linux-kernel - Re: [PATCH v2 2/9] clk: mstar: msc313 cpupll clk driver

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Date:   Sat, 8 Jan 2022 20:49:49 +0900
From:   Daniel Palmer <daniel@...f.com>
To:     Stephen Boyd <sboyd@...nel.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Michael Turquette <mturquette@...libre.com>,
        Rob Herring <robh+dt@...nel.org>,
        Romain Perier <romain.perier@...il.com>,
        linux-clk <linux-clk@...r.kernel.org>,
        DTML <devicetree@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Willy Tarreau <w@....eu>
Subject: Re: [PATCH v2 2/9] clk: mstar: msc313 cpupll clk driver

Hi Stephen,

Thank you for looking at this for us.

On Sat, 8 Jan 2022 at 10:25, Stephen Boyd <sboyd@...nel.org> wrote:

> > +static void msc313_cpupll_reg_write32(struct msc313_cpupll *cpupll, unsigned int reg, u32 value)
> > +{
> > +       u16 l = value & 0xffff, h = (value >> 16) & 0xffff;
> > +
> > +       iowrite16(l, cpupll->base + reg);
>
> We don't usually see 16-bit accesses but if that's what the hardware
> wants then OK.

This hardware is weird and most of the registers are like this where
they are 32bit spaced but only 16 bits are used in each.
32bit registers are split across 2 16 bit registers spaced 32bits
apart. Writing the two parts has to be in the right order to get the
right result.

> > +       iowrite16(h, cpupll->base + reg + 4);
> > +}
> > +
> > +static void msc313_cpupll_setfreq(struct msc313_cpupll *cpupll, u32 regvalue)
> > +{
> > +       msc313_cpupll_reg_write32(cpupll, REG_LPF_HIGH_BOTTOM, regvalue);
> > +
> > +       iowrite16(0x1, cpupll->base + REG_LPF_MYSTERYONE);
> > +       iowrite16(0x6, cpupll->base + REG_LPF_MYSTERYTWO);
> > +       iowrite16(0x8, cpupll->base + REG_LPF_UPDATE_COUNT);
> > +       iowrite16(BIT(12), cpupll->base + REG_LPF_TRANSITIONCTRL);
> > +
> > +       iowrite16(0, cpupll->base + REG_LPF_TOGGLE);
> > +       iowrite16(1, cpupll->base + REG_LPF_TOGGLE);
> > +
> > +       while (!(ioread16(cpupll->base + REG_LPF_LOCK)))
> > +               cpu_relax();
>
> Any timeout? Can this use the io read timeout APIs?

Good point. I never saw a situation where the lock didn't happen but I
think Willy did when he was poking at it.
I guess if it doesn't lock we should timeout, warn that something
isn't working and return an error.

> > +static long msc313_cpupll_round_rate(struct clk_hw *hw, unsigned long rate,
> > +                                    unsigned long *parent_rate)
> > +{
> > +       u32 reg = msc313_cpupll_regforfrequecy(rate, *parent_rate);
> > +       long rounded = msc313_cpupll_frequencyforreg(reg, *parent_rate);
> > +
> > +       /*
> > +        * This is my poor attempt at making sure the resulting
> > +        * rate doesn't overshoot the requested rate.
>
> If you want better bounds you can use determine_rate and then look at
> the min/max constraints to make sure you don't overshoot. But otherwise
> round_rate implementation is up to the provider to figure out what
> should happen, i.e. overshooting could be OK if the provider intends for
> it.

This clock is basically only used by cpufreq-dt. I'm not sure what it
would do with determine_rate. I'll take a look.
The main thing I wanted to do here was make sure the resulting clock
wasn't higher than what we have in the opp table and end up with the
CPU locking up.

> > +       clk_init.name = dev_name(dev);
> > +       clk_init.ops = &msc313_cpupll_ops;
> > +       clk_init.flags = CLK_IS_CRITICAL;
>
> Why is it critical? Can we have a comment? The clk ops don't have enable
> or disable so it seems like the flag won't do anything.

This clock is critical in the sense that once the DDR memory is setup
by the bootloader you must not turn it off even if you switch the CPU
to the other clock source. If you disable it the system locks up.
I think it can be dropped as does nothing without enable or disable
like you wrote.

Cheers,

Daniel