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

Drew Fustini
2025-11-26T14:39:33Z

On Tue, Nov 25, 2025 at 03:19:30AM +0000, Yao Zi wrote:
> On Mon, Nov 24, 2025 at 02:08:00PM -0800, 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(-)
> >
> > Thanks for working on this patch series.
> >
> > [...]
> > > @@ -299,9 +310,21 @@ static void ccu_pll_disable(struct clk_hw *hw)
> > > static int ccu_pll_enable(struct clk_hw *hw)
> > > {
> > > struct ccu_pll *pll = hw_to_ccu_pll(hw);
> > > + u32 reg;
> > > + int ret;
> > >
> > > - return regmap_clear_bits(pll->common.map, pll->common.cfg1,
> > > - TH1520_PLL_VCO_RST);
> > > + regmap_clear_bits(pll->common.map, pll->common.cfg1,
> > > + TH1520_PLL_VCO_RST);
> > > +
> > > + ret = regmap_read_poll_timeout_atomic(pll->common.map, TH1520_PLL_STS,
> > > + reg, reg & pll->lock_sts_mask,
> > > + 5, TH1520_PLL_LOCK_TIMEOUT_US);
> >
> > Is there a reason for the specific value of 5 uS polling delay?
>
> No, it was picked randomly. A smaller value would reduce latency of
> PLL enabling, and I could tune it more carefully by some testing. But
> it's hard to predict how much improvement it will bring.

Okay, I was just curious. I think it is okay to stick with that current
value if it is working correctly.

> > > + if (ret)
> > > + return ret;
> > > +
> > > + udelay(TH1520_PLL_STABLE_DELAY_US);
> >
> > Is it the case that the 30 uS delay after the lock bit is set is just so
> > that it has the same behavior as the vendor's code? Or did you notice
> > stability problems without this?
>
> This aligns with the vendor code, and I haven't yet observed stability
> issues without the delay. But I think it's more safe to keep the
> behavior similar since it's hard to test all working conditions.

Okay, that seems reasonable to play it safe.

Thanks,
Drew