Re: [PATCH] clk: meson: g12a: Fix kernel warnings when no display attached

[Jerome Brunet <jbrunet@baylibre.com>]

On Thu 13 Feb 2025 at 23:17, Martijn van Deventer <linux@martijnvandeventer.nl> wrote:

> When booting SM1 or G12A boards without a dislay attached to HDMI,
> the kernel shows the following warning:
>
> [CRTC:46:meson_crtc] vblank wait timed out
> WARNING: CPU: 2 PID: 265 at drivers/gpu/drm/drm_atomic_helper.c:1682 drm_atomic_helper_wait_for_vblanks.part.0+0x240/0x264
> CPU: 2 UID: 0 PID: 265 Comm: setfont Tainted: G         C
> Tainted: [C]=CRAP
> pstate: 60400009 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> pc : drm_atomic_helper_wait_for_vblanks.part.0+0x240/0x264
> lr : drm_atomic_helper_wait_for_vblanks.part.0+0x240/0x264
> Call trace:
>  drm_atomic_helper_wait_for_vblanks.part.0+0x240/0x264
>  drm_atomic_helper_commit_tail_rpm+0x84/0xa0
>  commit_tail+0xa4/0x18c
>  drm_atomic_helper_commit+0x164/0x178
>  drm_atomic_commit+0xb4/0xec
>  drm_client_modeset_commit_atomic+0x210/0x270
>  drm_client_modeset_commit_locked+0x5c/0x188
>  drm_fb_helper_pan_display+0xb8/0x1d4
>  fb_pan_display+0x7c/0x120
>  bit_update_start+0x20/0x48
>  fbcon_switch+0x418/0x54c
>  el0t_64_sync+0x194/0x198
>
> This happens when the kernel disables the unused clocks.
> Sometimes this causes the boot to hang.
>
> By (re)adding the flag CLK_IGNORE_UNUSED to the VCLK2 clocks, these
> clocks will not be disabled.
>
> This partially reverts commit b70cb1a21a54 ("clk: meson: g12a:
> make VCLK2 and ENCL clock path configurable by CCF").

It looks like DRM needs those clock enabled regardless of connection
status on HDMI. Even with this change applied, you would get the same
problem again if the bootloader does not take of turning the clock on,
which is not a given.

CLK_IGNORE_UNUSED gives not guarantee a clock will be enabled or stay
enabled at any point.

A proper fix to this issue should be done in DRM, IMO.

>
> Fixes: b70cb1a21a54 ("clk: meson: g12a: make VCLK2 and ENCL clock path configurable by CCF").
> Signed-off-by: Martijn van Deventer <linux@martijnvandeventer.nl>
> ---
>  drivers/clk/meson/g12a.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/clk/meson/g12a.c b/drivers/clk/meson/g12a.c
> index cfffd434e998..1651898658f5 100644
> --- a/drivers/clk/meson/g12a.c
> +++ b/drivers/clk/meson/g12a.c
> @@ -3234,7 +3234,7 @@ static struct clk_regmap g12a_vclk2_div = {
>  			&g12a_vclk2_input.hw
>  		},
>  		.num_parents = 1,
> -		.flags = CLK_SET_RATE_GATE,
> +		.flags = CLK_SET_RATE_GATE | CLK_IGNORE_UNUSED,
>  	},
>  };
>  
> @@ -3270,7 +3270,7 @@ static struct clk_regmap g12a_vclk2 = {
>  		.ops = &meson_vclk_gate_ops,
>  		.parent_hws = (const struct clk_hw *[]) { &g12a_vclk2_div.hw },
>  		.num_parents = 1,
> -		.flags = CLK_SET_RATE_PARENT,
> +		.flags = CLK_SET_RATE_PARENT | CLK_IGNORE_UNUSED,
>  	},
>  };
>  
> @@ -3354,7 +3354,7 @@ static struct clk_regmap g12a_vclk2_div1 = {
>  		.ops = &clk_regmap_gate_ops,
>  		.parent_hws = (const struct clk_hw *[]) { &g12a_vclk2.hw },
>  		.num_parents = 1,
> -		.flags = CLK_SET_RATE_PARENT,
> +		.flags = CLK_SET_RATE_PARENT | CLK_IGNORE_UNUSED,
>  	},
>  };
>  
> @@ -3368,7 +3368,7 @@ static struct clk_regmap g12a_vclk2_div2_en = {
>  		.ops = &clk_regmap_gate_ops,
>  		.parent_hws = (const struct clk_hw *[]) { &g12a_vclk2.hw },
>  		.num_parents = 1,
> -		.flags = CLK_SET_RATE_PARENT,
> +		.flags = CLK_SET_RATE_PARENT | CLK_IGNORE_UNUSED,
>  	},
>  };
>  
> @@ -3382,7 +3382,7 @@ static struct clk_regmap g12a_vclk2_div4_en = {
>  		.ops = &clk_regmap_gate_ops,
>  		.parent_hws = (const struct clk_hw *[]) { &g12a_vclk2.hw },
>  		.num_parents = 1,
> -		.flags = CLK_SET_RATE_PARENT,
> +		.flags = CLK_SET_RATE_PARENT | CLK_IGNORE_UNUSED,
>  	},
>  };
>  
> @@ -3396,7 +3396,7 @@ static struct clk_regmap g12a_vclk2_div6_en = {
>  		.ops = &clk_regmap_gate_ops,
>  		.parent_hws = (const struct clk_hw *[]) { &g12a_vclk2.hw },
>  		.num_parents = 1,
> -		.flags = CLK_SET_RATE_PARENT,
> +		.flags = CLK_SET_RATE_PARENT | CLK_IGNORE_UNUSED,
>  	},
>  };

-- 
Jerome

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