linux-kernel - Re: [PATCH 1/4] dt-bindings: reset: Add binding for Sophgo CV1800B reset controller

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Message-ID: <42ad3f60-2755-4d3b-a766-8e4404f76a7c@linaro.org>
Date:   Wed, 15 Nov 2023 22:00:17 +0100
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Jisheng Zhang <jszhang@...nel.org>, Conor Dooley <conor@...nel.org>
Cc:     Samuel Holland <samuel.holland@...ive.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Chao Wei <chao.wei@...hgo.com>,
        Chen Wang <unicorn_wang@...look.com>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-riscv@...ts.infradead.org
Subject: Re: [PATCH 1/4] dt-bindings: reset: Add binding for Sophgo CV1800B
 reset controller

On 15/11/2023 16:15, Jisheng Zhang wrote:
> On Wed, Nov 15, 2023 at 03:02:21PM +0000, Conor Dooley wrote:
>> On Wed, Nov 15, 2023 at 09:56:07AM -0500, Samuel Holland wrote:
>>> On 2023-11-15 7:27 AM, Jisheng Zhang wrote:
>>>> On Tue, Nov 14, 2023 at 10:12:35PM +0100, Krzysztof Kozlowski wrote:
>>>>> On 13/11/2023 01:55, Jisheng Zhang wrote:
>>>>> ...
>>>>>
>>>>>> diff --git a/include/dt-bindings/reset/sophgo,cv1800b-reset.h b/include/dt-bindings/reset/sophgo,cv1800b-reset.h
>>>>>> new file mode 100644
>>>>>> index 000000000000..28dda71369b4
>>>>>> --- /dev/null
>>>>>> +++ b/include/dt-bindings/reset/sophgo,cv1800b-reset.h
>>>>>> @@ -0,0 +1,96 @@
>>>>>> +/* SPDX-License-Identifier: GPL-2.0 OR MIT */
>>>>>> +/*
>>>>>> + * Copyright (C) 2023 Sophgo Technology Inc. All rights reserved.
>>>>>> + * Copyright (C) 2023 Jisheng Zhang <jszhang@...nel.org>
>>>>>> + */
>>>>>> +
>>>>>> +#ifndef _DT_BINDINGS_CV1800B_RESET_H
>>>>>> +#define _DT_BINDINGS_CV1800B_RESET_H
>>>>>> +
>>>>>> +/*				0-1	*/
>>>>>> +#define RST_DDR			2
>>>>>> +#define RST_H264C		3
>>>>>> +#define RST_JPEG		4
>>>>>> +#define RST_H265C		5
>>>>>> +#define RST_VIPSYS		6
>>>>>> +#define RST_TDMA		7
>>>>>> +#define RST_TPU			8
>>>>>> +#define RST_TPUSYS		9
>>>>>> +/*				10	*/
>>>>>
>>>>> Why do you have empty IDs? IDs start at 0 and are incremented by 1.
>>>>
>>>> there's 1:1 mapping between the ID and bit. Some bits are reserved, I.E
>>>> no actions at all. Is "ID start at 0 and increment by 1" documented
>>>> in some docs? From another side, I also notice some SoCs especially
>>>> those which make use of reset-simple driver don't strictly follow
>>>> this rule, for example, amlogic,meson-a1-reset.h and so on. What
>>>> happened?
>>>>
>>>> And I'd like to ask a question here before cooking 2nd version:
>>>> if the HW programming logic is the same as reset-simple, but some
>>>> or many bits are reserved, what's the can-be-accepted way to support
>>>> the reset controller? Use reset-simple? Obviously if we want the
>>>> "ID start at 0 and increment by 1" rule, then we have to write
>>>> a custom driver which almost use the reset-simple but with a
>>>> customized mapping.
>>>
>>> There are two possible situations. Either the reset specifier maps directly to
>>> something in the hardware, or you are inventing some brand new enumeration to
>>> use as a specifier.
>>>
>>> In the first situation, you do not need a header. We assume the user will look
>>> to the SoC documentation if they want to know what the numbers mean. (You aren't
>>> _creating_ an ABI, since the ABI is already defined by the hardware.)
>>>
>>> In the second situation, since we are inventing something new, the numbers
>>> should be contiguous. This is what Krzysztof's comment was about.
>>>
>>> For this reset device, the numbers are hardware bit offsets, so you are in the
>>> first situation. So I think the recommended solution here is to remove the
>>> header entirely and use the bit numbers directly in the SoC devicetree.
>>>
>>> It's still appropriate to use reset-simple. Adding some new mapping would make
>>> things more complicated for no benefit.
>>
>> Further, I think it is fine in that case to have a header, just the
>> header doesn't belong as a binding, and can instead go in the dts
>> directory.
> 
> Hi Samuel, Conor,
> 
> thanks a lot for the suggestion!

There is actually a thing here I missed. If this is a reset-simple
driver with dedicated SoC-specific compatible, you could want to have a
binding header to have IDs. This way later you can easily replace the
driver with another implementation, which does no rely on 1-1 mapping to
reset bits.

Therefore the reset-simple could be the exception where reset-bits could
have a meaning as binding header.

Best regards,
Krzysztof