kernel: register H.264 DECODE_MODE + START_CODE menu controls

libva-v4l2-request sets V4L2_CID_STATELESS_H264_DECODE_MODE and
V4L2_CID_STATELESS_H264_START_CODE on the device fd at context init
(see libva-v4l2-request-fourier src/context.c:577 — best-effort call,
result is (void)cast).  Our ctrl_handler did not advertise either
control, so v4l2-core returned EINVAL on validate; userspace logged
the noisy

    v4l2-request: Unable to set control(s): Invalid argument
                  (error_idx=2/2 ioctl-level)

at every Firefox/ffmpeg context creation, despite decode itself
succeeding (the daemon already operates as FRAME_BASED + ANNEX_B and
the per-request SPS/PPS/SCALING_MATRIX/DECODE_PARAMS batch lands
fine).

Register the two as v4l2_ctrl_new_std_menu with the only value each
the daemon actually supports — FRAME_BASED for DECODE_MODE,
ANNEX_B for START_CODE — and mask out the unsupported alternates
(SLICE_BASED, NONE).  Pattern matches rkvdec / hantro.  Update the
handler-init capacity hint to ARRAY_SIZE(daedalus_stateless_ctrls)
+ 2 to cover the additions.

Verified: builds clean on 6.18.29+rpt-rpi-2712 (Pi CM5) DKMS source
tree.

kernel/daedalus_v4l2_main.c

@@ -287,6 +287,44 @@ static int daedalus_register_stateless_ctrls(struct v4l2_ctrl_handler *hdl)
 		}
 		(void) ctrl;
 	}
+
+	/*
+	 * Device-wide H.264 mode controls.  libva-v4l2-request sets these
+	 * on the device fd (request_fd=-1) at context init via
+	 * VIDIOC_S_EXT_CTRLS before any per-request controls are bound;
+	 * without entries on our ctrl_handler v4l2-core returns EINVAL
+	 * and userspace logs a noisy "Unable to set control(s)" warning
+	 * (cosmetic — libva already treats this as best-effort).  Expose
+	 * the single value each that the daemon actually accepts:
+	 *
+	 *   DECODE_MODE: FRAME_BASED only — the daemon receives a full
+	 *     frame's worth of slice data per REQ_DECODE and calls
+	 *     avcodec_send_packet / avcodec_receive_frame once per
+	 *     submission; partial-slice (SLICE_BASED) decode is not
+	 *     wired into the daemon pipeline.
+	 *   START_CODE: ANNEX_B only — the daemon's H.264 SPS/PPS
+	 *     synthesiser prepends 0x00000001-delimited NAL units
+	 *     (Annex B); a NONE start-code variant would need a
+	 *     separate emit path.
+	 *
+	 * Pattern matches rkvdec / hantro (skip_mask = BIT(unsupported)).
+	 */
+	v4l2_ctrl_new_std_menu(hdl, &daedalus_ctrl_ops,
+			       V4L2_CID_STATELESS_H264_DECODE_MODE,
+			       V4L2_STATELESS_H264_DECODE_MODE_FRAME_BASED,
+			       BIT(V4L2_STATELESS_H264_DECODE_MODE_SLICE_BASED),
+			       V4L2_STATELESS_H264_DECODE_MODE_FRAME_BASED);
+	v4l2_ctrl_new_std_menu(hdl, &daedalus_ctrl_ops,
+			       V4L2_CID_STATELESS_H264_START_CODE,
+			       V4L2_STATELESS_H264_START_CODE_ANNEX_B,
+			       BIT(V4L2_STATELESS_H264_START_CODE_NONE),
+			       V4L2_STATELESS_H264_START_CODE_ANNEX_B);
+	if (hdl->error) {
+		pr_debug("daedalus_v4l2: H.264 menu ctrls registration err=%d\n",
+			 hdl->error);
+		hdl->error = 0;
+	}
+
 	return 0;
 }
 
@@ -1130,7 +1168,10 @@ static int daedalus_open(struct file *file)
 	v4l2_fh_init(&ctx->fh, &dev->vdev);
 	file->private_data = &ctx->fh;
 
-	v4l2_ctrl_handler_init(&ctx->hdl, ARRAY_SIZE(daedalus_stateless_ctrls));
+	/* +2 covers the H.264 DECODE_MODE + START_CODE menu controls
+	 * registered alongside daedalus_stateless_ctrls[]. */
+	v4l2_ctrl_handler_init(&ctx->hdl,
+			       ARRAY_SIZE(daedalus_stateless_ctrls) + 2);
 	daedalus_register_stateless_ctrls(&ctx->hdl);
 	/*
 	 * v4l2_ctrl_handler_setup runs s_ctrl for every registered