[PATCH AUTOSEL 6.16] media: i2c: vd55g1: Setup sensor external clock before patching

[Sasha Levin <sashal@kernel.org>]

From: Benjamin Mugnier <benjamin.mugnier@foss.st.com>

[ Upstream commit df2f8fd91bde57d5d5aca6adddf7e988f2e8c60e ]

Proper clock configuration is required to advance through FSM states.
Prior than this having a different clock value than default sensor's
value was used (12 MHz) could prevent the sensor from booting.

Signed-off-by: Benjamin Mugnier <benjamin.mugnier@foss.st.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

LLM Generated explanations, may be completely bogus:

**Backport Status: YES**

This commit should be backported to stable kernel trees for the
following reasons:

## Bug Fix Analysis

1. **Critical Boot Issue**: The commit fixes a sensor initialization
   failure where the sensor could fail to boot if a non-default clock
   frequency was used. The commit message explicitly states: "Prior than
   this having a different clock value than default sensor's value was
   used (12 MHz) could prevent the sensor from booting."

2. **Clear Root Cause**: The issue is a timing/sequencing problem where
   the external clock must be configured BEFORE the patch is applied to
   allow proper FSM (Finite State Machine) state transitions. The commit
   message states: "Proper clock configuration is required to advance
   through FSM states."

3. **Small and Contained Fix**: The fix is minimal - it simply moves one
   line of code (`vd55g1_write(sensor, VD55G1_REG_EXT_CLOCK,
   sensor->xclk_freq, &ret);`) from the streaming enable function to the
   power-on sequence, specifically between sensor detection and
   patching.

## Code Analysis

Looking at the code changes:
- **REMOVED** from `vd55g1_enable_streams()` at line 1040 (after power-
  on)
- **ADDED** to `vd55g1_power_on()` at line 1614 (after detect, before
  patch)

The new sequence in `vd55g1_power_on()` becomes:
1. Enable power/clock → FSM state: READY_TO_BOOT
2. `vd55g1_detect()` - verify sensor ID
3. **NEW: Configure external clock** ← Critical for FSM advancement
4. `vd55g1_patch()` - apply firmware patch
5. Wait for FSM state: SW_STBY

## Stable Criteria Met

1. **Fixes a real bug**: Yes - sensor initialization failure with non-
   default clocks
2. **Small change**: Yes - single line moved between functions
3. **No new features**: Correct - just fixes sequencing
4. **Low regression risk**: Yes - moves existing code to proper location
5. **Hardware enablement**: Fixes broken hardware initialization for
   VD55G1 sensor
6. **Recently added driver**: The driver was added in April 2025 (commit
   e56616d7b23c), making this an early fix for a new driver that likely
   needs to work properly in stable kernels

The fix addresses a fundamental initialization issue that would prevent
the sensor from working with any clock frequency other than the default
12 MHz, making it a critical fix for proper hardware support.

 drivers/media/i2c/vd55g1.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/media/i2c/vd55g1.c b/drivers/media/i2c/vd55g1.c
index 25e2fc88a036..8552ce75e1aa 100644
--- a/drivers/media/i2c/vd55g1.c
+++ b/drivers/media/i2c/vd55g1.c
@@ -1038,8 +1038,6 @@ static int vd55g1_enable_streams(struct v4l2_subdev *sd,
 	if (ret < 0)
 		return ret;
 
-	vd55g1_write(sensor, VD55G1_REG_EXT_CLOCK, sensor->xclk_freq, &ret);
-
 	/* Configure output */
 	vd55g1_write(sensor, VD55G1_REG_MIPI_DATA_RATE,
 		     sensor->mipi_rate, &ret);
@@ -1613,6 +1611,9 @@ static int vd55g1_power_on(struct device *dev)
 		goto disable_clock;
 	}
 
+	/* Setup clock now to advance through system FSM states */
+	vd55g1_write(sensor, VD55G1_REG_EXT_CLOCK, sensor->xclk_freq, &ret);
+
 	ret = vd55g1_patch(sensor);
 	if (ret) {
 		dev_err(dev, "Sensor patch failed %d\n", ret);
-- 
2.39.5