Re: [PATCH v4 6/7] dt-bindings: phy-qcom-usb2: Add support to override tuning values

[Manu Gautam <mgautam@codeaurora.org>]

On 4/10/2018 1:48 AM, Rob Herring wrote:
> On Thu, Mar 29, 2018 at 01:38:23PM -0700, Doug Anderson wrote:
>> Hi,
>>
>> On Thu, Mar 29, 2018 at 4:04 AM, Manu Gautam <mgautam@codeaurora.org> wrote:
>>> To improve eye diagram for PHYs on different boards of same SOC,
>>> some parameters may need to be changed. Provide device tree
>>> properties to override these from board specific device tree
>>> files. While at it, replace "qcom,qusb2-v2-phy" with compatible
>>> string for USB2 PHY on sdm845 which was earlier added for
>>> sdm845 only.
>>>
>>> Signed-off-by: Manu Gautam <mgautam@codeaurora.org>
>>> ---
>>>  .../devicetree/bindings/phy/qcom-qusb2-phy.txt        | 19 ++++++++++++++++++-
>>>  1 file changed, 18 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/Documentation/devicetree/bindings/phy/qcom-qusb2-phy.txt b/Documentation/devicetree/bindings/phy/qcom-qusb2-phy.txt
>>> index 42c9742..0ed140a 100644
>>> --- a/Documentation/devicetree/bindings/phy/qcom-qusb2-phy.txt
>>> +++ b/Documentation/devicetree/bindings/phy/qcom-qusb2-phy.txt
>>> @@ -6,7 +6,7 @@ QUSB2 controller supports LS/FS/HS usb connectivity on Qualcomm chipsets.
>>>  Required properties:
>>>   - compatible: compatible list, contains
>>>                "qcom,msm8996-qusb2-phy" for 14nm PHY on msm8996,
>>> -              "qcom,qusb2-v2-phy" for QUSB2 V2 PHY.
>>> +              "qcom,sdm845-qusb2-phy" for 10nm PHY on sdm845.
>>>
>>>   - reg: offset and length of the PHY register set.
>>>   - #phy-cells: must be 0.
>>> @@ -27,6 +27,23 @@ Optional properties:
>>>                 tuning parameter value for qusb2 phy.
>>>
>>>   - qcom,tcsr-syscon: Phandle to TCSR syscon register region.
>> Just to confirm: the new properties below work just fine on the old
>> msm8996 PHY too, right?
No it won't as register layouts and bit fields are different. Also IMP_CTRL
register doesnt exist for msm8996. I will mention that explicitly here.

>>
>>
>>> + - qcom,imp-res-offset-value: It is a 6 bit value that specifies offset to be
>>> +               added to PHY refgen RESCODE via IMP_CTRL1 register. It is a PHY
>>> +               tuning paramter that may vary for different boards of same SOC.
>>> + - qcom,hstx-trim-value: It is a 4 bit value that specifies tuning for HSTX
>>> +               output current. 0x0 value corresponding to 24mA which is maximum
>>> +               current and 0xf corresponds to lowest current which is 15mA.
>>> + - qcom,preemphasis-level: It is a 2 bit value that specifies pre-emphasis level.
>>> +               Possible values are:
>>> +               00: NONE
>>> +               01: +5%
>>> +               10: +10%
>>> +               11: +15%
>> The user of the device tree will expect to specify this in decimal,
>> right?  So list the above as 0, 1, 2, 3.  ...not 00, 01, 10, 11.
Fine.
>> (though below I suggest that specifying 0, 1, 2, 3 is probably not
>> quite the right way to describe this property).
>>
>>
>>> +- qcom,preemphasis-width: It is a 1 bit value that specifies how long the HSTX
>>> +               pre-emphasis (specified using qcom,preemphasis-level) must be in
>>> +               effect. Possible values are:
>>> +               0: Full-bit width
>>> +               1: Half-bit width
>> Perhaps just make this a boolean property.  If it exists then you get
>> the non-default case.  AKA: if the default is full bit width, then
>> you'd allow a boolean property "qcom,preemphasis-half-width" to
>> override.  If the default is half bit width then you'd allow
>> "qcom,preemphasis-full-width" to override.

Default property value for an SOC is specified in driver and could vary from
soc to soc. Hence, from board devicetree for different SOCs we might need
to select separate widths overriding default driver values.
Alternative is to have two bool properties each for half and full-width. Did
you actually mean that?

>>
>>
>> For all the above optional bits, please indicate what the default is
>> if they aren't specified.  If you have to specify a different default
>> for sdm845 vs. msm8996 then so be it (though it would be nice to avoid
>> it by changing the default for sdm845 unless that's totally crazy).
Sure.
>>
>>
>> Overall this looks pretty good to me.  Certainly the descriptions are
>> very understandable and this will make tuning on a per-board /
>> per-port basis much easier!  Thanks!
>>
>>
>> One last overall comment is that ideally we could make the device tree
>> a little bit more human readable.  Right now we'll have a set of
>> properties that looks like this (numbers made up):
>>
>> qcom,imp-res-offset-value = <0x13>;
>> qcom,hstx-trim-value = <0x7>;
>> qcom,preemphasis-level = <1>;
>> qcom,preemphasis-width = <0>;
>>
>>
>> One option to make that more readable would be to change the units, AKA:
>>
>> qcom,hstx-trim-ma = <18>;
>> qcom,preemphasis-percent = <5>;
>>
>> ...then the code would translate from these human-readable values to
>> the real numbers.
> Yes, we often do that. However, there properties are very specific to 
> this device and making the driver translate back to h/w values doesn't 
> add much.

Thanks for review Rob. I too agree with both the viewpoints.
Doug, if it is not of much concern then can I stick with current approach?

>
>> Another option would be to add a #include to the bindings.  I'd defer
>> to the wisdom of the bindings guys about if this is better or worse
>> than adding the units, but personally I like it better because:
>> * You get a compile-time error if you use an unsupported value.
>> * You don't need to add the code to translate.
> This is fine for me. Really, I'm fine with it as-is, but #defines make 
> some people happy.
>
> Rob

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