Re: [PATCH v12 4/7] drm/meson: gate px_clk when setting rate

From: Neil Armstrong
Date: Mon Apr 22 2024 - 12:51:50 EST


Hi Martin,

On 10/04/2024 21:34, Martin Blumenstingl wrote:
Hi Neil,

On Wed, Apr 3, 2024 at 9:46 AM Neil Armstrong <neil.armstrong@xxxxxxxxxx> 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@xxxxxxxxxx>
---
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.

I checked and tested your patches and it doesn't work because the pc_clk
needs to be disabled & prepared in order to correctly reset and setup again
the video clock tree.

dw_mipi_dsi_phy_init() is called at each DSI mode change, but it requires a
full clock tree recalc and reset, so it's safer to keep the current design.

I'll try to send a change to better handle the disable_unprepare() failure, but
definitely only calling clk_disable() wasn't enough.

Thanks,
Neil



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