linux-kernel - Re: [PATCH 3/6] clk: mvebu: add CPU clock driver for Armada 7K/8K

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Date:   Wed, 28 Nov 2018 13:58:11 -0800
From:   Stephen Boyd <sboyd@...nel.org>
To:     Gregory CLEMENT <gregory.clement@...tlin.com>
Cc:     Mike Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...eaurora.org>, linux-clk@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        linux-arm-kernel@...ts.infradead.org,
        Jason Cooper <jason@...edaemon.net>,
        Andrew Lunn <andrew@...n.ch>,
        Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>,
        Antoine Tenart <antoine.tenart@...tlin.com>,
        Miquèl Raynal <miquel.raynal@...tlin.com>,
        Maxime Chevallier <maxime.chevallier@...tlin.com>
Subject: Re: [PATCH 3/6] clk: mvebu: add CPU clock driver for Armada 7K/8K

Quoting Gregory CLEMENT (2018-11-15 15:22:59)
>  On mer., oct. 17 2018, Stephen Boyd <sboyd@...nel.org> wrote:
> 
> > Quoting Gregory CLEMENT (2018-09-22 11:17:06)
> >> + * @cluster: Cluster clock controller index
> >> + * @clk_name: Cluster clock controller name
> >> + * @dev : Cluster clock device
> >> + * @hw: HW specific structure of Cluster clock controller
> >> + * @pll_cr_base: CA72MP2 Register base (Device Sample at Reset register)
> >> + */
> >> +struct ap_cpu_clk {
> >> +       int cluster;
> >
> > unsigned?
> 
> I don't expect to have so many clusters that a signed int would be too
> short :)
> 
> Actually I used int to let the compiler choose what ever he wants.
> 
> But if you really want I can use a unsigned int

It's not just about opening up a wider range of values. It's also about
being more explicit with the type by indicating a negative value is
impossible/doesn't make sense for the code. So yes please, change it to
unsigned int.

> 
> >
> >> +       const char *clk_name;
> >> +       struct device *dev;
> >> +       struct clk_hw hw;
> >> +       struct regmap *pll_cr_base;
> >> +};
> >> +
> >> +static struct clk **cluster_clks;
> >> +static struct clk_onecell_data clk_data;
> >
> > Can you give these some better names that are ap_cpu specific? Grepping
> > for clk_data will make lots of hits otherwise because they're so
> > generic.
> 
> OK I will try to be more creative ;)

Thanks.

> >> +                          BIT(AP806_PLL_CR_0_CPU_CLK_RELOAD_RATIO_OFFSET),
> >> +                          BIT(AP806_PLL_CR_0_CPU_CLK_RELOAD_RATIO_OFFSET));
> >> +
> >> +
> >> +       /* 4. Wait for stabilizing CPU Clock */
> >> +       do {
> >> +               regmap_read(clk->pll_cr_base,
> >> +                           AP806_CA72MP2_0_PLL_SR_REG_OFFSET,
> >> +                           &reg);
> >> +       } while (!(reg & BIT(clk->cluster * AP806_CA72MP2_0_PLL_RATIO_STATE)));
> >
> > Use regmap_read_poll_timeout() API here and give some large timeout
> > value so that we don't get stuck waiting here forever?
> 
> Indeed regmap_read_poll_timeout() seems a nice improvement
> 
> >
> >> +
> >> +       /* 5. Clear Reload ratio */
> >
> > I'm not sure we really need these comments. They're just saying what the
> > code is doing, so they don't add much value.
> 
> Actually, these 5 steps directly come from the datasheet.

Ok, still doesn't mean we need to copy the datasheet into code comments
though.

> 
> >
> >> +       regmap_update_bits(clk->pll_cr_base, cpu_ratio_reg,
> >> +                          BIT(AP806_PLL_CR_0_CPU_CLK_RELOAD_RATIO_OFFSET), 0);
> >> +
> >> +       return 0;
> >> +}
> >> +
> >> +
> >> +static long ap_cpu_clk_round_rate(struct clk_hw *hw, unsigned long rate,
> >> +                                unsigned long *parent_rate)
> >> +{
> >> +       int divider = *parent_rate / rate;
> >> +
> >> +       if (divider > APN806_MAX_DIVIDER)
> >> +               divider = APN806_MAX_DIVIDER;
> >
> >       divider = min(divider, APN806_MAX_DIVIDER);
> 
> OK
> >
> >> +
> >> +       return *parent_rate / divider;
> >> +}
> >> +
> >> +static const struct clk_ops ap_cpu_clk_ops = {
> >> +       .recalc_rate    = ap_cpu_clk_recalc_rate,
> >> +       .round_rate     = ap_cpu_clk_round_rate,
> >> +       .set_rate       = ap_cpu_clk_set_rate,
> >> +};
> >> +
> >> +static int ap_cpu_clock_probe(struct platform_device *pdev)
> >> +{
> >> +       int ret, nclusters = 0, cluster_index = 0;
> >> +       struct device *dev = &pdev->dev;
> >> +       struct device_node *dn, *np = dev->of_node;
> >> +       struct ap_cpu_clk *ap_cpu_clk;
> >> +       struct regmap *regmap;
> >> +
> >> +       regmap = syscon_node_to_regmap(np->parent);
> >
> > Can we just call dev_get_remap() on pdev->dev.parent?
> 
> we could do regmap = dev_get_regmap(pdev->dev.parent, NULL); instead of
> this line. But is it really better?

It allows us to ignore 'syscon' in this driver and drop the include of
that header file? 

> 
> >
> >> +       if (IS_ERR(regmap)) {
> >> +               pr_err("cannot get pll_cr_base regmap\n");
> >> +               return PTR_ERR(regmap);
> >> +       }
> >> +
> >> +       /*
> >> +        * AP806 has 4 cpus and DFS for AP806 is controlled per
> >> +        * cluster (2 CPUs per cluster), cpu0 and cpu1 are fixed to
> >> +        * cluster0 while cpu2 and cpu3 are fixed to cluster1 whether
> >> +        * they are enabled or not.  Since cpu0 is the boot cpu, then
> >> +        * cluster0 must exist.  If cpu2 or cpu3 is enabled, cluster1
> >> +        * will exist and the cluster number is 2; otherwise the
> >> +        * cluster number is 1.
> >> +        */
> >> +       nclusters = 1;
> >> +       for_each_node_by_type(dn, "cpu") {
> >
> > Isn't there some sort of for_each_cpu_node() API that can be used
> > here?
> 
> Not when I wrote the code but since v4.20-rc1 we have
> for_each_of_cpu_node(). So I am going to use it now.

Awesome!

> >
> >> +               cluster_index >>= APN806_CLUSTER_NUM_OFFSET;
> >> +
> >> +               /* Initialize once for one cluster */
> >> +               if (cluster_clks[cluster_index])
> >> +                       continue;
> >> +
> >> +               parent = of_clk_get(np, cluster_index);
> >> +               if (IS_ERR(parent)) {
> >> +                       dev_err(dev, "Could not get the clock parent\n");
> >> +                       return -EINVAL;
> >> +               }
> >> +               parent_name =  __clk_get_name(parent);
> >> +               clk_name[12] += cluster_index;
> >> +               ap_cpu_clk[cluster_index].clk_name =
> >> +                       ap_cp_unique_name(dev, np->parent, clk_name);
> >> +               ap_cpu_clk[cluster_index].cluster = cluster_index;
> >> +               ap_cpu_clk[cluster_index].pll_cr_base = regmap;
> >> +               ap_cpu_clk[cluster_index].hw.init = &init;
> >> +               ap_cpu_clk[cluster_index].dev = dev;
> >> +
> >> +               init.name = ap_cpu_clk[cluster_index].clk_name;
> >> +               init.ops = &ap_cpu_clk_ops;
> >> +               init.num_parents = 1;
> >> +               init.parent_names = &parent_name;
> >> +               init.flags = CLK_GET_RATE_NOCACHE;
> >
> > Can you add a comment why we need NOCACHE here? It isn't clear why this
> > is important.
> 
> I think there was the idea that the clock rate could be modified by
> something else that the clk driver, but now I am not sure it is the
> case. I will check it and either add a comment or just remove it.

Ok.