linux-kernel - Re: [PATCH] clk: sunxi-ng: sun50i: h6: Modify GPU clock configuration to support DFS

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Date:   Sun, 3 Jul 2022 01:49:58 -0500
From:   Samuel Holland <samuel@...lland.org>
To:     Roman Stratiienko <r.stratiienko@...il.com>, peron.clem@...il.com,
        jernej.skrabec@...il.com
Cc:     mturquette@...libre.com, sboyd@...nel.org, mripard@...nel.org,
        wens@...e.org, linux-clk@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-sunxi@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] clk: sunxi-ng: sun50i: h6: Modify GPU clock configuration
 to support DFS

On 6/24/22 11:52 AM, Roman Stratiienko wrote:
> 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.

The GPU module clock has only one parent declared, so it is surprising that the
mux would get set. Did this happen while the kernel driver was changing the
frequency?

> 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.

The manual lists PLL_GPU's maximum frequency as 800 MHz. I assume you chose 756
MHz because that is the highest OPP. That should be okay, too.

> After all the changes made DVFS started to work seamlessly.
> 
> Signed-off-by: Roman Stratiienko <r.stratiienko@...il.com>
> ---
>  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);

These changes look fine to me. You also need to set the initial value for the
fixed fields in the driver's probe function.

Regards,
Samuel