On Mon, Aug 28, 2023 at 11:42 AM Marco Felsch <m.felsch@xxxxxxxxxxxxxx> wrote:
>
> On 23-08-28, Michael Tretter wrote:
> > 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.
> >
> > Signed-off-by: Michael Tretter <m.tretter@xxxxxxxxxxxxxx>
> > ---
> > 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;
>
> Really just a cosmetic nit: fin /= 2;
>
> Reviewed-by: Marco Felsch <m.felsch@xxxxxxxxxxxxxx>
>
> > + clk_set_rate(dsi->pll_clk, fin);
> > +
> > fin = clk_get_rate(dsi->pll_clk);
> > - else
> > + } else {
> > fin = dsi->pll_clk_rate;
> > + }
I don't have all the code style rules memorized. Did you run it
through checkpatch? I wonder if the braces here are appropriate for a
1-line after the else. If checkpatch is happy, I am fine with it.
adam
> > 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
> >
> >
> >
