v4l2: add QUERYCTRL/QUERYMENU capability-probe helpers

Pure utility additions, no behaviour change. Three helpers in
src/v4l2.{c,h}:

  - v4l2_query_ext_ctrl(): wraps VIDIOC_QUERY_EXT_CTRL by CID.
    Returns 0 if the control exists, -1 if not. Caller passes NULL
    qec to test existence only.

  - v4l2_query_menu(): wraps VIDIOC_QUERYMENU at a given index.
    Returns 0 if a menu item exists at that index, -1 otherwise.

  - v4l2_ctrl_menu_has_value(): convenience layered on the above.
    For a menu/intmenu-type control, walks all menu items between
    minimum and maximum and returns true iff `value` is a valid
    entry. Used by callers that ask "does this driver accept menu
    value X for this CID?" without caring about iteration details.

These unblock commit 3 (request_pool — needs ext-ctrl probing for
codec-ops dispatch) and commit 4 (probe-then-set DECODE_MODE/
START_CODE — replaces 0002's unconditional set with a real probe)
of the upstreamable design's six-commit series.

Forward-declarations in v4l2.h keep the header lean: existing
prototypes already use opaque struct v4l2_ext_control * pointers
without including <linux/videodev2.h>; we follow the same
convention for struct v4l2_query_ext_ctrl and struct v4l2_querymenu.

No call sites added in this commit. Compile-only verification:
the .so links cleanly with three new exports.

Signed-off-by: Markus Fritsche <fritsche.markus@gmail.com>

src/v4l2.c

@@ -508,3 +508,63 @@ int v4l2_set_stream(int video_fd, unsigned int type, bool enable)
 
 	return 0;
 }
+
+int v4l2_query_ext_ctrl(int video_fd, unsigned int id,
+			struct v4l2_query_ext_ctrl *qec)
+{
+	struct v4l2_query_ext_ctrl local;
+	struct v4l2_query_ext_ctrl *target = qec ? qec : &local;
+	int rc;
+
+	memset(target, 0, sizeof(*target));
+	target->id = id;
+
+	rc = ioctl(video_fd, VIDIOC_QUERY_EXT_CTRL, target);
+	if (rc < 0)
+		return -1;
+
+	return 0;
+}
+
+int v4l2_query_menu(int video_fd, unsigned int id, unsigned int index,
+		    struct v4l2_querymenu *qm)
+{
+	int rc;
+
+	if (qm == NULL)
+		return -1;
+
+	memset(qm, 0, sizeof(*qm));
+	qm->id = id;
+	qm->index = index;
+
+	rc = ioctl(video_fd, VIDIOC_QUERYMENU, qm);
+	if (rc < 0)
+		return -1;
+
+	return 0;
+}
+
+bool v4l2_ctrl_menu_has_value(int video_fd, unsigned int id,
+			      unsigned int value)
+{
+	struct v4l2_query_ext_ctrl qec;
+	struct v4l2_querymenu qm;
+	long long i;
+
+	if (v4l2_query_ext_ctrl(video_fd, id, &qec) < 0)
+		return false;
+
+	if (qec.type != V4L2_CTRL_TYPE_MENU &&
+	    qec.type != V4L2_CTRL_TYPE_INTEGER_MENU)
+		return false;
+
+	for (i = qec.minimum; i <= qec.maximum; i += qec.step ? qec.step : 1) {
+		if (v4l2_query_menu(video_fd, id, (unsigned int)i, &qm) < 0)
+			continue;
+		if ((unsigned int)i == value)
+			return true;
+	}
+
+	return false;
+}

src/v4l2.h

@@ -64,4 +64,37 @@ int v4l2_set_control(int video_fd, int request_fd, unsigned int id, void *data,
 		     unsigned int size);
 int v4l2_set_stream(int video_fd, unsigned int type, bool enable);
 
+/*
+ * Capability-probe helpers. These let calling code discover what the
+ * backing kernel driver supports rather than hardcoding assumptions
+ * about specific decoder hardware.
+ */
+
+/*
+ * Query the metadata of an extended control by CID. Fills *qec on
+ * success. Returns 0 if the control exists, -1 (errno=EINVAL) if the
+ * driver does not expose this CID. Pass qec=NULL to test existence
+ * only.
+ */
+struct v4l2_query_ext_ctrl;
+int v4l2_query_ext_ctrl(int video_fd, unsigned int id,
+			struct v4l2_query_ext_ctrl *qec);
+
+/*
+ * Query a single menu item of a menu/intmenu control at the given
+ * index. Fills *qm on success. Returns 0 if the menu item exists at
+ * this index, -1 otherwise.
+ */
+struct v4l2_querymenu;
+int v4l2_query_menu(int video_fd, unsigned int id, unsigned int index,
+		    struct v4l2_querymenu *qm);
+
+/*
+ * Convenience: for a menu-type control, return true iff `value` is a
+ * valid menu entry (i.e. the driver accepts it). Walks all menu items
+ * up to the control's maximum to check.
+ */
+bool v4l2_ctrl_menu_has_value(int video_fd, unsigned int id,
+			      unsigned int value);
+
 #endif