Re: [PATCH 3/5] drm/msm/dsi_pll_10nm: Fix bad VCO rate calculation and prescaler

[Rob Clark <robdclark@gmail.com>]

On Sat, Jan 9, 2021 at 5:51 AM AngeloGioacchino Del Regno
<angelogioacchino.delregno@somainline.org> wrote:
>
> The VCO rate was being miscalculated due to a big overlook during
> the process of porting this driver from downstream to upstream:
> here we are really recalculating the rate of the VCO by reading
> the appropriate registers and returning a real frequency, while
> downstream the driver was doing something entirely different.
>
> In our case here, the recalculated rate was wrong, as it was then
> given back to the set_rate function, which was erroneously doing
> a division on the fractional value, based on the prescaler being
> either enabled or disabled: this was actually producing a bug for
> which the final VCO rate was being doubled, causing very obvious
> issues when trying to drive a DSI panel because the actual divider
> value was multiplied by two!
>
> To make things work properly, remove the multiplication of the
> reference clock by two from function dsi_pll_calc_dec_frac and
> account for the prescaler enablement in the vco_recalc_rate (if
> the prescaler is enabled, then the hardware will divide the rate
> by two).
>
> This will make the vco_recalc_rate function to pass the right
> frequency to the (clock framework) set_rate function when called,
> which will - in turn - program the right values in both the
> DECIMAL_DIV_START_1 and the FRAC_DIV_START_{LOW/MID/HIGH}_1
> registers, finally making the PLL to output the right clock.
>
> Also, while at it, remove the prescaler TODO by also adding the
> possibility of disabling the prescaler on the PLL (it is in the
> PLL_ANALOG_CONTROLS_ONE register).
> Of course, both prescaler-ON and OFF cases were tested.

This somehow breaks things on sc7180 (display gets stuck at first
frame of splash screen).  (This is a setup w/ an ti-sn65dsi86 dsi->eDP
bridge)

Also, something (I assume DSI related) that I was testing on
msm-next-staging seems to have effected the colors on the panel (ie.
they are more muted).. which seems to persist across reboots (ie. when
switching back to a good kernel), and interestingly if I reboot from a
good kernel I see part of the login prompt (or whatever was previously
on-screen) in the firmware ui screen !?!  (so maybe somehow triggered
the display to think it is in PSR mode??)

Not sure if that is caused by these patches, but if I can figure out
how to get the panel back to normal I can bisect.  I think for now
I'll drop this series.  Possibly it could be a
two-wrongs-makes-a-right situation that had things working before, but
I think someone from qcom who knows the DSI IP should take a look.

BR,
-R


> Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@somainline.org>
> ---
>  drivers/gpu/drm/msm/dsi/pll/dsi_pll_10nm.c | 22 +++++++++-------------
>  1 file changed, 9 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/gpu/drm/msm/dsi/pll/dsi_pll_10nm.c b/drivers/gpu/drm/msm/dsi/pll/dsi_pll_10nm.c
> index 8b66e852eb36..5be562dfbf06 100644
> --- a/drivers/gpu/drm/msm/dsi/pll/dsi_pll_10nm.c
> +++ b/drivers/gpu/drm/msm/dsi/pll/dsi_pll_10nm.c
> @@ -165,11 +165,7 @@ static void dsi_pll_calc_dec_frac(struct dsi_pll_10nm *pll)
>
>         pll_freq = pll->vco_current_rate;
>
> -       if (config->disable_prescaler)
> -               divider = fref;
> -       else
> -               divider = fref * 2;
> -
> +       divider = fref;
>         multiplier = 1 << config->frac_bits;
>         dec_multiple = div_u64(pll_freq * multiplier, divider);
>         dec = div_u64_rem(dec_multiple, multiplier, &frac);
> @@ -266,9 +262,11 @@ static void dsi_pll_ssc_commit(struct dsi_pll_10nm *pll)
>
>  static void dsi_pll_config_hzindep_reg(struct dsi_pll_10nm *pll)
>  {
> +       struct dsi_pll_config *config = &pll->pll_configuration;
>         void __iomem *base = pll->mmio;
> +       u32 val = config->disable_prescaler ? 0x0 : 0x80;
>
> -       pll_write(base + REG_DSI_10nm_PHY_PLL_ANALOG_CONTROLS_ONE, 0x80);
> +       pll_write(base + REG_DSI_10nm_PHY_PLL_ANALOG_CONTROLS_ONE, val);
>         pll_write(base + REG_DSI_10nm_PHY_PLL_ANALOG_CONTROLS_TWO, 0x03);
>         pll_write(base + REG_DSI_10nm_PHY_PLL_ANALOG_CONTROLS_THREE, 0x00);
>         pll_write(base + REG_DSI_10nm_PHY_PLL_DSM_DIVIDER, 0x00);
> @@ -499,17 +497,15 @@ static unsigned long dsi_pll_10nm_vco_recalc_rate(struct clk_hw *hw,
>         frac |= ((pll_read(base + REG_DSI_10nm_PHY_PLL_FRAC_DIV_START_HIGH_1) &
>                   0x3) << 16);
>
> -       /*
> -        * TODO:
> -        *      1. Assumes prescaler is disabled
> -        */
>         multiplier = 1 << config->frac_bits;
> -       pll_freq = dec * (ref_clk * 2);
> -       tmp64 = (ref_clk * 2 * frac);
> +       pll_freq = dec * ref_clk;
> +       tmp64 = ref_clk * frac;
>         pll_freq += div_u64(tmp64, multiplier);
> -
>         vco_rate = pll_freq;
>
> +       if (config->disable_prescaler)
> +               vco_rate = div_u64(vco_rate, 2);
> +
>         DBG("DSI PLL%d returning vco rate = %lu, dec = %x, frac = %x",
>             pll_10nm->id, (unsigned long)vco_rate, dec, frac);
>
> --
> 2.29.2
>