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

[Michael Tretter]

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;
+		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