linux-kernel - Re: [PATCH v2 RESEND 1/5] dt-bindings: soc: samsung: Add Exynos USI bindings

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Message-ID: <e80b2fc7-038a-e05e-6ca1-2bfcffda906b@canonical.com>
Date:   Sat, 4 Dec 2021 12:31:08 +0100
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>
To:     Sam Protsenko <semen.protsenko@...aro.org>,
        Rob Herring <robh@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Jaewon Kim <jaewon02.kim@...sung.com>,
        Chanho Park <chanho61.park@...sung.com>,
        David Virag <virag.david003@...il.com>,
        Youngmin Nam <youngmin.nam@...sung.com>,
        devicetree@...r.kernel.org, linux-serial@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-samsung-soc@...r.kernel.org
Subject: Re: [PATCH v2 RESEND 1/5] dt-bindings: soc: samsung: Add Exynos USI
 bindings

On 04/12/2021 01:18, Sam Protsenko wrote:
> On Fri, 3 Dec 2021 at 22:40, Rob Herring <robh@...nel.org> wrote:
>>
>> On Fri, Dec 3, 2021 at 1:36 PM Sam Protsenko <semen.protsenko@...aro.org> wrote:
>>>
>>> On Tue, 30 Nov 2021 at 21:31, Rob Herring <robh@...nel.org> wrote:
>>>>
>>>> On Tue, Nov 30, 2021 at 01:13:21PM +0200, Sam Protsenko wrote:
>>>>> Add constants for choosing USIv2 configuration mode in device tree.
>>>>> Those are further used in USI driver to figure out which value to write
>>>>> into SW_CONF register. Also document USIv2 IP-core bindings.
>>>>>
>>>>> Signed-off-by: Sam Protsenko <semen.protsenko@...aro.org>
>>>>> ---
>>>>> Changes in v2:
>>>>>   - Combined dt-bindings doc and dt-bindings header patches
>>>>>   - Added i2c node to example in bindings doc
>>>>>   - Added mentioning of shared internal circuits
>>>>>   - Added USI_V2_NONE value to bindings header
>>>>>
>>>>>  .../bindings/soc/samsung/exynos-usi.yaml      | 135 ++++++++++++++++++
>>>>>  include/dt-bindings/soc/samsung,exynos-usi.h  |  17 +++
>>>>>  2 files changed, 152 insertions(+)
>>>>>  create mode 100644 Documentation/devicetree/bindings/soc/samsung/exynos-usi.yaml
>>>>>  create mode 100644 include/dt-bindings/soc/samsung,exynos-usi.h
>>>>>
>>>>> diff --git a/Documentation/devicetree/bindings/soc/samsung/exynos-usi.yaml b/Documentation/devicetree/bindings/soc/samsung/exynos-usi.yaml
>>>>> new file mode 100644
>>>>> index 000000000000..a822bc62b3cd
>>>>> --- /dev/null
>>>>> +++ b/Documentation/devicetree/bindings/soc/samsung/exynos-usi.yaml
>>>>> @@ -0,0 +1,135 @@
>>>>> +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
>>>>> +%YAML 1.2
>>>>> +---
>>>>> +$id: http://devicetree.org/schemas/soc/samsung/exynos-usi.yaml#
>>>>> +$schema: http://devicetree.org/meta-schemas/core.yaml#
>>>>> +
>>>>> +title: Samsung's Exynos USI (Universal Serial Interface) binding
>>>>> +
>>>>> +maintainers:
>>>>> +  - Sam Protsenko <semen.protsenko@...aro.org>
>>>>> +  - Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>
>>>>> +
>>>>> +description: |
>>>>> +  USI IP-core provides selectable serial protocol (UART, SPI or High-Speed I2C).
>>>>> +  USI shares almost all internal circuits within each protocol, so only one
>>>>> +  protocol can be chosen at a time. USI is modeled as a node with zero or more
>>>>> +  child nodes, each representing a serial sub-node device. The mode setting
>>>>> +  selects which particular function will be used.
>>>>> +
>>>>> +  Refer to next bindings documentation for information on protocol subnodes that
>>>>> +  can exist under USI node:
>>>>> +
>>>>> +  [1] Documentation/devicetree/bindings/serial/samsung_uart.yaml
>>>>> +  [2] Documentation/devicetree/bindings/i2c/i2c-exynos5.txt
>>>>> +  [3] Documentation/devicetree/bindings/spi/spi-samsung.txt
>>>>> +
>>>>> +properties:
>>>>> +  $nodename:
>>>>> +    pattern: "^usi@[0-9a-f]+$"
>>>>> +
>>>>> +  compatible:
>>>>> +    const: samsung,exynos-usi-v2
>>>>
>>>> Use SoC based compatibles.
>>>>
>>>
>>> In this particular case, I'd really prefer to have it like this. Most
>>> likely we'll only have USIv1 and USIv1 in the end, and I think that
>>> would be more clear to have USI version in compatible, rather than SoC
>>> name. Please let me know if you have a strong opinion on this one --
>>> if so I'll re-send.
>>
>> Fine if you have some evidence the ratio of versions to SoC are much
>> more than 1:1 and the versions correspond to something (IOW, you
>> aren't making them up).
>>
> 
> Yes, it's documented in TRM for different SoCs (USI version 2), and
> there are even dedicated registers where you can read the USI IP-core
> version. Right now we only know about two USI versions: v1 and v2 (can
> be found for example from different published Samsung downstream
> kernels, and from TRMs). So the USI block is standardized and
> versioned.

There is no version register for USIv1 and it does not look at all as
standardized. At least not documented. Just because later Samsung
introduced some logic behind it, it's not a proof it is standardized.
It's not. Standard comes with specification and there is no such.
Description of current implementation is not a specification.

Best regards,
Krzysztof