linux-kernel - Re: [linux-sunxi] [PATCH] clk: sunxi-ng: sun8i: r40: Use sigma delta modulation for audio PLL

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Message-ID: <21600331.3yVNq0iq4W@jernej-laptop>
Date:   Tue, 25 Aug 2020 18:45:53 +0200
From:   Jernej Škrabec <jernej.skrabec@...l.net>
To:     Chen-Yu Tsai <wens@...e.org>
Cc:     Maxime Ripard <mripard@...nel.org>,
        Mike Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        linux-clk <linux-clk@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-sunxi <linux-sunxi@...glegroups.com>
Subject: Re: [linux-sunxi] [PATCH] clk: sunxi-ng: sun8i: r40: Use sigma delta modulation for audio PLL

Dne torek, 25. avgust 2020 ob 16:46:31 CEST je Chen-Yu Tsai napisal(a):
> On Tue, Aug 25, 2020 at 9:11 PM Jernej Skrabec <jernej.skrabec@...l.net> 
wrote:
> > Audio cores need specific clock rates which can't be simply obtained by
> > adjusting integer multipliers and dividers. HW for such cases supports
> > delta-sigma modulation which enables fractional multipliers.
> > 
> > Port H3 delta-sigma table to R40. They have identical audio PLLs.
> > 
> > Signed-off-by: Jernej Skrabec <jernej.skrabec@...l.net>
> > ---
> > 
> >  drivers/clk/sunxi-ng/ccu-sun8i-r40.c | 37 ++++++++++++++++++----------
> >  1 file changed, 24 insertions(+), 13 deletions(-)
> > 
> > diff --git a/drivers/clk/sunxi-ng/ccu-sun8i-r40.c
> > b/drivers/clk/sunxi-ng/ccu-sun8i-r40.c index 23bfe1d12f21..84153418453f
> > 100644
> > --- a/drivers/clk/sunxi-ng/ccu-sun8i-r40.c
> > +++ b/drivers/clk/sunxi-ng/ccu-sun8i-r40.c
> > @@ -45,18 +45,29 @@ static struct ccu_nkmp pll_cpu_clk = {
> > 
> >   * the base (2x, 4x and 8x), and one variable divider (the one true
> >   * pll audio).
> >   *
> > 
> > - * We don't have any need for the variable divider for now, so we just
> > - * hardcode it to match with the clock names
> > + * With sigma-delta modulation for fractional-N on the audio PLL,
> > + * we have to use specific dividers. This means the variable divider
> > + * can no longer be used, as the audio codec requests the exact clock
> > + * rates we support through this mechanism. So we now hard code the
> > + * variable divider to 1. This means the clock rates will no longer
> > + * match the clock names.
> > 
> >   */
> >  
> >  #define SUN8I_R40_PLL_AUDIO_REG        0x008
> > 
> > -static SUNXI_CCU_NM_WITH_GATE_LOCK(pll_audio_base_clk, "pll-audio-base",
> > -                                  "osc24M", 0x008,
> > -                                  8, 7,        /* N */
> > -                                  0, 5,        /* M */
> > -                                  BIT(31),     /* gate */
> > -                                  BIT(28),     /* lock */
> > -                                  CLK_SET_RATE_UNGATE);
> > +static struct ccu_sdm_setting pll_audio_sdm_table[] = {
> > +       { .rate = 22579200, .pattern = 0xc0010d84, .m = 8, .n = 7 },
> > +       { .rate = 24576000, .pattern = 0xc000ac02, .m = 14, .n = 14 },
> 
> The user manual has an additional requirement: 3 <= N/M <= 21.
> Though it then says 72 <= 24*N/P <= 504. Not sure which one is
> right...
> 
> Did you run into any glitches or audio distortions?

No, I tested HDMI audio and it seems to work fine.

BSP driver also uses those values:
https://github.com/BPI-SINOVOIP/BPI-M2U-bsp/blob/master/linux-sunxi/drivers/
clk/sunxi/clk-sun8iw11.c#L160

Best regards,
Jernej