Re: [PATCH 1/4] dt-bindings: input: Add binding for Qualcomm SPMI PMIC haptics

[Krzysztof Kozlowski <krzk@kernel.org>]

On 22/06/2026 04:28, Fenglin Wu wrote:
> 
> On 6/19/2026 12:18 PM, Krzysztof Kozlowski wrote:
>> On 17/06/2026 13:02, Fenglin Wu wrote:
>>> On 6/17/2026 6:35 PM, Krzysztof Kozlowski wrote:
>>>> On Tue, Jun 16, 2026 at 03:08:24AM -0700, Fenglin Wu wrote:
>>>>> ....
>>>>> +
>>>>> +  qcom,lra-period-us:
>>>>> +    description:
>>>>> +      LRA actuator initial resonance period in microseconds
>>>>> +      (1,000,000 / resonant_freq_hz).  Used to configure T_LRA-based play
>>>>> +      rates and the auto-resonance zero-crossing window.
>>>> This does not feel like static characteristic. Isn't period depending on
>>>> intensity of vibration you want to have? Why would that be fixed per
>>>> board?
>>> This period is specifically used for playbacks that require
>>> auto-resonance to be enabled, which I referred to as "T_LRA-based" and
>>> "auto-resonance zero-crossing window." It plays a key role in the
>>> "DIRECT_PLAY" mode, which produces a constant vibration effect. To
>>> adjust the vibration intensity during this constant effect, the hardware
>>> does it by scaling the peak voltage of the driver signals, rather than
>>> changing the frequency.
>> But maybe changing frequency runtime still would be useful?
> It could be, but the LRA F0 (resonant frequency) still needs to be the 
> starting point. You can control vibration intensity by driving the LRA 
> slightly off resonance by a given percentage—for example, to reach 50% 
> vibration, you could probably drive it 10% off resonant frequency, and 
> that mapping also depends on the LRA characteristic. Keep in mind that 
> LRA is a spring-mass resonant system, so its output is not linear with 
> driving frequency; it is a High_Q system, and its output actually shows 
> a sharp peak at the resonance point. By contrast, the relationship 
> between driving voltage and its output is much more linear, so scaling 
> vibration intensity by adjusting the driving voltage is easier to 
> control. Qcom haptics HW scales vibration intensity in DIRECT_PLAY mode 
> (for constant vibration effect) by scaling the driving voltage instead. 
> That said, the HW can also change the driving waveform frequency by 
> updating the T-LRA registers, and this property has to be specified as 
> an initial value; otherwise, you won't have a baseline to achieve that.
> 

OK, property is fine.

Best regards,
Krzysztof