iter2 step3: HEVC EXT_SPS_*_RPS UAPI header + runtime probe

src/hevc-ctrls/v4l2-hevc-ext-controls.h (NEW, MIT, ~95 LOC):
  Verbatim mirror of Linux 7.0 V4L2_CID_STATELESS_HEVC_EXT_SPS_ST_RPS
  and _LT_RPS control IDs + struct definitions + flag macros. Each
  symbol is ifndef-guarded so when ampere's linux-api-headers
  eventually bumps to 7.0+, the kernel header takes precedence and
  this shim silently no-ops. Citation block links the upstream
  Casanova v8 series.

  Per LGPL section 3.b, kernel UAPI struct definitions are excepted
  from GPL inheritance, so copying them into MIT userspace is fine.

src/request.h: added has_hevc_ext_sps_rps_rkvdec + _hantro bool
  fields on struct request_data — pair-of-flags layout mirrors
  video_fd_rkvdec / video_fd_hantro (iter38 multi-device-probe
  pattern, per feedback_multi_device_probe_design). Phase 5 review
  identified single-scalar storage as a silent-misbehavior risk
  across device-switch boundaries.

src/request.c:
  - new probe_hevc_ext_sps_rps_controls(fd) helper: queries the two
    new CIDs via VIDIOC_QUERYCTRL; returns true iff both register.
    RK3399 rkvdec (linux 6.x or 7.x without VDPU381/383 bindings)
    returns false; RK3588 rkvdec (VDPU381/383) returns true.
  - probe each driver_data->video_fd_rkvdec / _hantro after the
    iter38 multi-device-probe block at VA_DRIVER_INIT time
  - log-line if rkvdec supports it - diagnostic for Phase 7

src/meson.build: added the new UAPI header to the headers list.

Build verified: ninja -C build clean, .so produced. The new probe
runs at driver init and stores the result, but nothing CONSUMES the
result yet — that's Step 4 (h265_set_controls wiring).

Per ampere-kernel-decoders campaign iter2 Phase 4 step 3 (amended
by Phase 5 review item 'per-fd storage').

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/request.c

@@ -57,6 +57,8 @@
 #include <linux/media.h>
 #include <linux/videodev2.h>
 
+#include "hevc-ctrls/v4l2-hevc-ext-controls.h"
+
 /*
  * fresnel-fourier iter4 Phase 6 commit Z + iter7 Phase 6 (B1a): device-path
  * auto-detect via media controller topology with decoder-entity discrimination.
@@ -286,6 +288,43 @@ out:
  *   - non-NULL → match only that exact driver name
  *   - NULL     → match any name in known_decoder_drivers[]
  */
+/*
+ * iter2 (ampere-kernel-decoders campaign) — runtime probe for the
+ * V4L2 stateless HEVC EXT_SPS_{ST,LT}_RPS controls added in
+ * Linux 7.0 (Casanova VDPU381/VDPU383 series). Returns true iff BOTH
+ * controls are registered on the given fd. Stored per-fd on
+ * driver_data so the multi-device-probe model (iter38) doesn't
+ * silently misbehave when codec routing switches devices.
+ *
+ * The two CIDs together are the gate — neither alone is meaningful
+ * without the other (st-RPS + lt-RPS arrays both need to be set to
+ * match the SPS num_short_term_ref_pic_sets / num_long_term_ref_pics_sps
+ * counts). Old kernels (RK3399 rkvdec on linux 6.x) register neither;
+ * RK3588 rkvdec (VDPU381/383 path) registers both.
+ *
+ * Reference: phase4_plan_iter2.md §Step 3 in
+ * ~/src/ampere-kernel-decoders/.
+ */
+static bool probe_hevc_ext_sps_rps_controls(int video_fd)
+{
+	struct v4l2_queryctrl q;
+
+	if (video_fd < 0)
+		return false;
+
+	memset(&q, 0, sizeof(q));
+	q.id = V4L2_CID_STATELESS_HEVC_EXT_SPS_ST_RPS;
+	if (ioctl(video_fd, VIDIOC_QUERYCTRL, &q) < 0)
+		return false;
+
+	memset(&q, 0, sizeof(q));
+	q.id = V4L2_CID_STATELESS_HEVC_EXT_SPS_LT_RPS;
+	if (ioctl(video_fd, VIDIOC_QUERYCTRL, &q) < 0)
+		return false;
+
+	return true;
+}
+
 static int find_decoder_device_by_driver(const char *want_driver,
 					 char *video_out, size_t video_out_sz,
 					 char *media_out, size_t media_out_sz)
@@ -644,6 +683,22 @@ VAStatus VA_DRIVER_INIT_FUNC(VADriverContextP context)
 		(void)primary_driver;
 	}
 
+	/*
+	 * iter2 (ampere-kernel-decoders): probe the new HEVC EXT_SPS_RPS
+	 * controls on each rkvdec/hantro fd. Result is consumed by
+	 * h265_set_controls per-codec gate. Per-fd storage matches the
+	 * iter38 multi-device-probe pattern (Phase 5 review item).
+	 */
+	driver_data->has_hevc_ext_sps_rps_rkvdec =
+		probe_hevc_ext_sps_rps_controls(driver_data->video_fd_rkvdec);
+	driver_data->has_hevc_ext_sps_rps_hantro =
+		probe_hevc_ext_sps_rps_controls(driver_data->video_fd_hantro);
+	if (driver_data->has_hevc_ext_sps_rps_rkvdec) {
+		request_log("iter2: kernel registers HEVC EXT_SPS_{ST,LT}_RPS "
+			    "controls on rkvdec fd (will route through "
+			    "vendored GStreamer parser)\n");
+	}
+
 	status = VA_STATUS_SUCCESS;
 	goto complete;