Date:   Thu, 28 Jul 2022 09:57:02 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Ben Dooks <ben.dooks@...ive.com>, linux-pwm@...r.kernel.org
Cc:     devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        Lee Jones <lee.jones@...aro.org>,
        u.kleine-koenig@...gutronix.de,
        Thierry Reding <thierry.reding@...il.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Greentime Hu <greentime.hu@...ive.com>
Subject: Re: [[PATCH v2] 1/9] dt-bindings: pwm: Document Synopsys DesignWare
 snps,pwm

On 27/07/2022 15:21, Ben Dooks wrote:
> 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@...ive.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@...ive.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.

I don't have the datasheets/spec/manual for this, so I have no clue what
is it. I know though that calling it generic "pwm" is a bit too generic.

For example "snps,dw-apb-timer" is not called "snps,timer" but
DesignWare APB Timer.

>>> 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.

Yeah, but I still don't understand what is the problem to solve in
bindings for that.

>>> 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.

Several of them might be actually compatible, so you might not need 100
different compatibles. Patterns are not allowed.

I doubt that PWM block is much more complicated than for example DW MAC,
which somehow can exist with few versions defined...

> 
> 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.

Auto-detection is then preferred, so just call it -v2.02d which will
cover all known v2 for you and the rest is done via autodetection.

Best regards,
Krzysztof

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