Re: [PATCH V2 3/6] clk: qcom: clk-alpha-pll: Remove explicit CAL_L configuration for EVO PLL

[Jagadeesh Kona <quic_jkona@quicinc.com>]

On 6/14/2023 5:56 PM, Dmitry Baryshkov wrote:
> On Wed, 14 Jun 2023 at 14:53, Jagadeesh Kona <quic_jkona@quicinc.com> wrote:
>>
>>
>>
>> On 6/9/2023 5:55 PM, Dmitry Baryshkov wrote:
>>> On Fri, 9 Jun 2023 at 14:50, Jagadeesh Kona <quic_jkona@quicinc.com> wrote:
>>>>
>>>> Hi Dmitry,
>>>>
>>>> Thanks for your review!
>>>>
>>>> On 6/1/2023 8:13 PM, Dmitry Baryshkov wrote:
>>>>> On 01/06/2023 17:33, Jagadeesh Kona wrote:
>>>>>> Hi Dmitry, Konrad,
>>>>>>
>>>>>> On 5/26/2023 9:23 PM, Dmitry Baryshkov wrote:
>>>>>>> On 26/05/2023 12:33, Konrad Dybcio wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>> On 25.05.2023 19:21, Jagadeesh Kona wrote:
>>>>>>>>> In lucid evo pll, the CAL_L field is part of L value register
>>>>>>>>> itself, and
>>>>>>>>> the l value configuration passed from clock controller driver includes
>>>>>>>>> CAL_L and L values as well. Hence remove explicit configuration of
>>>>>>>>> CAL_L
>>>>>>>>> for evo pll.
>>>>>>>>>
>>>>>>>>> Fixes: 260e36606a03 ("clk: qcom: clk-alpha-pll: add Lucid EVO PLL
>>>>>>>>> configuration interfaces")
>>>>>>>>> Signed-off-by: Taniya Das <quic_tdas@quicinc.com>
>>>>>>>>> Signed-off-by: Jagadeesh Kona <quic_jkona@quicinc.com>
>>>>>>>>> ---
>>>>>>>> Oh that isn't obvious at first sight, nice find!
>>>>>>>>
>>>>>>>> I'd suggest a different solution though:
>>>>>>>>
>>>>>>>> #define LUCID_EVO_PLL_L_LVAL    GENMASK(..
>>>>>>>> #define LUCID_EVO_PLL_L_CAL_L    GENMASK(..
>>>>>>>>
>>>>>>>> lval = FIELD_PREP(LUCID_EVO_PLL_L_LVAL, config->l) |
>>>>>>>>           FIELD_PREP(LUCID_EVO_PLL_L_CAL_L, config->cal_l);
>>>>>>>>
>>>>>>>> This would make the separation between the two parts more explicit
>>>>>>>>
>>>>>>>> however
>>>>>>>>
>>>>>>>> config->l would then represent the L value and not the end value
>>>>>>>> written to the L register
>>>>>>>
>>>>>>> Yes. I think there should be separate config->l and config->cal_l
>>>>>>> values (and probably ringosc_cal_l, basing on the comment in the
>>>>>>> source).
>>>>>>> Thanks for your suggestions. In all recent chipsets, L & CAL_L fields
>>>>>> are encapsulated in the same register, so we feel it is better to
>>>>>> directly pass the combined configuration value in config->l itself and
>>>>>> program it directly into register without any additional handling
>>>>>> required in pll driver code.
>>>>>
>>>>> My feeling is that it is better to split it, since these are the
>>>>> different fields. The value .l = 0x4444003e doesn't mean anything per se.
>>>>>
>>>>> Three values are much more meaningful:
>>>>> .l = 0x3e,
>>>>> .cal_l = 0x44,
>>>>> .ringosc_cal_l = 0x44,
>>>>>
>>>>> Not to mention that this way you don't have to touch pll configuration
>>>>> for the existing Lucid EVO PLL. Not to mention that for the Lucid ole
>>>>> PLLs the cal_l and ringosc_cal_l values seem to be static (0x44), so
>>>>> there is no need to put them to the variable data.
>>>>>
>>>>
>>>> Sure, will keep the existing code as is and will remove this patch in
>>>> the next series.
>>>>
>>>>>>
>>>>>> Also the evo pll code is currently reused for both lucid evo and ole
>>>>>> pll's. Lucid ole PLL has an additional RINGOSC_CAL_L field along with
>>>>>> L, CAL_L fields in the same L register. By passing combined
>>>>>> configuration value in config->l itself, we feel we can avoid all the
>>>>>> additional handling required in PLL code.
>>>>>>
>>>>>>> Just a question: is camcc-sm8550 using the same PLL type or is it
>>>>>>> some kind of subtype of lucid_evo PLL?
>>>>>>>
>>>>>> No, it is not the same lucid evo PLL. It uses lucid ole PLL.
>>>>>
>>>>> Then please don't reuse the clk_lucid_evo_pll_configure() call.
>>>>> You can add a new one, which will handle L/CAL_L/RINGOSC_CAL_L differences.
>>>>>
>>>>
>>>> The only difference between evo and ole pll configure is extra
>>>> RINGOSC_CAL_L programming needed only for ole pll. We can achieve the
>>>> same with clk_lucid_evo_pll_configure() itself by directly including
>>>> RINGOSC_CAL_L field in L configuration for OLE PLL's.
>>>
>>> Please don't, that's all I can say. Those are different fields. By
>>> looking at the config->l one can calculate PLL rate. If you overload
>>> the config->l with CAL_L and RINGOSC_CAL_L, the purpose of this field
>>> is gone.
>>>
>>> As the CAL_L and RINGOSC_CAL_L fields are static, just move them to
>>> the clk_lucid_ole_pll_configure().
>>>
>>
>> We feel it is better to include them in config->l and reuse existing
>> code than adding separate function for lucid ole pll configure. Even the
>> other pll configurations(like .user_ctl_val) contains multiple fields
>> but are passed as a single value from driver.
> 
> I suppose it was done this way because these fields are pretty much
> not documented in the openly published data. And sometimes this
> strikes, one can not easily check PLL's configuration. Or tune
> it.There was a discussion whether we should start handling PLL outputs
> properly (in CCF) rather than configuring them in a static way.
> 
> Also mentioned registers differ from PLL to PLL. For the RISCOSC_CAL_L
> and CAL_L the value is static, if I'm not mistaken. Having them in the
> configurable field doesn't sound correct.
> 
> Last, but not least. We are already handling CAL_L value completely in
> the clock-alpha-pll.c for triton, lucid and lucid evo PLLs. What would
> be the _reason_ to change that?
> 

Yes, will follow the approach similar to other existing PLL's and will 
add a separate function for clk_lucid_ole_pll_configure() in next series.

Thanks,
Jagadeesh

>>
>> We also added a comment in code stating all the fields included in
>> config->l value, so user will be aware while calculating PLL frequency.
>>
>> Thanks,
>> Jagadeesh
> 
> 
>