clk: meson: meson8b: do not use cpu_div3 for cpu_scale_out_sel

The cpu_div3 clock (cpu_in divided by 3) generates a signal with a duty
cycle of 33%. The CPU clock however requires a clock signal with a duty
cycle of 50% to run stable.
cpu_div3 was observed to be problematic when cycling through all
available CPU frequencies (with additional patches on top of this one)
while running "stress --cpu 4" in the background. This caused sporadic
hangs where the whole system would fully lock up.

Amlogic's 3.10 kernel code also does not use the cpu_div3 clock either
when changing the CPU clock.
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://lkml.kernel.org/r/20181115224048.13511-3-martin.blumenstingl@googlemail.com

drivers/clk/meson/meson8b.c

@@ -608,20 +608,27 @@ static struct clk_regmap meson8b_cpu_scale_div = {
 	},
 };
 
+static u32 mux_table_cpu_scale_out_sel[] = { 0, 1, 3 };
 static struct clk_regmap meson8b_cpu_scale_out_sel = {
 	.data = &(struct clk_regmap_mux_data){
 		.offset = HHI_SYS_CPU_CLK_CNTL0,
 		.mask = 0x3,
 		.shift = 2,
+		.table = mux_table_cpu_scale_out_sel,
 	},
 	.hw.init = &(struct clk_init_data){
 		.name = "cpu_scale_out_sel",
 		.ops = &clk_regmap_mux_ro_ops,
+		/*
+		 * NOTE: We are skipping the parent with value 0x2 (which is
+		 * "cpu_div3") because it results in a duty cycle of 33% which
+		 * makes the system unstable and can result in a lockup of the
+		 * whole system.
+		 */
 		.parent_names = (const char *[]) { "cpu_in_sel",
 						   "cpu_div2",
-						   "cpu_div3",
 						   "cpu_scale_div" },
-		.num_parents = 4,
+		.num_parents = 3,
 		.flags = CLK_SET_RATE_PARENT,
 	},
 };