v4l2: surface error_idx + errno on VIDIOC_S_EXT_CTRLS failure

ampere-av1 Phase 2.1 + 3 diagnostic: log which control failed validation
on S_EXT_CTRLS rejection so debug iterations can identify the offending
CID without strace. Pre-validation failures (error_idx >= count) log as
"<pre-validation>" with the syscall errno surfacing the root reason.

Already informative on ampere — surfaces the pre-existing benign H264 +
HEVC device-init failures on the vpu981 AV1 fd as count=2 / failed_cid=0
(those go through (void)cast at context.c:450/473 by design).

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

src/v4l2.c

@@ -433,6 +433,7 @@ static int v4l2_ioctl_controls(int video_fd, int request_fd, unsigned long ioc,
 			       unsigned int num_controls)
 {
 	struct v4l2_ext_controls controls;
+	int rc;
 
 	memset(&controls, 0, sizeof(controls));
 
@@ -444,7 +445,28 @@ static int v4l2_ioctl_controls(int video_fd, int request_fd, unsigned long ioc,
 		controls.request_fd = request_fd;
 	}
 
-	return ioctl(video_fd, ioc, &controls);
+	rc = ioctl(video_fd, ioc, &controls);
+	if (rc < 0) {
+		/* ampere-av1 Phase 2.1 diag: surface error_idx so the caller's
+		 * error path knows which CID failed validation. error_idx >=
+		 * count means the failure was pre-validation (e.g., bad
+		 * request_fd). errno carries the syscall-level reason. */
+		const char *failed_cid_label = "<pre-validation>";
+		unsigned int failed_size = 0;
+		if (controls.error_idx < num_controls) {
+			failed_size = control_array[controls.error_idx].size;
+			(void)failed_cid_label;  /* keep symbol if logger truncates */
+		}
+		request_log("v4l2_ioctl_controls: rc=%d errno=%d (%s) "
+			    "ioc=0x%lx error_idx=%u count=%u "
+			    "failed_cid=0x%x failed_size=%u\n",
+			    rc, errno, strerror(errno), ioc,
+			    controls.error_idx, num_controls,
+			    controls.error_idx < num_controls
+			        ? control_array[controls.error_idx].id : 0,
+			    failed_size);
+	}
+	return rc;
 }
 
 int v4l2_get_controls(int video_fd, int request_fd,