[PATCH v2] media: intel/ipu6: Improve DWC PHY HSFREQRANGE band selection for overlapping ranges

[Marco Nenciarini <mnencia@kcore.it>]

The get_hsfreq_by_mbps() function searches the freqranges[] table
backward (from highest to lowest index). Because adjacent frequency
bands overlap, a data rate that falls in the overlap region always
lands on the higher-indexed band.

For data rates up to 1500 Mbps (index 42) every band uses
osc_freq_target 335. Starting at index 43 (1461-1640 Mbps) the
osc_freq_target drops to 208. A sensor running at 1498 Mbps sits in
the overlap between index 42 (1414-1588, osc 335) and index 43
(1461-1640, osc 208). The backward search picks index 43, programming
the lower osc_freq_target of 208 instead of the optimal 335.

This causes DDL lock instability and CSI-2 CRC errors on affected
configurations, such as the OmniVision OV08X40 sensor on Intel Arrow
Lake platforms (Dell Pro Max 16).

Rewrite get_hsfreq_by_mbps() to select the optimal band:

1. Prefer an exact default_mbps match (returned immediately).
2. Among bands whose min/max range covers the data rate, prefer
   the one with the higher osc_freq_target.
3. If osc_freq_target is equal, prefer the band whose default_mbps
   is closest to the requested rate.

For 1498 Mbps this now correctly selects index 42 (osc_freq_target
335, range 1414-1588) instead of index 43 (osc_freq_target 208,
range 1461-1640).

Fixes: 1e7eeb301696 ("media: intel/ipu6: add the CSI2 DPHY implementation")
Cc: stable@vger.kernel.org
Cc: Sakari Ailus <sakari.ailus@linux.intel.com>
Cc: Bingbu Cao <bingbu.cao@intel.com>
Signed-off-by: Marco Nenciarini <mnencia@kcore.it>
---
Changes in v2:
- Rewrote get_hsfreq_by_mbps() with a proper selection algorithm instead
  of patching after the call, as suggested by Sakari Ailus.
- Added Fixes tag and Cc stable.

 .../media/pci/intel/ipu6/ipu6-isys-dwc-phy.c  | 22 ++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/drivers/media/pci/intel/ipu6/ipu6-isys-dwc-phy.c b/drivers/media/pci/intel/ipu6/ipu6-isys-dwc-phy.c
index db28748..4c9e50c 100644
--- a/drivers/media/pci/intel/ipu6/ipu6-isys-dwc-phy.c
+++ b/drivers/media/pci/intel/ipu6/ipu6-isys-dwc-phy.c
@@ -288,15 +288,27 @@ static const struct dwc_dphy_freq_range freqranges[DPHY_FREQ_RANGE_NUM] = {
 
 static u16 get_hsfreq_by_mbps(u32 mbps)
 {
-	unsigned int i = DPHY_FREQ_RANGE_NUM;
+	int best = DPHY_FREQ_RANGE_INVALID_INDEX;
+	unsigned int i;
 
-	while (i--) {
-		if (freqranges[i].default_mbps == mbps ||
-		    (mbps >= freqranges[i].min && mbps <= freqranges[i].max))
+	for (i = 0; i < DPHY_FREQ_RANGE_NUM; i++) {
+		if (freqranges[i].default_mbps == mbps)
 			return i;
+
+		if (mbps < freqranges[i].min || mbps > freqranges[i].max)
+			continue;
+
+		if (best == DPHY_FREQ_RANGE_INVALID_INDEX ||
+		    freqranges[i].osc_freq_target >
+		    freqranges[best].osc_freq_target ||
+		    (freqranges[i].osc_freq_target ==
+		     freqranges[best].osc_freq_target &&
+		     abs((int)mbps - (int)freqranges[i].default_mbps) <
+		     abs((int)mbps - (int)freqranges[best].default_mbps)))
+			best = i;
 	}
 
-	return DPHY_FREQ_RANGE_INVALID_INDEX;
+	return best;
 }
 
 static int ipu6_isys_dwc_phy_config(struct ipu6_isys *isys,
-- 
2.47.3