drm: bridge: icn6211: Add generic DSI-to-DPI PLL configuration

The chip contains fractional PLL, however the driver currently hard-codes
one specific PLL setting. Implement generic PLL parameter calculation code,
so any DPI panel with arbitrary pixel clock can be attached to this bridge.

The datasheet for this bridge is not available, the PLL behavior has been
inferred from [1] and [2] and by analyzing the DPI pixel clock with scope.
The PLL limits might be wrong, but at least the calculated values match all
the example code available. This is better than one hard-coded pixel clock
value anyway.

[1] https://github.com/rockchip-linux/kernel/blob/develop-4.19/drivers/gpu/drm/bridge/icn6211.c
[2] https://github.com/tdjastrzebski/ICN6211-ConfiguratorAcked-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Jagan Teki <jagan@amarulasolutions.com>
Cc: Maxime Ripard <maxime@cerno.tech>
Cc: Robert Foss <robert.foss@linaro.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
To: dri-devel@lists.freedesktop.org
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220331150509.9838-5-marex@denx.de

drivers/gpu/drm/bridge/chipone-icn6211.c

@@ -136,6 +136,7 @@ struct chipone {
 	struct drm_bridge bridge;
 	struct drm_display_mode mode;
 	struct drm_bridge *panel_bridge;
+	struct mipi_dsi_device *dsi;
 	struct gpio_desc *enable_gpio;
 	struct regulator *vdd1;
 	struct regulator *vdd2;
@@ -161,6 +162,87 @@ static inline int chipone_dsi_write(struct chipone *icn,  const void *seq,
 		chipone_dsi_write(icn, d, ARRAY_SIZE(d));	\
 	}
 
+static void chipone_configure_pll(struct chipone *icn,
+				  const struct drm_display_mode *mode)
+{
+	unsigned int best_p = 0, best_m = 0, best_s = 0;
+	unsigned int delta, min_delta = 0xffffffff;
+	unsigned int freq_p, freq_s, freq_out;
+	unsigned int p_min, p_max;
+	unsigned int p, m, s;
+	unsigned int fin;
+
+	/*
+	 * DSI clock lane frequency (input into PLL) is calculated as:
+	 *  DSI_CLK = mode clock * bpp / dsi_data_lanes / 2
+	 * the 2 is there because the bus is DDR.
+	 *
+	 * DPI pixel clock frequency (output from PLL) is mode clock.
+	 *
+	 * The chip contains fractional PLL which works as follows:
+	 *  DPI_CLK = ((DSI_CLK / P) * M) / S
+	 * P is pre-divider, register PLL_REF_DIV[3:0] is 2^(n+1) divider
+	 *                   register PLL_REF_DIV[4] is extra 1:2 divider
+	 * M is integer multiplier, register PLL_INT(0) is multiplier
+	 * S is post-divider, register PLL_REF_DIV[7:5] is 2^(n+1) divider
+	 *
+	 * It seems the PLL input clock after applying P pre-divider have
+	 * to be lower than 20 MHz.
+	 */
+	fin = mode->clock * mipi_dsi_pixel_format_to_bpp(icn->dsi->format) /
+	      icn->dsi->lanes / 2; /* in kHz */
+
+	/* Minimum value of P predivider for PLL input in 5..20 MHz */
+	p_min = ffs(fin / 20000);
+	p_max = (fls(fin / 5000) - 1) & 0x1f;
+
+	for (p = p_min; p < p_max; p++) {	/* PLL_REF_DIV[4,3:0] */
+		freq_p = fin / BIT(p + 1);
+		if (freq_p == 0)		/* Divider too high */
+			break;
+
+		for (s = 0; s < 0x7; s++) {	/* PLL_REF_DIV[7:5] */
+			freq_s = freq_p / BIT(s + 1);
+			if (freq_s == 0)	/* Divider too high */
+				break;
+
+			m = mode->clock / freq_s;
+
+			/* Multiplier is 8 bit */
+			if (m > 0xff)
+				continue;
+
+			/* Limit PLL VCO frequency to 1 GHz */
+			freq_out = (fin * m) / BIT(p + 1);
+			if (freq_out > 1000000)
+				continue;
+
+			/* Apply post-divider */
+			freq_out /= BIT(s + 1);
+
+			delta = abs(mode->clock - freq_out);
+			if (delta < min_delta) {
+				best_p = p;
+				best_m = m;
+				best_s = s;
+				min_delta = delta;
+			}
+		}
+	}
+
+	dev_dbg(icn->dev,
+		"PLL: P[3:0]=2^%d P[4]=2*%d M=%d S[7:5]=2^%d delta=%d => DSI f_in=%d kHz ; DPI f_out=%ld kHz\n",
+		best_p, !!best_p, best_m, best_s + 1, min_delta, fin,
+		(fin * best_m) / BIT(best_p + best_s + 2));
+
+	/* Clock source selection fixed to MIPI DSI clock lane */
+	ICN6211_DSI(icn, PLL_CTRL(6), PLL_CTRL_6_MIPI_CLK);
+	ICN6211_DSI(icn, PLL_REF_DIV,
+		    (best_p ? PLL_REF_DIV_Pe : 0) | /* Prefer /2 pre-divider */
+		    PLL_REF_DIV_P(best_p) | PLL_REF_DIV_S(best_s));
+	ICN6211_DSI(icn, PLL_INT(0), best_m);
+}
+
 static void chipone_atomic_enable(struct drm_bridge *bridge,
 				  struct drm_bridge_state *old_bridge_state)
 {
@@ -221,9 +303,9 @@ static void chipone_atomic_enable(struct drm_bridge *bridge,
 	      ((bus_flags & DRM_BUS_FLAG_DE_HIGH) ? BIST_POL_DE_POL : 0);
 	ICN6211_DSI(icn, BIST_POL, pol);
 
-	ICN6211_DSI(icn, PLL_CTRL(6), PLL_CTRL_6_MIPI_CLK);
-	ICN6211_DSI(icn, PLL_REF_DIV, 0x71);
-	ICN6211_DSI(icn, PLL_INT(0), 0x2b);
+	/* Configure PLL settings */
+	chipone_configure_pll(icn, mode);
+
 	ICN6211_DSI(icn, SYS_CTRL(0), 0x40);
 	ICN6211_DSI(icn, SYS_CTRL(1), 0x98);
 
@@ -376,6 +458,7 @@ static int chipone_probe(struct mipi_dsi_device *dsi)
 	icn->bridge.funcs = &chipone_bridge_funcs;
 	icn->bridge.type = DRM_MODE_CONNECTOR_DPI;
 	icn->bridge.of_node = dev->of_node;
+	icn->dsi = dsi;
 
 	drm_bridge_add(&icn->bridge);