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

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Message-ID: <1jplj3g21q.fsf@starbuckisacylon.baylibre.com>
Date: Thu, 27 Feb 2025 10:38:41 +0100
From: Jerome Brunet <jbrunet@...libre.com>
To: <linux@...tijnvandeventer.nl>
Cc: "'Neil Armstrong'" <neil.armstrong@...aro.org>,  "'Michael Turquette'"
 <mturquette@...libre.com>,  "'Stephen Boyd'" <sboyd@...nel.org>,  "'Kevin
 Hilman'" <khilman@...libre.com>,  "'Martin Blumenstingl'"
 <martin.blumenstingl@...glemail.com>,
  <linux-amlogic@...ts.infradead.org>,  <linux-clk@...r.kernel.org>,
  <linux-arm-kernel@...ts.infradead.org>,  <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] clk: meson: g12a: Fix kernel warnings when no display
 attached

On Wed 26 Feb 2025 at 21:39, <linux@...tijnvandeventer.nl> wrote:

> Hi Jerome,
>
> Thank you for reviewing, and apologies for my late response due to a holiday.
>
>> On Thu 13 Feb 2025 at 23:17, Martijn van Deventer
>> <linux@...tijnvandeventer.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.
>
> I know and I totally agree. Unfortunately, I don't have access to any vendor 
> documentation, nor do I have any real knowledge about the DRM/HDMI 
> subsystem to fix that.

You have identified which clocks are not properly claimed, by what they
are not claimed and even when. 50% of the job is done. Thanks for this.

>
> And I guess if it were as easy as adding a clock to the DT and calling 
> clk_prepare_enable on it in the probe function, Neil would have done that 
> already.
>
> So, all I can do, for now, is revert to the previous situation when it did  work
> for (probably) most boards.

Maybe so, but it does not make this change appropriate. The problem
is the DRM driver which does not enable what it needs to properly
operate. This should be fixed.

>
>> >
>> > Fixes: b70cb1a21a54 ("clk: meson: g12a: make VCLK2 and ENCL clock path
>> configurable by CCF").
>> > Signed-off-by: Martijn van Deventer <linux@...tijnvandeventer.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