linux-kernel - Re: [PATCH v2] clk: sunxi-ng: sun50i: h6: Modify GPU clock configuration to support DFS

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Message-ID: <4748270.31r3eYUQgx@jernej-laptop>
Date:   Sun, 03 Jul 2022 20:43:48 +0200
From:   Jernej Škrabec <jernej.skrabec@...il.com>
To:     samuel@...lland.org, Roman Stratiienko <r.stratiienko@...il.com>
Cc:     peron.clem@...il.com, 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,
        Roman Stratiienko <r.stratiienko@...il.com>
Subject: Re: [PATCH v2] clk: sunxi-ng: sun50i: h6: Modify GPU clock configuration to support DFS

Dne nedelja, 03. julij 2022 ob 18:45:14 CEST je Roman Stratiienko napisal(a):
> 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   |
> 
> DVFS started to work seamlessly once dividers which caused the
> glitches were set to fixed values.
> 
> Signed-off-by: Roman Stratiienko <r.stratiienko@...il.com>
> 
> ---
> 
> Changelog:
> 
> V2:
> - Drop changes related to mux
> - Drop frequency limiting
> - Add unused dividers initialization
> ---
>  drivers/clk/sunxi-ng/ccu-sun50i-h6.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/clk/sunxi-ng/ccu-sun50i-h6.c
> b/drivers/clk/sunxi-ng/ccu-sun50i-h6.c index 2ddf0a0da526f..1b0205ff24108
> 100644
> --- a/drivers/clk/sunxi-ng/ccu-sun50i-h6.c
> +++ b/drivers/clk/sunxi-ng/ccu-sun50i-h6.c
> @@ -95,13 +95,13 @@ 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 
*/

Having minimum (288 MHz) as per vendor GPU driver and maximum, either max. opp 
or max. from datasheet is equally good. I know that both are basically limited 
with opp table, but people like to play with these, so it's good to have them 
in.

>  	.common		= {
>  		.reg		= 0x030,
>  		.hw.init	= CLK_HW_INIT("pll-gpu", "osc24M",
> @@ -294,9 +294,9 @@ 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);
> 
> +/* Keep GPU_CLK divider const to avoid DFS instability. */
>  static const char * const gpu_parents[] = { "pll-gpu" };
> -static SUNXI_CCU_M_WITH_MUX_GATE(gpu_clk, "gpu", gpu_parents, 0x670,
> -				       0, 3,	/* M */
> +static SUNXI_CCU_MUX_WITH_GATE(gpu_clk, "gpu", gpu_parents, 0x670,
>  				       24, 1,	/* mux */
>  				       BIT(31),	/* gate */
>  				       CLK_SET_RATE_PARENT);
> @@ -1193,6 +1193,16 @@ static int sun50i_h6_ccu_probe(struct platform_device
> *pdev) if (IS_ERR(reg))
>  		return PTR_ERR(reg);
> 
> +	/* Force PLL_GPU output divider to 0 */

Divider  0 here

> +	val = readl(reg + SUN50I_H6_PLL_GPU_REG);
> +	val &= ~BIT(0);
> +	writel(val, reg + SUN50I_H6_PLL_GPU_REG);
> +
> +	/* Force GPU_CLK divider to 0 */

and here sounds wrong, since division by zero is not defined. Using 1 is more 
intuitive and correct, since that's what HW actually uses.

Patch looks good otherwise.

Best regards,
Jernej

> +	val = readl(reg + gpu_clk.common.reg);
> +	val &= ~GENMASK(3, 0);
> +	writel(val, reg + gpu_clk.common.reg);
> +
>  	/* Enable the lock bits on all PLLs */
>  	for (i = 0; i < ARRAY_SIZE(pll_regs); i++) {
>  		val = readl(reg + pll_regs[i]);