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

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Date:   Wed, 15 Nov 2023 15:02:21 +0000
From:   Conor Dooley <conor@...nel.org>
To:     Samuel Holland <samuel.holland@...ive.com>
Cc:     Jisheng Zhang <jszhang@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        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 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.

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