tech@,

here the last step in refactoring: replace magic constant by offest and
sizeof.

Tested on all my devices, no regression.

Ok?

Index: sys/dev/usb/uvideo.c
===================================================================
RCS file: /cvs/src/sys/dev/usb/uvideo.c,v
diff -u -p -r1.264 uvideo.c
--- sys/dev/usb/uvideo.c	4 Sep 2025 07:43:29 -0000	1.264
+++ sys/dev/usb/uvideo.c	4 Sep 2025 07:44:32 -0000
@@ -1046,45 +1046,33 @@ uvideo_vs_parse_desc_format(struct uvide
 		/* Crossed device function boundary. */
 		if (desc->bDescriptorType == UDESC_IFACE_ASSOC)
 			break;
-		if (desc->bDescriptorType != UDESC_CS_INTERFACE) {
-			desc = usbd_desc_iter_next(&iter);
-			continue;
-		}
+
+		if (desc->bDescriptorType != UDESC_CS_INTERFACE)
+			goto next;
+
+		if (desc->bLength != UVIDEO_FORMAT_LEN(desc))
+			goto next;
 
 		switch (desc->bDescriptorSubtype) {
 		case UDESCSUB_VS_COLORFORMAT:
-			if (desc->bLength == 6) {
-				uvideo_vs_parse_desc_colorformat(
-				    sc, desc);
-			}
+			uvideo_vs_parse_desc_colorformat(sc, desc);
 			break;
 		case UDESCSUB_VS_FORMAT_MJPEG:
-			if (desc->bLength == 11) {
-				uvideo_vs_parse_desc_format_mjpeg(
-				    sc, desc);
-			}
+			uvideo_vs_parse_desc_format_mjpeg(sc, desc);
 			break;
 		case UDESCSUB_VS_FORMAT_UNCOMPRESSED:
-			if (desc->bLength == 27) {
-				uvideo_vs_parse_desc_format_uncompressed(
-				    sc, desc);
-			}
+			uvideo_vs_parse_desc_format_uncompressed(sc, desc);
 			break;
 		case UDESCSUB_VS_FORMAT_FRAME_BASED:
-			if (desc->bLength == 28) {
-				uvideo_vs_parse_desc_format_frame_based(
-				    sc, desc);
-			}
+			uvideo_vs_parse_desc_format_frame_based(sc, desc);
 			break;
 		case UDESCSUB_VS_FORMAT_H264:
 		case UDESCSUB_VS_FORMAT_H264_SIMULCAST:
-			if (desc->bLength == 52) {
-				uvideo_vs_parse_desc_format_h264(
-				    sc, desc);
-			}
+			uvideo_vs_parse_desc_format_h264(sc, desc);
 			break;
 		}
 
+next:
 		desc = usbd_desc_iter_next(&iter);
 	}
 
@@ -2873,7 +2861,7 @@ uvideo_dump_desc_all(struct uvideo_softc
 			case UDESCSUB_VC_SELECTOR_UNIT:
 				printf("bDescriptorSubtype=0x%02x",
 				    desc->bDescriptorSubtype);
-				if (desc->bLength == 27) {
+				if (desc->bLength == UVIDEO_FORMAT_LEN(desc)) {
 					printf(" (UDESCSUB_VS_FORMAT_"
 					    "UNCOMPRESSED)\n");
 					uvideo_dump_desc_format_uncompressed(
@@ -2901,7 +2889,7 @@ uvideo_dump_desc_all(struct uvideo_softc
 			case UDESCSUB_VC_EXTENSION_UNIT:
 				printf("bDescriptorSubtype=0x%02x",
 				    desc->bDescriptorSubtype);
-				if (desc->bLength == 11) {
+				if (desc->bLength == UVIDEO_FORMAT_LEN(desc)) {
 					printf(" (UDESCSUB_VS_FORMAT_MJPEG)\n");
 					printf("|\n");
 					uvideo_dump_desc_format_mjpeg(sc, desc);
@@ -2925,7 +2913,7 @@ uvideo_dump_desc_all(struct uvideo_softc
 				printf("bDescriptorSubtype=0x%02x",
 				    desc->bDescriptorSubtype);
 				printf(" (UDESCSUB_VS_FORMAT_FRAME_BASED)\n");
-				if (desc->bLength == 28) {
+				if (desc->bLength == UVIDEO_FORMAT_LEN(desc)) {
 					printf("|\n");
 					uvideo_dump_desc_format_frame_based(sc, desc);
 				}
@@ -2943,7 +2931,7 @@ uvideo_dump_desc_all(struct uvideo_softc
 				printf("bDescriptorSubtype=0x%02x",
 				    desc->bDescriptorSubtype);
 				printf(" (UDESCSUB_VS_FORMAT_H264)\n");
-				if (desc->bLength == 52) {
+				if (desc->bLength == UVIDEO_FORMAT_LEN(desc)) {
 					printf("|\n");
 					uvideo_dump_desc_format_h264(sc, desc);
 				}
@@ -2961,7 +2949,7 @@ uvideo_dump_desc_all(struct uvideo_softc
 				printf("bDescriptorSubtype=0x%02x",
 				    desc->bDescriptorSubtype);
 				printf(" (UDESCSUB_VS_FORMAT_H264_SIMULCAST)\n");
-				if (desc->bLength == 52) {
+				if (desc->bLength == UVIDEO_FORMAT_LEN(desc)) {
 					printf("|\n");
 					uvideo_dump_desc_format_h264(sc, desc);
 				}
Index: sys/dev/usb/uvideo.h
===================================================================
RCS file: /cvs/src/sys/dev/usb/uvideo.h,v
diff -u -p -r1.70 uvideo.h
--- sys/dev/usb/uvideo.h	4 Sep 2025 07:43:29 -0000	1.70
+++ sys/dev/usb/uvideo.h	4 Sep 2025 07:44:32 -0000
@@ -589,6 +589,25 @@ struct usb_video_format_desc {
 			uWord	wMaxMBperSecFourResolutionsFullScalability;
 		} h264;
 	} u;
+
+#define UVIDEO_FORMAT_LEN(fmt)							\
+	(									\
+	(((fmt)->bDescriptorSubtype == UDESCSUB_VS_FORMAT_H264) ||		\
+	 ((fmt)->bDescriptorSubtype == UDESCSUB_VS_FORMAT_H264_SIMULCAST)) ?	\
+		(offsetof(struct usb_video_format_desc, u) +			\
+		 sizeof(((struct usb_video_format_desc *)0)->u.h264)) :		\
+	((fmt)->bDescriptorSubtype == UDESCSUB_VS_FORMAT_FRAME_BASED) ?		\
+		(offsetof(struct usb_video_format_desc, u) +			\
+		 sizeof(((struct usb_video_format_desc *)0)->u.fb)) :		\
+	((fmt)->bDescriptorSubtype == UDESCSUB_VS_FORMAT_UNCOMPRESSED) ?	\
+		(offsetof(struct usb_video_format_desc, u) +			\
+		 sizeof(((struct usb_video_format_desc *)0)->u.uc)) :		\
+	((fmt)->bDescriptorSubtype == UDESCSUB_VS_FORMAT_MJPEG) ?		\
+		(offsetof(struct usb_video_format_desc, u) +			\
+		 sizeof(((struct usb_video_format_desc *)0)->u.mjpeg)) :	\
+	sizeof(struct usb_video_colorformat_desc)				\
+	)
+
 } __packed;
 
 /*