drm/bridge: samsung-dsim: update PLL reference clock

The PLL requires a clock frequency in a certain platform-dependent range
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
the upper limit 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.
Reviewed-by: Marco Felsch <m.felsch@pengutronix.de>
Tested-by: Frieder Schrempf <frieder.schrempf@kontron.de> # Kontron BL i.MX8MM + Waveshare 10.1inch HDMI LCD (E)
Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Link: https://lore.kernel.org/r/20230818-samsung-dsim-v2-3-846603df0e0a@pengutronix.deSigned-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230818-samsung-dsim-v2-3-846603df0e0a@pengutronix.de

drivers/gpu/drm/bridge/samsung-dsim.c

@@ -410,6 +410,8 @@ static const struct samsung_dsim_driver_data exynos3_dsi_driver_data = {
 	.num_bits_resol = 11,
 	.pll_p_offset = 13,
 	.reg_values = reg_values,
+	.pll_fin_min = 6,
+	.pll_fin_max = 12,
 	.m_min = 41,
 	.m_max = 125,
 	.min_freq = 500,
@@ -427,6 +429,8 @@ static const struct samsung_dsim_driver_data exynos4_dsi_driver_data = {
 	.num_bits_resol = 11,
 	.pll_p_offset = 13,
 	.reg_values = reg_values,
+	.pll_fin_min = 6,
+	.pll_fin_max = 12,
 	.m_min = 41,
 	.m_max = 125,
 	.min_freq = 500,
@@ -442,6 +446,8 @@ static const struct samsung_dsim_driver_data exynos5_dsi_driver_data = {
 	.num_bits_resol = 11,
 	.pll_p_offset = 13,
 	.reg_values = reg_values,
+	.pll_fin_min = 6,
+	.pll_fin_max = 12,
 	.m_min = 41,
 	.m_max = 125,
 	.min_freq = 500,
@@ -457,6 +463,8 @@ static const struct samsung_dsim_driver_data exynos5433_dsi_driver_data = {
 	.num_bits_resol = 12,
 	.pll_p_offset = 13,
 	.reg_values = exynos5433_reg_values,
+	.pll_fin_min = 6,
+	.pll_fin_max = 12,
 	.m_min = 41,
 	.m_max = 125,
 	.min_freq = 500,
@@ -472,6 +480,8 @@ static const struct samsung_dsim_driver_data exynos5422_dsi_driver_data = {
 	.num_bits_resol = 12,
 	.pll_p_offset = 13,
 	.reg_values = exynos5422_reg_values,
+	.pll_fin_min = 6,
+	.pll_fin_max = 12,
 	.m_min = 41,
 	.m_max = 125,
 	.min_freq = 500,
@@ -491,6 +501,8 @@ static const struct samsung_dsim_driver_data imx8mm_dsi_driver_data = {
 	 */
 	.pll_p_offset = 14,
 	.reg_values = imx8mm_dsim_reg_values,
+	.pll_fin_min = 2,
+	.pll_fin_max = 30,
 	.m_min = 64,
 	.m_max = 1023,
 	.min_freq = 1050,
@@ -614,10 +626,21 @@ 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.
+		 */
+		fin = clk_get_rate(clk_get_parent(dsi->pll_clk));
+		while (fin > driver_data->pll_fin_max * MHZ)
+			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);

include/drm/bridge/samsung-dsim.h

@@ -61,6 +61,8 @@ struct samsung_dsim_driver_data {
 	unsigned int num_bits_resol;
 	unsigned int pll_p_offset;
 	const unsigned int *reg_values;
+	unsigned int pll_fin_min;
+	unsigned int pll_fin_max;
 	u16 m_min;
 	u16 m_max;
 };