Re: [PATCH 2/7] clk: thead: th1520-ap: Poll for PLL lock and wait for stability

Yao Zi
2025-11-26T15:17:21Z

On Wed, Nov 26, 2025 at 08:52:00AM -0600, Drew Fustini wrote:
> On Thu, Nov 20, 2025 at 01:14:11PM +0000, Yao Zi wrote:
> > All PLLs found on TH1520 SoC take 21250ns at maximum to lock, and their
> > lock status is indicated by register PLL_STS (offset 0x80 inside AP
> > clock controller). We should poll the register to ensure the PLL
> > actually locks after enabling it.
> >
> > Furthermore, a 30us delay is added after enabling the PLL, after which
> > the PLL could be considered stable as stated by vendor clock code.
> >
> > Fixes: 56a48c1833aa ("clk: thead: add support for enabling/disabling PLLs")
> > Signed-off-by: Yao Zi <ziyao@disroot.org>
> > ---
> > drivers/clk/thead/clk-th1520-ap.c | 34 +++++++++++++++++++++++++++++--
> > 1 file changed, 32 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/clk/thead/clk-th1520-ap.c b/drivers/clk/thead/clk-th1520-ap.c
> [...]
> > +/*
> > + * All PLLs in TH1520 take 21250ns at maximum to lock, let's take its double
> > + * for safety.
> > + */
> > +#define TH1520_PLL_LOCK_TIMEOUT_US 44
> > +#define TH1520_PLL_STABLE_DELAY_US 30
>
> I'm taking a second look at this and I think it might be best to add a
> define for the polling loop delay of 5. It could be helpful when other
> people read the code later.
>
> [...]
> > + ret = regmap_read_poll_timeout_atomic(pll->common.map, TH1520_PLL_STS,
> > + reg, reg & pll->lock_sts_mask,
> > + 5, TH1520_PLL_LOCK_TIMEOUT_US);
>
> The loop delay is only used here but I think using a #define would make
> it more readable.

There are TH1520_PLL_LOCK_TIMEOUT_US and TH1520_PLL_STABLE_DELAY_US
defined because they're meaningful constants, either specified by TRM or
implied by vendor code, however the 5us delay is only a randomly-picked
value, as what I've mentioned before.

Anyway, I'm fine with a separate definition. So please go ahead if it
looks better to you.

> Other than that:
> Reviewed-by: Drew Fustini <fustini@kernel.org>
>
> If no other changes are needed I could fix this up on apply. Let's see
> what other comments there may be. It's too late for me to send a 6.19
> clk pull request so this will have to target the next merge window. I
> can put it into linux-next once 6.19-rc1 is released.

Many thanks for it.

> Thanks,
> Drew

Best regards,
Yao Zi