linux-kernel - Re: [v4 1/3] dt-bindings: phy: qcom,usb-snps-femto-v2: Add phy override params bindings

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Date:   Sat, 14 May 2022 21:41:42 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Krishna Kurapati PSSNV <quic_kriskura@...cinc.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Doug Anderson <dianders@...omium.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        Wesley Cheng <quic_wcheng@...cinc.com>,
        Vinod Koul <vkoul@...nel.org>
Cc:     devicetree@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-phy@...ts.infradead.org, quic_pkondeti@...cinc.com,
        quic_ppratap@...cinc.com, quic_vpulyala@...cinc.com
Subject: Re: [v4 1/3] dt-bindings: phy: qcom,usb-snps-femto-v2: Add phy
 override params bindings

On 14/05/2022 08:24, Krishna Kurapati PSSNV wrote:
> 
> On 5/12/2022 4:00 PM, Krzysztof Kozlowski wrote:
>> On 12/05/2022 07:57, Krishna Kurapati PSSNV wrote:
>>> On 5/11/2022 11:49 PM, Krzysztof Kozlowski wrote:
>>>> On 11/05/2022 17:26, Krishna Kurapati wrote:
>>>>> From: Sandeep Maheswaram <quic_c_sanm@...cinc.com>
>>>>>
>>>>> Add device tree bindings for SNPS phy tuning parameters.
>>>>>
>>>>> Signed-off-by: Sandeep Maheswaram <quic_c_sanm@...cinc.com>
>>>>> Signed-off-by: Krishna Kurapati <quic_kriskura@...cinc.com>
>>>>> ---
>>>>>    .../bindings/phy/qcom,usb-snps-femto-v2.yaml       | 87 ++++++++++++++++++++++
>>>>>    1 file changed, 87 insertions(+)
>>>>>
>>>>> diff --git a/Documentation/devicetree/bindings/phy/qcom,usb-snps-femto-v2.yaml b/Documentation/devicetree/bindings/phy/qcom,usb-snps-femto-v2.yaml
>>>>> index 1ce251d..70efffe 100644
>>>>> --- a/Documentation/devicetree/bindings/phy/qcom,usb-snps-femto-v2.yaml
>>>>> +++ b/Documentation/devicetree/bindings/phy/qcom,usb-snps-femto-v2.yaml
>>>>> @@ -53,6 +53,93 @@ properties:
>>>>>      vdda33-supply:
>>>>>        description: phandle to the regulator 3.3V supply node.
>>>>>    
>>>>> +  qcom,hs-disconnect-bps:
>>>>> +    $ref: /schemas/types.yaml#/definitions/int32
>>>>> +    description:
>>>>> +      This adjusts the voltage level for the threshold used to
>>>>> +      detect a disconnect event at the host. Possible values are.
>>>>> +      The values defined are in multiples of basis points (1bp = 0.01%).
>>>> This means there is some minimum and maximum (100%)?
>>> Hi Krzystof,
>>>
>>> Yes there are max and min for each parameter (not necessarily 0%/100%)
>>>
>>> As an example if we take squelch detector threshold, the register value
>>> vs actual percentage changer as per data book is as follows :
>>>
>>> % change in voltage    |     corresponding reg value
>>>
>>>    -20.90%                        |    7
>>>    -15.60%                        |    6
>>> -10.30%                         |    5
>>> -5.30%                           |    4
>>> 0%                                  |    3
>>> 5.30%                            |    2
>>> 10.60%                          |    1
>>> 15.90%                          |    0
>>>
>>> Here the min and max are 15.9% to -20.9%
>>>
>>> The min and max differ for each parameter and might not be necessarily
>>> 0% and 100%
>> Then it seems possible to define minimum and maximum values - please add
>> them ("minimum: xxxx").
>>
>>
>> Best regards,
>> Krzysztof
> 
> Hi Krzysztof,
> 
>   Sorry for the late reply, missed this mail.
> 
> Currently, these values have a fixed maximum and minimum. But if these 
> limits change in the
> 
> future (say on a per target basis) , would it be appropriate to add them 
> here in bindings file ?

Per "target" you mean compatible? Then yes, the same as customizing
number of interrupts, clocks etc.

> 
> Also in the driver file for sc7280 target, we have added parameter 
> mapping : (map b/w register value
> 
> and bps passed from device tree). For squelch detector, it is as follows:
> 
> +static struct override_param squelch_det_threshold_sc7280[] = {
> +	OVERRIDE_PARAM(-2090, 7),
> +	OVERRIDE_PARAM(-1560, 6),
> +	OVERRIDE_PARAM(-1030, 5),
> +	OVERRIDE_PARAM(-530, 4),
> +	OVERRIDE_PARAM(0, 3),
> +	OVERRIDE_PARAM(530, 2),
> +	OVERRIDE_PARAM(1060, 1),
> +	OVERRIDE_PARAM(1590, 0),
> +};
> 
> And the code is written such that if we give a bps value in dt greater than max value in
> table, we would automatically choose max value. And if we provide bps value lesser than
> minimum value, we would choose the min value.
> 
> So, would it be appropriate to add the min and max in dt-bindings when there is a
> slight chance of them changing in the future ?

The kernel behavior should not matter here, because the bindings are
about the hardware. Therefore if hardware comes with maximum/minimum
values, it would be better to document them here.


Best regards,
Krzysztof