linux-kernel - Re: [PATCH 5/7] cpufreq: qcom-hw: Use regmap for accessing hardware registers

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Date:   Tue, 8 Sep 2020 17:18:35 +0530
From:   Amit Kucheria <amitk@...nel.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Rob Herring <robh+dt@...nel.org>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Linux PM list <linux-pm@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Taniya Das <tdas@...eaurora.org>
Subject: Re: [PATCH 5/7] cpufreq: qcom-hw: Use regmap for accessing hardware registers

On Tue, Sep 8, 2020 at 4:48 PM Viresh Kumar <viresh.kumar@...aro.org> wrote:
>
> On 08-09-20, 16:41, Manivannan Sadhasivam wrote:
> > On 0908, Viresh Kumar wrote:
> > > On 08-09-20, 13:27, Manivannan Sadhasivam wrote:
> > > > Use regmap for accessing cpufreq registers in hardware.
> > >
> > > Why ? Please mention why a change is required in the log.
> > >
> >
> > Only because it is recommended to use regmap for abstracting the hw access.
>
> Yes it can be very useful in abstracting the hw access in case of
> busses like SPI/I2C, others, but in this case there is only one way of
> doing it with the exact same registers. I am not sure it is worth it
> here. FWIW, I have never played with regmaps personally, and so every
> chance I can be wrong here.

One could handle the reg offsets through a struct initialisation, but
then you end up with lots of #defines for bitmasks and bits for each
version of the IP. And the core code becomes a bit convoluted IMO,
trying to handle the differences.

regmap hides the differences of the bit positions and register offsets
between several IP versions.

> > Moreover it handles the proper locking for us in the core (spinlock vs mutex).
>
> What locking do you need here ?

Right, locking isn't the main reason here.

>
> > I've seen many subsystem maintainers prefer regmap over plain readl/writel
> > calls. I'll add the reason in commit log.
>
> I am not sure if it is worth it here.