Re: [PATCH 2/2] cpufreq: qcom-hw: Add support for QCOM cpufreq HW driver

[Taniya Das]

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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