[PATCH 3/5] drm/bridge: samsung-dsim: update PLL reference clock

[Inki Dae]

2023년 8월 29일 (화) 오전 12:59, Michael Tretter <m.tretter at pengutronix.de>님이 작성:
>
> The PLL requires a clock between 2 MHz and 30 MHz after the pre-divider.
> The reference clock for the PLL may change due to changes to it's parent
> clock. Thus, the frequency may be out of range or unsuited for
> generating the high speed clock for MIPI DSI.
>
> Try to keep the pre-devider small, and set the reference clock close to
> 30 MHz before recalculating the PLL configuration. Use a divider with a
> power of two for the reference clock as this seems to work best in
> my tests.

Clock frequency is specific to SoC architecture so we have to handle
it carefully because samsung-dsim.c is a common module for I.MX and
Exynos series.
You mentioned "The PLL requires a clock between 2 MHz and 3MHz after
pre-divider", and the clock means that fin_pll - PFD input frequency -
which can be calculated with oscillator clock frequency / P value?
According to Exynos datasheet, the fin_pll should be 6 ~ 12Mhz.

For example,
In case of Exyhos, we use 24MHz as oscillator clock frequency, so
fin_pll frequency, 8MHz = 24MHz / P(3).

Can you tell me the source of the constraint that clocks must be
between 2MHz and 30MHz?

To other I.MX and Exynos engineers, please do not merge this patch
until two SoC platforms are tested correctly.

Thanks,
Inki Dae

>
> Signed-off-by: Michael Tretter <m.tretter at pengutronix.de>
> ---
>  drivers/gpu/drm/bridge/samsung-dsim.c | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/bridge/samsung-dsim.c b/drivers/gpu/drm/bridge/samsung-dsim.c
> index da90c2038042..4de6e4f116db 100644
> --- a/drivers/gpu/drm/bridge/samsung-dsim.c
> +++ b/drivers/gpu/drm/bridge/samsung-dsim.c
> @@ -611,10 +611,21 @@ static unsigned long samsung_dsim_set_pll(struct samsung_dsim *dsi,
>         u16 m;
>         u32 reg;
>
> -       if (dsi->pll_clk)
> +       if (dsi->pll_clk) {
> +               /*
> +                * Ensure that the reference clock is generated with a power of
> +                * two divider from its parent, but close to the PLLs upper
> +                * limit of the valid range of 2 MHz to 30 MHz.
> +                */
> +               fin = clk_get_rate(clk_get_parent(dsi->pll_clk));
> +               while (fin > 30 * MHZ)
> +                       fin = fin / 2;
> +               clk_set_rate(dsi->pll_clk, fin);
> +
>                 fin = clk_get_rate(dsi->pll_clk);
> -       else
> +       } else {
>                 fin = dsi->pll_clk_rate;
> +       }
>         dev_dbg(dsi->dev, "PLL ref clock freq %lu\n", fin);
>
>         fout = samsung_dsim_pll_find_pms(dsi, fin, freq, &p, &m, &s);
>
> --
> 2.39.2
>