video: ssd2828: Allow using 'pclk' as the PLL clock source

Instead of using the internal 'tx_clk' clock source, it is also
possible to use the pixel clock signal from the parallel LCD
interface ('pclk') as the reference clock for PLL.

The 'tx_clk' clock speed may be different on different boards/devices
(the allowed range is 8MHz - 30MHz). Which is not very convenient,
especially considering the need to know the exact 'tx_clk' clock
speed. This clock speed may be difficult to identify without having
device schematics and/or accurate documentation/sources every time.

Using 'pclk' is free from all these problems.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
Acked-by: Anatolij Gustschin <agust@denx.de>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

drivers/video/Kconfig

@@ -19,12 +19,14 @@ config VIDEO_LCD_SSD2828
 config VIDEO_LCD_SSD2828_TX_CLK
 	int "SSD2828 TX_CLK frequency (in MHz)"
 	depends on VIDEO_LCD_SSD2828
+	default 0
 	---help---
 	The frequency of the crystal, which is clocking SSD2828. It may be
 	anything in the 8MHz-30MHz range and the exact value should be
 	retrieved from the board schematics. Or in the case of Allwinner
 	hardware, it can be usually found as 'lcd_xtal_freq' variable in
-	FEX files.
+	FEX files. It can be also set to 0 for selecting PCLK from the
+	parallel LCD interface instead of TX_CLK as the PLL clock source.
 
 config VIDEO_LCD_SSD2828_RESET
 	string "RESET pin of SSD2828"

drivers/video/ssd2828.c

@@ -340,7 +340,7 @@ static int ssd2828_configure_video_interface(const struct ssd2828_config *cfg,
 int ssd2828_init(const struct ssd2828_config *cfg,
 		 const struct ctfb_res_modes *mode)
 {
-	u32 lp_div, pll_freq_kbps, pll_config;
+	u32 lp_div, pll_freq_kbps, reference_freq_khz, pll_config;
 	/* The LP clock speed is limited by 10MHz */
 	const u32 mipi_dsi_low_power_clk_khz = 10000;
 	/*
@@ -373,6 +373,20 @@ int ssd2828_init(const struct ssd2828_config *cfg,
 		}
 	}
 
+	/*
+	 * Pick the reference clock for PLL. If we know the exact 'tx_clk'
+	 * clock speed, then everything is good. If not, then we can fallback
+	 * to 'pclk' (pixel clock from the parallel LCD interface). In the
+	 * case of using this fallback, it is necessary to have parallel LCD
+	 * already initialized and running at this point.
+	 */
+	reference_freq_khz = cfg->ssd2828_tx_clk_khz;
+	if (reference_freq_khz  == 0) {
+		reference_freq_khz = mode->pixclock_khz;
+		/* Use 'pclk' as the reference clock for PLL */
+		cfgr_reg |= SSD2828_CFGR_CSS;
+	}
+
 	/*
 	 * Setup the parallel LCD timings in the appropriate registers.
 	 */
@@ -390,10 +404,10 @@ int ssd2828_init(const struct ssd2828_config *cfg,
 	/* PLL Configuration Register */
 	pll_config = construct_pll_config(
 				cfg->mipi_dsi_bitrate_per_data_lane_mbps * 1000,
-				cfg->ssd2828_tx_clk_khz);
+				reference_freq_khz);
 	write_hw_register(cfg, SSD2828_PLCR, pll_config);
 
-	pll_freq_kbps = decode_pll_config(pll_config, cfg->ssd2828_tx_clk_khz);
+	pll_freq_kbps = decode_pll_config(pll_config, reference_freq_khz);
 	lp_div = DIV_ROUND_UP(pll_freq_kbps, mipi_dsi_low_power_clk_khz * 8);
 
 	/* VC Control Register */

drivers/video/ssd2828.h

@@ -47,8 +47,12 @@ struct ssd2828_config {
 	 * to TX_CLK_XIO/TX_CLK_XIN pins), which is necessary at least for
 	 * clocking SPI after reset. The exact clock speed is not strictly,
 	 * defined, but the datasheet says that it must be somewhere in the
-	 * 8MHz - 30MHz range (see "TX_CLK Timing" section). It is used as
-	 * a reference clock for PLL and must be set correctly.
+	 * 8MHz - 30MHz range (see "TX_CLK Timing" section). It can be also
+	 * used as a reference clock for PLL. If the exact clock frequency
+	 * is known, then it can be specified here. If it is unknown, or the
+	 * information is not trustworthy, then it can be set to 0.
+	 *
+	 * If unsure, set to 0.
 	 */
 	int ssd2828_tx_clk_khz;
 
@@ -115,7 +119,8 @@ struct ssd2828_config {
  * The right place to insert this function call is after the parallel LCD
  * interface is initialized and before turning on the backlight. This is
  * advised in the "Example for system sleep in and out" section of the
- * SSD2828 datasheet.
+ * SSD2828 datasheet. And also SS2828 may use 'pclk' as the clock source
+ * for PLL, which means that the input signal must be already there.
  */
 int ssd2828_init(const struct ssd2828_config *cfg,
 		 const struct ctfb_res_modes *mode);