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Subject: Re: [PATCH 2/2] cpufreq: qcom-hw: Add support for QCOM cpufreq HW
 driver
To:     Stephen Boyd <swboyd@chromium.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Viresh Kumar <viresh.kumar@linaro.org>,
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Cc:     Rajendra Nayak <rnayak@codeaurora.org>, devicetree@vger.kernel.org,
        robh@kernel.org, skannan@codeaurora.org,
        linux-arm-msm@vger.kernel.org, amit.kucheria@linaro.org,
        evgreen@google.com
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From:   Taniya Das <tdas@codeaurora.org>
Message-ID: <3aa7b871-9cf9-9626-11fe-b9aa6009b477@codeaurora.org>
Date:   Sun, 11 Nov 2018 18:12:29 +0530
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Hello Stephen,

Thanks for your comments.

On 11/4/2018 9:50 AM, Stephen Boyd wrote:
> Quoting Taniya Das (2018-11-02 20:06:00)
>> Hello Stephen,
>>
>> On 10/18/2018 5:02 AM, Stephen Boyd wrote:
>>> Quoting Taniya Das (2018-10-11 04:36:01)
>>>> --- a/drivers/cpufreq/Kconfig.arm
>>>> +++ b/drivers/cpufreq/Kconfig.arm
>>>> @@ -121,6 +121,17 @@ config ARM_QCOM_CPUFREQ_KRYO
>>>>
>>>>             If in doubt, say N.
>>>>
>>>> +config ARM_QCOM_CPUFREQ_HW
>>>> +       bool "QCOM CPUFreq HW driver"
>>>
>>> Is there any reason this can't be a module?
>>>
>>
>> We do not have any use cases where we need to support it as module.
> 
> Ok, so it could easily be tristate then? Why not allow it?
> 

I have checked other vendors CPUfreq drivers and those too support only 
"bool".

>>
>>>> diff --git a/drivers/cpufreq/qcom-cpufreq-hw.c b/drivers/cpufreq/qcom-cpufreq-hw.c
>>>> new file mode 100644
>>>> index 0000000..fe1c264
>>>> --- /dev/null
>>>> +++ b/drivers/cpufreq/qcom-cpufreq-hw.c
>>>> @@ -0,0 +1,354 @@
>>>> +// SPDX-License-Identifier: GPL-2.0
>>>> +/*
>>>> + * Copyright (c) 2018, The Linux Foundation. All rights reserved.
>>>> + */
> [...]
>>>> +
>>>> +static const u16 cpufreq_qcom_std_offsets[REG_ARRAY_SIZE] = {
>>>
>>> Is this going to change in the future?
>>>
>>
>> Yes, they could change and that was the reason to introduce the offsets.
>> This was discussed earlier too with Sudeep and was to add them.
>>
>>>> +       [REG_ENABLE]            = 0x0,
> 
> This is only used once? Maybe it could be removed.
> 
>>>> +       [REG_LUT_TABLE]         = 0x110,
> 
> And this is only used during probe to figure out the supported
> frequencies. So we definitely don't need to store around the registers
> after probe in an array of iomem pointers. The only one that we need
> after probe is the one below.
> 
>>>> +       [REG_PERF_STATE]        = 0x920,
>>>> +};
>>>> +

As these address offsets could change, so I am of the opinion to leave 
them as it is.

>>>> +static struct cpufreq_qcom *qcom_freq_domain_map[NR_CPUS];
>>>> +
>>>> +static int
>>>> +qcom_cpufreq_hw_target_index(struct cpufreq_policy *policy,
>>>> +                            unsigned int index)
>>>> +{
>>>> +       struct cpufreq_qcom *c = policy->driver_data;
>>>> +
>>>> +       writel_relaxed(index, c->reg_bases[REG_PERF_STATE]);
>>>
>>> Why can't we avoid the indirection here and store the perf_state pointer
>>> in probe? Then we don't have to indirect through a table to perform the
>>> register write.
>>>
>>
>> As the offsets could change and that was the reason to add this.
> 
> With fast switching we can avoid incurring any extra instructions, so
> please make another iomem pointer in the cpufreq_qcom struct just for
> writing the index or if possible, just pass the iomem pointer that
> points to the REG_PERF_STATE as the policy->driver_data variable here.
> Then we have the address in hand without any extra load. If my
> understanding is correct, we don't need to keep around anything besides
> this register address anyway so we should be able to just load it and
> write it immediately.
>

The c->reg_bases[] is just an index to the updated bases addresses. I am 
not clear as to why it would incur an extra instruction.

The below code would already take care of it.

+	for (i = REG_ENABLE; i < REG_ARRAY_SIZE; i++)
+		c->reg_bases[i] = base + offsets[i];
+

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