RSDSO-21151: Make HW reset SERDES recovery conditional (light path)

When the GMSL link recovers naturally after HW reset (the common case
for D457), skip SERDES recovery and stability verification entirely.
This eliminates ~1300ms of unnecessary overhead per reset that was
causing timing-sensitive CI test failures (rec-play-got-playback-frames,
t2ff-sensor, hdr-performance).

Changes:
- Step 6: Probe I2C link after Step 5 polling succeeds. If alive, skip
  ds5_hw_reset_serdes_recovery() (no max9295_init_settings, no Phase 1
  stability reads). Only invoke SERDES recovery when I2C is actually
  broken post-reset (e.g. D401 secondary FW init phase).
- Step 10: Skip the 3x200ms stability verification loop and Phase 2
  escalation when SERDES recovery was not performed. Steps 7-9 already
  validate the link via multiple I2C reads.
- Reduce DS5_HW_RESET_COOLDOWN_MS from 5000ms to 2000ms. With
  conditional recovery eliminating unnecessary SERDES disruption, the
  aggressive cooldown is no longer needed for the common case.

v1.0.1.33 (all CI tests passing) used fire-and-forget HW reset with
zero SERDES intervention. This change restores that lightweight behavior
for the normal path while keeping full tiered recovery as a fallback
when the link is genuinely broken.

Author: sareluzi

kernel/realsense/d4xx.c

@@ -2323,7 +2323,7 @@ static int ds5_set_calibration_data(struct ds5 *state,
  * serializer re-init (Phase 1) conflicts with the camera FW's own
  * I2C bus reconfiguration after reboot.
  */
-#define DS5_HW_RESET_COOLDOWN_MS	5000
+#define DS5_HW_RESET_COOLDOWN_MS	2000
 
 /*
  * Register 0x5020 status values (from firmware):
@@ -2641,6 +2641,9 @@ static int ds5_hw_reset_with_recovery(struct ds5 *state)
 	int retry;
 	int __maybe_unused i;
 	u16 status = 0;
+#ifdef CONFIG_VIDEO_D4XX_SERDES
+	bool serdes_recovery_ran = false;
+#endif
 
 	dev_info(&state->client->dev, "%s(): Initiating HW reset with recovery\n",
 		__func__);
@@ -2842,15 +2845,37 @@ static int ds5_hw_reset_with_recovery(struct ds5 *state)
 	}
 
 #ifdef CONFIG_VIDEO_D4XX_SERDES
-	/* 6. Tiered SERDES recovery:
-	 *    Phase 1: serializer-only re-init (per-camera, non-disruptive).
-	 *    Phase 2: full deserializer reset only if ALL siblings are also dead.
+	/* 6. Tiered SERDES recovery (conditional).
+	 *    Step 5 polled the device over I2C until 0xDEAD was returned,
+	 *    which means the GMSL link was already up at that point.  Probe
+	 *    the I2C link once more: if it is still alive, the GMSL link
+	 *    recovered naturally and SERDES re-init is unnecessary (this is
+	 *    the common case for D457).  Only invoke SERDES recovery when
+	 *    the link is actually broken — e.g. the camera FW's secondary
+	 *    init phase dropped the bus between Step 5 and now (observed on
+	 *    D401, occasionally on D457).
 	 */
-	ret = ds5_hw_reset_serdes_recovery(state, false);
-	if (ret < 0) {
-		dev_err(&state->client->dev,
-			"%s(): SERDES recovery failed: %d\n", __func__, ret);
-		return ret;
+	{
+		u16 link_probe = 0;
+
+		ret = ds5_read(state, DS5_FW_VERSION, &link_probe);
+		if (ret < 0 || link_probe == 0) {
+			dev_info(&state->client->dev,
+				"%s(): I2C link dead after Step 5 (ret=%d, val=0x%x), running SERDES recovery\n",
+				__func__, ret, link_probe);
+			ret = ds5_hw_reset_serdes_recovery(state, false);
+			if (ret < 0) {
+				dev_err(&state->client->dev,
+					"%s(): SERDES recovery failed: %d\n",
+					__func__, ret);
+				return ret;
+			}
+			serdes_recovery_ran = true;
+		} else {
+			dev_info(&state->client->dev,
+				"%s(): GMSL link recovered naturally (FW ver 0x%04x), skipping SERDES recovery\n",
+				__func__, link_probe);
+		}
 	}
 #endif
 
@@ -2969,7 +2994,14 @@ static int ds5_hw_reset_with_recovery(struct ds5 *state)
 	}
 
 #ifdef CONFIG_VIDEO_D4XX_SERDES
-	/* 10. Post-reset I2C stability verification.
+	/* 10. Post-reset I2C stability verification (conditional).
+	 *     Only needed when SERDES recovery was actively performed in Step 6,
+	 *     because the serializer/deserializer re-init can cause transient
+	 *     I2C instability.  When the GMSL link recovered naturally (Step 6
+	 *     was skipped), Steps 7-9 already verified the link via multiple
+	 *     I2C reads — an additional 600ms+ stability loop is unnecessary
+	 *     and causes timing-sensitive CI test failures.
+	 *
 	 *     Some camera SKUs have a secondary FW initialization phase that
 	 *     briefly drops the I2C bus ~65-110ms after the initial readiness
 	 *     checks (Steps 7-9) pass.  This has been observed on both D401
@@ -2984,7 +3016,11 @@ static int ds5_hw_reset_with_recovery(struct ds5 *state)
 	 *     keep waiting (up to DS5_HW_RESET_STABILITY_TIMEOUT_MS).  If it
 	 *     remains unstable, escalate to Phase 2 (full deserializer reset).
 	 */
-	{
+	if (!serdes_recovery_ran) {
+		dev_info(&state->client->dev,
+			"%s(): SERDES recovery was skipped (natural link recovery), "
+			"bypassing Step 10 stability verification\n", __func__);
+	} else {
 		int stable_count = 0;
 		unsigned long stab_ts = jiffies;
 		unsigned long stab_timeout =
@@ -3057,7 +3093,7 @@ static int ds5_hw_reset_with_recovery(struct ds5 *state)
 						__func__);
 			}
 		}
-	}
+	} /* if (serdes_recovery_ran) */
 #endif
 
 	dev_info(&state->client->dev,