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. Which 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 --- drivers/video/ssd2828.c | 20 +++++++++++++++++--- drivers/video/ssd2828.h | 11 ++++++++--- 2 files changed, 25 insertions(+), 6 deletions(-)
diff --git a/drivers/video/ssd2828.c b/drivers/video/ssd2828.c index 566ec19..702afd9 100644 --- a/drivers/video/ssd2828.c +++ b/drivers/video/ssd2828.c @@ -339,7 +339,7 @@ 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. */ if (ssd2828_configure_video_interface(cfg, mode) != 0) { @@ -389,10 +403,10 @@ 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 */ diff --git a/drivers/video/ssd2828.h b/drivers/video/ssd2828.h index 14b96c5..1af6fa4 100644 --- a/drivers/video/ssd2828.h +++ b/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);