Re: [RFC v1 2/7] clk: meson: meson8b: do not use cpu_div3 for cpu_scale_out_sel

[Jerome Brunet <jbrunet@baylibre.com>]

On Wed, 2018-11-14 at 23:57 +0100, Martin Blumenstingl wrote:
> 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>
> ---
>  drivers/clk/meson/meson8b.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/clk/meson/meson8b.c b/drivers/clk/meson/meson8b.c
> index 40e77fe4ba7c..8a3c346e110d 100644
> --- a/drivers/clk/meson/meson8b.c
> +++ b/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,
>  	},
>  };

Reviewed-by: Jerome Brunet <jbrunet@baylibre.com>