netboot: hold the screen on failure, dump the descriptor, shrink the block

Two separate problems, one of which was hiding the other.

1. The diagnostics were unreadable.  do_netboot() printed them and returned,
   and the caller's draw_main() redrew immediately and wiped the screen -- so
   two consecutive failures were reported only as "a short flash" with no
   detail, and the error code alone was not enough to act on.  nb_pause() now
   holds the screen until a button is pressed, on every failure path.

   This cost two blind round trips, each of which needs a hand copy of the
   .dol to the SD card.  Printing a diagnosis and then erasing it is worse
   than not printing one.

2. The default block was 1024 bytes, making ~1050-byte frames that span FIVE
   of the SPRAM ring's 15 pages.  The only multi-page frame ever proven on
   this hardware is the 342-byte DHCP broadcast from bring-up, which spans
   two.  Five-page frames are untested territory, and a transfer is the wrong
   place to discover a wrap bug.

   Default is now 256 bytes -> ~296-byte frame -> two pages, deliberately
   close to the proven case.  --block raises it once small blocks work; the GC
   still caps at NB_BLOCK_MAX = 1024.

ring_next() also keeps the four raw descriptor bytes and the page it read
them from, and the error path prints them.  When a length is rejected those
bytes ARE the diagnosis: 00 00 xx xx tells us the assembler wrote a
descriptor and the length is simply unexpected, whereas garbage tells us we
are reading the wrong page.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

gc_bba_test/source/main.c

@@ -125,6 +125,21 @@ static void nb_status(const char *msg)
 	printf("  %s\n", msg);
 }
 
+/* Hold the screen until a button is pressed.  Without this the caller's
+ * draw_main() redraws immediately and wipes every diagnostic line netboot
+ * just printed -- which is exactly why the first two failures were reported
+ * as "a short flash" with no detail. */
+static void nb_pause(const char *why)
+{
+	printf("\n" C_DIM "  %s -- press any button to return to the menu\n" C_RESET,
+	       why);
+	for (;;) {
+		VIDEO_WaitVSync();
+		PAD_ScanPads();
+		if (PAD_ButtonsDown(0)) return;
+	}
+}
+
 static void do_netboot(char *status, size_t nstatus)
 {
 	int len, rc;
@@ -147,11 +162,13 @@ static void do_netboot(char *status, size_t nstatus)
 	len = netboot_receive(s_stage, s_stage_max, nb_status);
 	if (len <= 0) {
 		snprintf(status, nstatus, "net-update failed (%d)", len);
+		nb_pause("transfer failed");
 		return;
 	}
 
 	rc = netboot_sd_write(netboot_dest_path(), s_stage, (u32)len, nb_status);
 	if (rc < 0) {
+		nb_pause("SD write failed");
 		/* Nothing has been launched and nothing is half-written that we
 		 * know of, so staying in the menu is the safe outcome. */
 		snprintf(status, nstatus,
@@ -166,6 +183,7 @@ static void do_netboot(char *status, size_t nstatus)
 	/* Reuse the staging region: the new build is already on the card and
 	 * verified, so the copy in RAM is finished with. */
 	rc = netboot_boot_file("/boot.dol", s_stage, s_stage_max, nb_status);
+	nb_pause("boot refused");
 
 	/* Only reached on refusal.  The new build IS on the card either way,
 	 * so this is recoverable by rebooting into Swiss by hand. */

gc_bba_test/source/netboot.c

@@ -26,6 +26,10 @@
 
 static u8 s_our_mac[6];
 static u8 s_peer_mac[6];
+/* Last descriptor ring_next() looked at, kept for the error path: when a
+ * length is rejected, the four raw bytes are the whole diagnosis. */
+static u8 s_last_desc[4];
+static u32 s_last_page;
 
 static u16 rd16(const u8 *p) { return ((u16)p[0] << 8) | p[1]; }
 static void wr16(u8 *p, u16 v) { p[0] = (u8)(v >> 8); p[1] = (u8)v; }
@@ -48,6 +52,8 @@ static int ring_next(u8 *buf, u32 cap)
 
 	addr = rpage << 8;
 	if (bba_stream((u16)addr, hdr, 4) < 0) return -1;
+	memcpy(s_last_desc, hdr, 4);
+	s_last_page = rpage;
 	dlen = ((u32)hdr[2] << 8) | hdr[3];
 	if (dlen <= 4 || dlen > 1518 + 4) return -3;  /* descriptor implausible */
 	flen = dlen - 4;
@@ -186,6 +192,11 @@ int netboot_receive(void *dst, u32 maxlen, void (*status)(const char *msg))
 				         n == -2 ? "RRP outside pages 1-15" :
 				                   "descriptor length implausible");
 				status(line);
+				snprintf(line, sizeof(line),
+				         "  page %u descriptor = %02X %02X %02X %02X",
+				         (unsigned)s_last_page, s_last_desc[0],
+				         s_last_desc[1], s_last_desc[2], s_last_desc[3]);
+				status(line);
 			}
 			return -2;
 		}