Date:   Tue, 10 Jan 2023 16:59:57 -0800
From:   William Zhang <william.zhang@...adcom.com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Linux SPI List <linux-spi@...r.kernel.org>,
        Broadcom Kernel List <bcm-kernel-feedback-list@...adcom.com>
Cc:     anand.gore@...adcom.com, tomer.yacoby@...adcom.com,
        dan.beygelman@...adcom.com, joel.peshkin@...adcom.com,
        f.fainelli@...il.com, jonas.gorski@...il.com,
        kursad.oney@...adcom.com, dregan@...l.com,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Mark Brown <broonie@...nel.org>,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 02/16] dt-bindings: spi: Add bcmbca-hsspi controller
 support



On 01/10/2023 12:40 AM, Krzysztof Kozlowski wrote:
> On 09/01/2023 20:13, William Zhang wrote:
>>
>>
>> On 01/09/2023 12:56 AM, Krzysztof Kozlowski wrote:
>>> On 09/01/2023 09:27, William Zhang wrote:
>>>> Hi Krzysztof,
>>>>
>>>> On 01/08/2023 06:51 AM, Krzysztof Kozlowski wrote:
>>>>> On 06/01/2023 21:07, William Zhang wrote:
>>>>>> The new Broadcom Broadband BCMBCA SoCs includes a updated HSSPI
>>>>>> controller. Add a new compatible string and required fields for the new
>>>>>> driver.  Also add myself and Kursad as the maintainers.
>>>>>>
>>>>>> Signed-off-by: William Zhang <william.zhang@...adcom.com>
>>>>>> ---
>>>>>>
>>>>>>     .../bindings/spi/brcm,bcm63xx-hsspi.yaml      | 84 +++++++++++++++++--
>>>>>>     1 file changed, 78 insertions(+), 6 deletions(-)
>>>>>>
>>>>>> diff --git a/Documentation/devicetree/bindings/spi/brcm,bcm63xx-hsspi.yaml b/Documentation/devicetree/bindings/spi/brcm,bcm63xx-hsspi.yaml
>>>>>> index 45f1417b1213..56e69d4a1faf 100644
>>>>>> --- a/Documentation/devicetree/bindings/spi/brcm,bcm63xx-hsspi.yaml
>>>>>> +++ b/Documentation/devicetree/bindings/spi/brcm,bcm63xx-hsspi.yaml
>>>>>> @@ -4,22 +4,51 @@
>>>>>>     $id: http://devicetree.org/schemas/spi/brcm,bcm63xx-hsspi.yaml#
>>>>>>     $schema: http://devicetree.org/meta-schemas/core.yaml#
>>>>>>     
>>>>>> -title: Broadcom BCM6328 High Speed SPI controller
>>>>>> +title: Broadcom Broadband SoC High Speed SPI controller
>>>>>>     
>>>>>>     maintainers:
>>>>>> +
>>>>>
>>>>> Drop blank line.
>>>> will fix in  v2.
>>>>
>>>>>
>>>>>> +  - William Zhang <william.zhang@...adcom.com>
>>>>>> +  - Kursad Oney <kursad.oney@...adcom.com>
>>>>>>       - Jonas Gorski <jonas.gorski@...il.com>
>>>>>
>>>>>>     
>>>>>> +description: |
>>>>>> +  Broadcom Broadband SoC supports High Speed SPI master controller since the
>>>>>> +  early MIPS based chips such as BCM6328 and BCM63268.  This controller was
>>>>>> +  carried over to recent ARM based chips, such as BCM63138, BCM4908 and BCM6858.
>>>>>> +
>>>>>> +  It has a limitation that can not keep the chip select line active between
>>>>>> +  the SPI transfers within the same SPI message. This can terminate the
>>>>>> +  transaction to some SPI devices prematurely. The issue can be worked around by
>>>>>> +  either the controller's prepend mode or using the dummy chip select
>>>>>> +  workaround. This controller uses the compatible string brcm,bcm6328-hsspi.
>>>>>> +
>>>>>> +  The newer SoCs such as BCM6756, BCM4912 and BCM6855 include an updated SPI
>>>>>> +  controller that add the capability to allow the driver to control chip select
>>>>>> +  explicitly. This solves the issue in the old controller. This new controller
>>>>>> +  uses the compatible string brcm,bcmbca-hsspi.
>>>>>> +
>>>>>>     properties:
>>>>>>       compatible:
>>>>>> -    const: brcm,bcm6328-hsspi
>>>>>> +    enum:
>>>>>> +      - brcm,bcm6328-hsspi
>>>>>> +      - brcm,bcmbca-hsspi
>>>>>
>>>>> bca seems quite unspecific. Your description above mentions several
>>>>> model numbers and "bca" is not listed as model. Compatibles cannot be
>>>>> generic.
>>>> "bca" is not model number, rather it is a group (broadband carrier
>>>> access) of chip that share the same spi host controller IP. Agree it is
>>>> not particularly specific but it differentiate from other broadcom spi
>>>> controller ip used by other groups.  We just don't have a specific name
>>>> for this spi host controller but can we treat bcmbca as the ip name?
>>>
>>> No, it is discouraged in such forms. Family or IP block compatibles
>>> should be prepended with a specific compatible. There were many issues
>>> when people insisted on generic or family compatibles...
>>>
>>>> Otherwise we will have to have a compatible string with chip model for
>>>> each SoC even they share the same IP. We already have more than ten of
>>>> SoCs and the list will increase.  I don't see this is a good solution too.
>>>
>>> You will have to do it anyway even with generic fallback, so I don't get
>>> what is here to gain... I also don't get why Broadcom should be here
>>> special, different than others. Why it is not a good solution for
>>> Broadcom SoCs but it is for others?
>>>
>> I saw a few other vendors like these qcom ones:
>>    qcom,spi-qup.yaml
>>        - qcom,spi-qup-v1.1.1 # for 8660, 8960 and 8064
>>        - qcom,spi-qup-v2.1.1 # for 8974 and later
>>        - qcom,spi-qup-v2.2.1 # for 8974 v2 and later
>>    qcom,spi-qup.yaml
>>        const: qcom,geni-spi
> 
> IP block version numbers are allowed when there is clear mapping between
> version and SoCs using it. This is the case for Qualcomm because there
> is such clear mapping documented and available for Qualcomm engineers
> and also some of us (although not public).
> 
>> I guess when individual who only has one particular board/chip and is
>> not aware of the IP family,  it is understandable to use the chip
>> specific compatible string.
> 
> Family of devices is not a versioned IP block.
> 
>> But when company works on it, we have the
>> visibility and access to all the chips and ip blocks in the family and
>> IMHO it is very reasonable to use the IP family name for the same IP as
>> these examples shows.
> 
> No, because family of devices is not a versioned IP block. I wrote
> before that families and wildcards are not allowed.
> 
>> Are you saying these are not good example to
>> follow?
> 
> It's nothing related to your case.
> 
>> What are the issues with generic or family compatibles?
>>   Could
>> you please elaborate?
> 
> They stop matching and some point and cause ABI breaks. We had several
> cases where engineer believed "I have here family of devices" and then
> later it turned out that the family is different.
> 
> 
>>
>>>
>>>
>>>>
>>>>>
>>>>>>     
>>>>>>       reg:
>>>>>> -    maxItems: 1
>>>>>> +    items:
>>>>>> +      - description: main registers
>>>>>> +      - description: miscellaneous control registers
>>>>>> +    minItems: 1
>>>>>> +
>>>>>> +  reg-names:
>>>>>> +    items:
>>>>>> +      - const: hsspi
>>>>>> +      - const: spim-ctrl
>>>>>
>>>>> This does not match reg
>>>> Do you mean it does not match the description?
>>>
>>> No. reg can be 1 item but you state reg-names cannot. These are always
>>> the same. If one is 1 item, second is as well.
>>>
>> I'll drop the "minItems: 1" from the reg property then.
> 
> Then it won't be correct, because it would mean two items are required
> always.
Ah you are right. Add minItems: 1 for reg-name then.
> 
>>
>>>>>
>>>>>>     
>>>>>>       clocks:
>>>>>>         items:
>>>>>> -      - description: spi master reference clock
>>>>>> -      - description: spi master pll clock
>>>>>> +      - description: SPI master reference clock
>>>>>> +      - description: SPI master pll clock
>>>>>
>>>>> Really? You just added it in previous patch, didn't you?
>>>> The previous patch was just word to word conversion of the text file.  I
>>>> will update that patch to include this change.
>>>>
>>>>>
>>>>>>     
>>>>>>       clock-names:
>>>>>>         items:
>>>>>> @@ -29,12 +58,43 @@ properties:
>>>>>>       interrupts:
>>>>>>         maxItems: 1
>>>>>>     
>>>>>> +  brcm,use-cs-workaround:
>>>>>> +    $ref: /schemas/types.yaml#/definitions/flag
>>>>>> +    description: |
>>>>>> +      Enable dummy chip select workaround for SPI transfers that can not be
>>>>>> +      supported by the default controller's prepend mode, i.e. delay or cs
>>>>>> +      change needed between SPI transfers.
>>>>>
>>>>> You need to describe what is the workaround.
>>>> Will do.
>>>>>
>>>>>> +
>>>>>>     required:
>>>>>>       - compatible
>>>>>>       - reg
>>>>>>       - clocks
>>>>>>       - clock-names
>>>>>> -  - interrupts
>>>>>> +
>>>>>> +allOf:
>>>>>> +  - $ref: "spi-controller.yaml#"
>>>>>
>>>>> No quotes. How this is related to this patch?
>>>> Will remove quote and put it in patch 1.
>>>>>
>>>>>> +  - if:
>>>>>> +      properties:
>>>>>> +        compatible:
>>>>>> +          contains:
>>>>>> +            enum:
>>>>>> +              - brcm,bcm6328-hsspi
>>>>>> +    then:
>>>>>> +      properties:
>>>>>> +        reg:
>>>>>> +          minItems: 1
>>>>>
>>>>> Drop.
>>>>>
>>>>> reg-names now do not match.
>>>> Don't quite understand your comment. What do I need to drop and what is
>>>> not matched?
>>>
>>> You need to add constraints for reg-names, same way as for reg.
>>> Disallowing the reg-names also could work, but there won't be benefit in
>>> it. Better to have uniform DTS.
>>>
>> I agree it is better to have the uniform DTS but the situation here is
>> that the brcm,bcm6328-hsspi does not require reg name since there is
>> only one register needed and it was already used in many chip dts for
>> long time.  If I enforce it to have the corresponding reg name, that
> 
> No one told you to enforce to have a reg-names.
> 
>> could potentially break the compatibility of those old device if the
>> driver change to use reg name, right?
> 
> How compatibility is broken by some optional, unrelated property?
> 
I think I misunderstand what you said.  You basically want the reg-name 
minItem/maxItem constraints for brcm,bcm6328-hsspi compatible as well so 
it is consistent for all the compatibles? I was confused and thought it 
is not needed as reg-name is not required for brcm,bcm6328-hsspi compatible.

> Best regards,
> Krzysztof
> 

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