[RFC 6/9] clk: ti: add support for omap4 module clocks

[Geert Uytterhoeven]

Hi Tero,

On Mon, Jan 4, 2016 at 8:36 AM, Tero Kristo <t-kristo at ti.com> wrote:
> On 01/01/2016 07:48 AM, Michael Turquette wrote:
>> Quoting Tero Kristo (2015-12-18 05:58:58)
>>> +static int _omap4_hwmod_clk_enable(struct clk_hw *hw)
>>> +{
>>> +       struct clk_hw_omap *clk = to_clk_hw_omap(hw);
>>> +       u32 val;
>>> +       int timeout = 0;
>>> +       int ret;
>>> +
>>> +       if (!clk->enable_bit)
>>> +               return 0;
>>> +
>>> +       if (clk->clkdm) {
>>> +               ret = ti_clk_ll_ops->clkdm_clk_enable(clk->clkdm,
>>> hw->clk);
>>> +               if (ret) {
>>> +                       WARN(1,
>>> +                            "%s: could not enable %s's clockdomain %s:
>>> %d\n",
>>> +                            __func__, clk_hw_get_name(hw),
>>> +                            clk->clkdm_name, ret);
>>> +                       return ret;
>>> +               }
>>> +       }
>>> +
>>> +       val = ti_clk_ll_ops->clk_readl(clk->enable_reg);
>>> +
>>> +       val &= ~OMAP4_MODULEMODE_MASK;
>>> +       val |= clk->enable_bit;
>>> +
>>> +       ti_clk_ll_ops->clk_writel(val, clk->enable_reg);
>>> +
>>> +       /* Wait until module is enabled */
>>> +       while (!_omap4_is_ready(val)) {
>>> +               udelay(1);
>>
>> This should really be a .prepare callback if you plan to keep the delays
>> in there.
>
> If this is changed to a .prepare, then all OMAP power management is
> effectively ruined as all clocks are going to be enabled all the time. hwmod
> core doesn't support .prepare/.enable at the moment that well, and changing
> that is going to be a big burden (educated guess, haven't checked this
> yet)... The call chain that comes here is:
>
> device driver -> pm_runtime -> hwmod_core -> hwmod_clk_enable / disable.
>
> The delay within this function should usually be pretty short, just to wait
> that the module comes up from idle.

Does it take multiple µs? Perhaps even one µs is much longer than needed?

> I recall the discussions regarding the udelays within clk_enable/disable
> calls, but what is the preferred approach then? Typically clk_enable/disable
> just becomes a NOP if it is not allowed to wait for hardware to complete
> transitioning before exiting the function.

FWIW, there are small loops with just a cpu_relax() in various clock drivers
under drivers/clk/shmobile/.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds