[PATCH] clk: mxs: source LCD clock from PLL instead of from ref_xtal

[Hector Palacios]

The lcd clock hierarchy is as follows:

    ref_clk (24MHz)-----\
                         +--+ lcdif_sel (MUX type)
    frac1_clk (480Mhz)--/   |
                            \--+ lcdif_div (DIV type)
                               |
                               \-- lcdif (GATE type)

The mxsfb driver retrieves the 'lcdif_div' clock from the DT, but the
'lcdif_sel' is by default sourced from 'ref_clk' which is a fixed 24MHz.
This means the pixel clock can only be 24MHz or a power-of-two divisor
of that frequency (12, 6, 3...) which is a very poor divisor granularity.

Since the driver doesn't know about the different clocks that can
source its clk parent from, we must select the PLL (480MHz) to be the
parent during the general clocks initialization. This allows for much
better divisor granularity and for more accurate frequencies for LCD
displays.

Signed-off-by: Hector Palacios <hector.palacios at digi.com>
Tested-by: Fabio Estevam <fabio.estevam at freescale.com>
CC: Maxime Rippard <maxime.ripard at free-electrons.com>
CC: Marek Vasut <marex at denx.de>
CC: Alexandre Belloni <alexandre.belloni at free-electrons.com>
CC: Gwenhael Goavec-Merou <gwenhael.goavec-merou at armadeus.com>
---
 drivers/clk/mxs/clk-imx28.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/clk/mxs/clk-imx28.c b/drivers/clk/mxs/clk-imx28.c
index 4faf0af..882ec3f 100644
--- a/drivers/clk/mxs/clk-imx28.c
+++ b/drivers/clk/mxs/clk-imx28.c
@@ -246,6 +246,12 @@ int __init mx28_clocks_init(void)
 	clk_data.clk_num = ARRAY_SIZE(clks);
 	of_clk_add_provider(np, of_clk_src_onecell_get, &clk_data);
 
+	/*
+	 * Select PLL as the parent source of lcdif_sel clk to have a finer
+	 * granularity when calculating the LCD pixelclock
+	 */
+	clk_set_parent(clks[lcdif_sel], clks[ref_pix]);
+
 	clk_register_clkdev(clks[enet_out], NULL, "enet_out");
 
 	for (i = 0; i < ARRAY_SIZE(clks_init_on); i++)