OMAPDSS: DSI: skip odd dividers when pck >= 100MHz

The DSI PLL and HSDivider can be used to generate the pixel clock for
LCD overlay manager, which then goes to DPI output. On the DPI output
pin the voltage of the signal is shifted from the OMAP's internal
minimal voltage to 1.8V range. The shifting is not instant, and the
higher the clock frequency, the less time there is to shift the signal
to nominal voltage.

If the HSDivider's divider is greater than one and odd, the resulting
pixel clock does not have 50% duty cycle. For example, with a divider of
3, the duty cycle is 33%.

When combining high frequency (in the area of 140MHz+) and non-50% duty
cycle, it has been observed the the shifter does not have enough time to
shift the voltage enough, and this leads to bad signal which is rejected
by monitors.

As a workaround this patch makes the divider calculation skip all odd
dividers when the required pixel clock is over 100MHz. The limit of
100MHz is a guesstimate.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/drivers/video/omap2/dss/dsi.c b/drivers/video/omap2/dss/dsi.c
index 2ba505b33153..ea5c6603a51d 100644
--- a/drivers/video/omap2/dss/dsi.c
+++ b/drivers/video/omap2/dss/dsi.c
@@ -1386,6 +1386,11 @@ retry:
 				cur.dsi_pll_hsdiv_dispc_clk =
 					cur.clkin4ddr / cur.regm_dispc;
 
+				if (cur.regm_dispc > 1 &&
+						cur.regm_dispc % 2 != 0 &&
+						req_pck >= 1000000)
+					continue;
+
 				/* this will narrow down the search a bit,
 				 * but still give pixclocks below what was
 				 * requested */