Commit dddccd69 authored by Siarhei Siamashka's avatar Siarhei Siamashka Committed by Hans de Goede

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: default avatarSiarhei Siamashka <>
Acked-by: default avatarAnatolij Gustschin <>
Acked-by: default avatarHans de Goede <>
Signed-off-by: default avatarHans de Goede <>
parent aaa6ac5e
......@@ -19,12 +19,14 @@ config VIDEO_LCD_SSD2828
int "SSD2828 TX_CLK frequency (in MHz)"
depends on VIDEO_LCD_SSD2828
default 0
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.
string "RESET pin of SSD2828"
......@@ -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,
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 */
......@@ -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);
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