linux-kernel - Re: [PATCH v12 4/7] drm/meson: gate px

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Date: Wed, 10 Apr 2024 21:34:56 +0200
From: Martin Blumenstingl <martin.blumenstingl@...glemail.com>
To: Neil Armstrong <neil.armstrong@...aro.org>
Cc: Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>, Conor Dooley <conor+dt@...nel.org>, 
	Jerome Brunet <jbrunet@...libre.com>, Kevin Hilman <khilman@...libre.com>, 
	Michael Turquette <mturquette@...libre.com>, Stephen Boyd <sboyd@...nel.org>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>, 
	Jagan Teki <jagan@...rulasolutions.com>, Nicolas Belin <nbelin@...libre.com>, 
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-amlogic@...ts.infradead.org, linux-clk@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v12 4/7] drm/meson: gate px_clk when setting rate

Hi Neil,

On Wed, Apr 3, 2024 at 9:46 AM Neil Armstrong <neil.armstrong@...aro.org> wrote:
>
> Disable the px_clk when setting the rate to recover a fully
> configured and correctly reset VCLK clock tree after the rate
> is set.
>
> Fixes: 77d9e1e6b846 ("drm/meson: add support for MIPI-DSI transceiver")
> Signed-off-by: Neil Armstrong <neil.armstrong@...aro.org>
> ---
>  drivers/gpu/drm/meson/meson_dw_mipi_dsi.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/drivers/gpu/drm/meson/meson_dw_mipi_dsi.c b/drivers/gpu/drm/meson/meson_dw_mipi_dsi.c
> index a6bc1bdb3d0d..a10cff3ca1fe 100644
> --- a/drivers/gpu/drm/meson/meson_dw_mipi_dsi.c
> +++ b/drivers/gpu/drm/meson/meson_dw_mipi_dsi.c
> @@ -95,6 +95,7 @@ static int dw_mipi_dsi_phy_init(void *priv_data)
>                 return ret;
>         }
>
> +       clk_disable_unprepare(mipi_dsi->px_clk);
nit-pick: clk_disable(mipi_dsi->px_clk); should be enough here as my
understanding is that we only need to {un,}prepare a clock once.

>         ret = clk_set_rate(mipi_dsi->px_clk, mipi_dsi->mode->clock * 1000);
>
>         if (ret) {
> @@ -103,6 +104,12 @@ static int dw_mipi_dsi_phy_init(void *priv_data)
>                 return ret;
>         }
>
> +       ret = clk_prepare_enable(mipi_dsi->px_clk);
> +       if (ret) {
> +               dev_err(mipi_dsi->dev, "Failed to enable DSI Pixel clock (ret %d)\n", ret);
> +               return ret;
If we ever hit this error case then there will be a lot of additional
errors in the kernel log:
- initially the clock is prepared and enabled in
meson_dw_mipi_dsi_probe() by calling devm_clk_get_enabled()
- we then disable the clock above (generally disabling a clock is
expected to always succeed)
- if the clock can NOT be re-enabled here we just log the error
- in case a user tries to rmmod the driver (to modprobe it again) to
try and recover from an error the automatic disabling of the pix_clk
(based on devm_clk_get_enabled() where it was enabled initially) there
will be a splat because the clock is already disabled (and enabled
count is zero, so it cannot be disabled any further)

For the 32-bit SoC video clocks I keep track of them being enabled or
disabled, see [0], [1] and [2].
In my case this is important because we can run into cases where the
PLL doesn't lock (I am not sure how likely this is for your case).

It *seems* like we need to do something similar as
dw_mipi_dsi_phy_init() can be called when changing the display
resolution (or whenever drm_bridge_funcs.atomic_pre_enable) is called.
To illustrate what I have in mind I attached a diff (it's based on
this patch) - it's compile tested only as I have no DSI hardware.
In case dw_mipi_dsi_phy_init() is called only once per device
lifecycle things may get easier.

PS: I'm so happy that we don't need any clock notifiers for this!
So: good work with the clock driver bits.


Let me know what you think,
Martin


[0] https://github.com/xdarklight/linux/blob/meson-mx-integration-6.9-20240323/drivers/gpu/drm/meson/meson_vclk.c#L1177-L1179
[1] https://github.com/xdarklight/linux/blob/meson-mx-integration-6.9-20240323/drivers/gpu/drm/meson/meson_vclk.c#L1077
[2] https://github.com/xdarklight/linux/blob/meson-mx-integration-6.9-20240323/drivers/gpu/drm/meson/meson_vclk.c#L1053

View attachment "meson_dw_mipi_dsi-clk-disable-enable.diff" of type "text/x-patch" (1837 bytes)