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Subject: Re: [PATCH v4 1/2] dt-bindings: cpufreq: Introduce QCOM CPUFREQ FW
 bindings
To:     Amit Kucheria <amit.kucheria@verdurent.com>
Cc:     Sudeep Holla <sudeep.holla@arm.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux PM list <linux-pm@vger.kernel.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Stephen Boyd <sboyd@kernel.org>,
        Rajendra Nayak <rnayak@codeaurora.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>, Rob Herring <robh@kernel.org>,
        Saravana Kannan <skannan@codeaurora.org>
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From:   Taniya Das <tdas@codeaurora.org>
Message-ID: <32e8f874-a58b-8ba3-7a53-dc89cb34f7d9@codeaurora.org>
Date:   Fri, 15 Jun 2018 23:10:07 +0530
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On 6/15/2018 5:29 PM, Amit Kucheria wrote:
> On Thu, Jun 14, 2018 at 9:24 PM, Taniya Das <tdas@codeaurora.org> 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 ENABLE_OFFSET               0x0
> #define LUT_OFFSET                      0x110
> #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.

> This will allow you add any new registers in the future w/o modifying
> the DT binding and reduce qcom_cpu_resources_init() to a handful of
> lines since you no longer need so many OF string matches, and
> devm_ioremap()s.
> 
> Regards,
> Amit
> 

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