Using simple bash script it was discovered that not all CCU registers
can be safely used for DFS, e.g.:

    while true
    do
        devmem 0x3001030 4 0xb0003e02
        devmem 0x3001030 4 0xb0001e02
    done

Script above changes the GPU_PLL multiplier register value. While the
script is running, the user should interact with the user interface.

Using this method the following results were obtained:

| Register  | Name           | Bits  | Values | Result |
| --        | --             | --    | --     | --     |
| 0x3001030 | GPU_PLL.MULT   | 15..8 | 20-62  | OK     |
| 0x3001030 | GPU_PLL.INDIV  |     1 | 0-1    | OK     |
| 0x3001030 | GPU_PLL.OUTDIV |     0 | 0-1    | FAIL   |
| 0x3001670 | GPU_CLK.DIV    |  3..0 | ANY    | FAIL   |

Once bits that caused system failure disabled (kept default 0),
it was discovered that GPU_CLK.MUX was used during DFS for some
reason and was causing the failure too.

After disabling GPU_PLL.OUTDIV the system started to fail during
booting for some reason until the maximum frequency of GPU_PLL
clock was limited to 756MHz.

After all the changes made DVFS started to work seamlessly.

Signed-off-by: Roman Stratiienko <redacted>
---
 drivers/clk/sunxi-ng/ccu-sun50i-h6.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/drivers/clk/sunxi-ng/ccu-sun50i-h6.c b/drivers/clk/sunxi-ng/ccu-sun50i-h6.c
index 2ddf0a0da526f..d941238cd178a 100644
--- a/drivers/clk/sunxi-ng/ccu-sun50i-h6.c
+++ b/drivers/clk/sunxi-ng/ccu-sun50i-h6.c

@@ -95,13 +95,14 @@ static struct ccu_nkmp pll_periph1_clk = {
 	},
 };
 
+/* For GPU PLL, using an output divider for DFS causes system to fail */
 #define SUN50I_H6_PLL_GPU_REG		0x030
 static struct ccu_nkmp pll_gpu_clk = {
 	.enable		= BIT(31),
 	.lock		= BIT(28),
 	.n		= _SUNXI_CCU_MULT_MIN(8, 8, 12),
 	.m		= _SUNXI_CCU_DIV(1, 1), /* input divider */
-	.p		= _SUNXI_CCU_DIV(0, 1), /* output divider */
+	.max_rate	= 756000000UL,
 	.common		= {
 		.reg		= 0x030,
 		.hw.init	= CLK_HW_INIT("pll-gpu", "osc24M",

@@ -294,12 +295,9 @@ static SUNXI_CCU_M_WITH_MUX_GATE(deinterlace_clk, "deinterlace",
 static SUNXI_CCU_GATE(bus_deinterlace_clk, "bus-deinterlace", "psi-ahb1-ahb2",
 		      0x62c, BIT(0), 0);
 
-static const char * const gpu_parents[] = { "pll-gpu" };
-static SUNXI_CCU_M_WITH_MUX_GATE(gpu_clk, "gpu", gpu_parents, 0x670,
-				       0, 3,	/* M */
-				       24, 1,	/* mux */
-				       BIT(31),	/* gate */
-				       CLK_SET_RATE_PARENT);
+/* GPU_CLK divider kept disabled to avoid interferences with DFS */
+static SUNXI_CCU_GATE(gpu_clk, "gpu", "pll-gpu", 0x670,
+		      BIT(31), CLK_SET_RATE_PARENT);
 
 static SUNXI_CCU_GATE(bus_gpu_clk, "bus-gpu", "psi-ahb1-ahb2",
 		      0x67c, BIT(0), 0);

-- 
2.34.1


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