Re: [PATCH v4 1/2] dt-bindings: cpufreq: Introduce QCOM CPUFREQ FW bindings

From: Sudeep Holla
Date: Fri Jun 15 2018 - 13:45:24 EST

On 15/06/18 18:40, Taniya Das wrote:
> On 6/15/2018 5:29 PM, Amit Kucheria wrote:
>> On Thu, Jun 14, 2018 at 9:24 PM, Taniya Das <tdas@xxxxxxxxxxxxxx> wrote:
>>>>> Sorry Sudeep I missed replying to your earlier query.
>>>>> The High level OS(HLOS) would require to access only these specific
>>>>> registers from this IP block and just mapping the whole block(huge
>>>>> region) is unnecessary from the OS point of View. As of now it is a
>>>>> generic binding for all using this IP block to manage frequency
>>>>> requests. The OS would only have to know the frequencies supported i.e
>>>>> to read the lookup table registers and put across the OS request using
>>>>> the performance state register.
>>>> I am not sure if you need to defining bindings to save OSPM IO mapping.
>>>> In-fact you may be adding more mapping unnecessarily. The mappings are
>>>> page aligned and spiting the registers and mapping them individually
>>>> may
>>>> result in more mappings.
>>>> I just need to know the rational for such specific choice of registers.
>>>> I assume it's aligned to some other standard specifications like CPPC
>>>> though not identical.
>>> I am not sure of the query but there is no other register that the OS is
>>> required to use other than the ones defined here.
>>>>>> Eg. Suppose you need some information on power curve for EAS energy
>>>>>> model, I really hate to update DT for that or even do a mix with
>>>>>> DT just
>>>>>> because f/w is no longer modifiable.
>>>>> For now we are safe.
>>>> What do you mean by that ?
>>> I meant here was currently there is no such known case where the f/w
>>> is no
>>> longer modifiable and we need to extend device tree bindings.
>>>> It should be easily extensible is what I am
>>>> trying to say. You can add more info and alter the information in the
>>>> driver with compatibles if you keep the register info as minimum as
>>>> possible. For now, you have enable, set and lut registers. What if you
>>>> want to provide power numbers ?
>>> Yes I do understand the intent of mapping the whole register space,
>>> but as
>>> per the HW specs these 3 registers would be the only ones required
>>> for now.
>>> I do not think this hardware engine has any information on the power
>>> numbers.
>> "For now" - I think this is exactly the point that Sudeep is trying to
>> make.
>> A future version of the HW engine, or more likely, a firmware
>> revision, will make more functionality available. Say, this needs
>> access to another register or two. This will require changing the DT
>> bindings. Instead, if you map the entire address space, you can just
>> add offsets to the new registers.
>> So in this case, I think you should define the following addresses
>> (size 0x1400) for the two frequency domains
>> 0x17d43000, 0x1400 (power cluster)
>> 0x17d45800, 0x1400 (perf cluster)
>> And in the driver simply add offsets as follows:
>> #define PERF_DESIRED_OFFSET 0x920
> The offsets could vary across versions of this IP and that is the reason
> to provide them through the DT and not define any such offsets.

How many versions do you have and how much has it varied already ?
I am now getting a sense that it's mostly decided and fixed my the
firmware rather than at the time of hardware design.