>> On 25/07/2022 23:21, Ben Dooks wrote:
>>>>>> Add documentation for the bindings for Synopsys' DesignWare PWM block
>>>>>> as we will be adding DT/platform support to the Linux driver soon.
>>>>>>
>>>>>> Signed-off-by: Ben Dooks <ben.dooks@sifive.com>
>>>>>> --
>>>>>
>>>>> This is not proper delimiter and causes the changelog to end up in commit.
>>>>>
>>>>> Correct also wrong formatting of subject PATCH.
>>>>
>>>> I realised that once sent and forgot the cover letter.
>>>> Maybe I'll try some more post covid recovery.
>>>>
>>>>>> v2:
>>>>>> - fix #pwm-cells to be 3
>>>>>> - fix indentation and ordering issues
>>>>>> ---
>>>>>>     .../devicetree/bindings/pwm/snps,pwm.yaml     | 40 +++++++++++++++++++
>>>>>>     1 file changed, 40 insertions(+)
>>>>>>     create mode 100644 Documentation/devicetree/bindings/pwm/snps,pwm.yaml
>>>>>>
>>>>>> diff --git a/Documentation/devicetree/bindings/pwm/snps,pwm.yaml b/Documentation/devicetree/bindings/pwm/snps,pwm.yaml
>>>>>> new file mode 100644
>>>>>> index 000000000000..594085e5e26f
>>>>>> --- /dev/null
>>>>>> +++ b/Documentation/devicetree/bindings/pwm/snps,pwm.yaml
>>>>>> @@ -0,0 +1,40 @@
>>>>>> +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
>>>>>> +# Copyright (C) 2022 SiFive, Inc.
>>>>>> +%YAML 1.2
>>>>>> +---
>>>>>> +$id: http://devicetree.org/schemas/pwm/snps,pwm.yaml#
>>>>>> +$schema: http://devicetree.org/meta-schemas/core.yaml#
>>>>>> +
>>>>>> +title: Synopsys PWM controller
>>>>>> +
>>>>>> +maintainers:
>>>>>> +  - Ben Dooks <ben.dooks@sifive.com>
>>>>>> +
>>>>>> +allOf:
>>>>>> +  - $ref: pwm.yaml#
>>>>>> +
>>>>>> +properties:
>>>>>> +  compatible:
>>>>>> +    const: snps,pwm
>>>>>
>>>>> This is very generic compatible. I doubt that you cover here all
>>>>> Synopsys PWM designs, past and future. You need a specific compatible.
>>>>
>>>>    From what I can get from the documentation (2.13a) there hasn't been
>>>> a huge external interface change and what has been added is all part
>>>> of synthesis time options.
>>>
>>> But you have some specific version, right? Usually these blocks are
>>> versioned, so you must include it. I would even argue that such generic
>>> compatible should not be used as fallback at all, because it is simply
>>> to generic (PWM is not some model name but common acronym),
>>
>> I suppose dw-apb-timers is the actual document name, but that's already
>> been used for the timer mode in a number of SoCs so probably isn't going
>> to be useful. dw-apb-timers-pwm might be a better prefix if snps,pwm is
>> not going to be acceptable. (Yes, the block can be built as either a
>> PWM or a generic interrupt generating timer at IP generation time)

The first thing I'd like to get sorted is should we rename this to
snps,dw-apb-timers-pwm so we can rename the file and the compatible
that goes with it.

>> As for the version numbers, we could have the -v.vv suffix for these
>> blocks, but the v2.xx log has 22 entries already and only one feature
>> for programming (which is also a configurable one so can't be just
>> enabled by default - it's the 0/100 mode flag in the control registers).
>>
>> I'm not sure what the v1.xx timers had, but I don't have access to this
>> information and we're getting these documents as second-generation so I
>> am not sure if we can get a v1.xx at-all (I suspect this is also going
>> to have a number of revisions and about 1 useful register api change
>> which would be the "new mode" double counter method which we currently
>> rely on having being implicitly enabled by the IP builder (again this
>> feature is still something that can be configured on IP genaration))
> 
> But why would you need v1.xx documentation?

I believe the driver should cover a large part of the v1.xx cores
as well, we just don't have any documentation for these to verify
this.

>>
>> Given the configurability of the core, the version numbers might be
>> usable at some point, but it does seem to be a lot of churn for what
>> currently can be described by one boolean for the 0/100 feature that
>> might-be available. Is there a way of saying the compatible string
>> can be dw-apb-timers-pwm-2.[0-9][0-9][a-z] ?
> 
> I don't understand why. Aren't you documenting here only v2.13a version?

The document as-such should cover everything I have a log for, we've not
had time to test the extension for 0or100% which was introduced in 2.11a
spec. The earliest history I have is 2.02d. I will go and see if I can
find someone who can go look for anything earlier.

As a note, it does look like all the v2.xx cores have the IP version
register in them so we can auto-detect the version from that, at least
for the DT/platform case.

> Best regards,
> Krzysztof