stub: render failure dump as giant hex digits

Round 8: the Round-7 bit-band dump was visible at the L0_ADDR scanout
(both the scanout substitution and the render path work), but 32 coarse
squares per row proved unreadable from a photo. Replace the rendering
with giant hex digits: one u32 per line, 8 digits of a 3x5 font scaled
x10 (30x50 px), white on the black background depthcharge left.
Readable in any panel orientation, transcribable as text.

Same data and markers as before; word 15 still carries the raw
OVL_L0_ADDR value. Payload 10760e3c... flashed and verified.

RESEARCH.md

@@ -385,6 +385,21 @@ The OVL register READ is device-only (qemu -M virt aborts on reads to
 unassigned MMIO; qemu never takes the failure/substitution path, so the
 test is unaffected).
 
+## Round 8 — hex-digit diagnostic dump
+
+The bit-band dump WAS visible at the L0_ADDR scanout (proving both the
+scanout substitution and the render path), but 32 coarse squares per row
+proved unreadable from a photo, and the apparent row/column confusion
+made transcription unreliable. The dump rendering is replaced with
+**giant hex digits**: one u32 per line, 8 digits of a 3x5 cell font
+scaled x10 (30x50 px), MSB nibble first, white on depthcharge's black
+background. Readable in any panel orientation, transcribable as text.
+Same data, same markers (`0xC0DE0001` DTB stage, `0xC0DE0002|rc` LBIO
+stage, word 15 = raw OVL_L0_ADDR value).
+
+Payload `10760e3c…` flashed (cmp + vbutil OK). host_test and qemu_test
+pass.
+
 Recovery: power-cycle, boot USB (unchanged), `dd if=mmcblk0p1-pmos-backup.img
 of=/dev/mmcblk0p1 bs=4M conv=fsync`.
 

main.c

@@ -313,34 +313,66 @@ static void stage0(void)
 
 /* ---- failure diagnostics -------------------------------------------------
  *
- * The parse failed. Paint the failing values as bit-bands directly into
- * the live scanout buffer (read from OVL_L0_ADDR — the same buffer the
- * menu was displayed from, and the one clear_screen() blackened at
- * handoff). No parse result is needed to find it. Format: one row per
- * u32 word, 32 cells of 32x32 px, MSB first, white=1 / black=0, rows
- * every 64 px. Photograph the panel; decode offline. Word 0 is the
- * marker 0xC0DE00xx identifying the failure stage.
+ * The parse failed. Paint the failing values as giant hex digits directly
+ * into the live scanout buffer (read from OVL_L0_ADDR — the same buffer
+ * the menu was displayed from and clear_screen() blackened at handoff).
+ * No parse result is needed to find it. One word per line, 8 digits of
+ * 3x5 cells scaled x10 (30x50 px), MSB nibble first, white on the black
+ * background depthcharge left. Readable in any panel orientation.
+ * Line 0 is the marker 0xC0DE00xx identifying the failure stage.
  */
 
 #define DIAG_FB_WIDTH	1200			/* menu geometry, 32bpp xRGB */
+#define DIAG_DIGIT_SCALE	10		/* 3x5 font -> 30x50 px */
+
+static const u16 hexfont[16] = {
+	0b111101101101111, /* 0 */
+	0b010110010010111, /* 1 */
+	0b111001111100111, /* 2 */
+	0b111001011001111, /* 3 */
+	0b101101111001001, /* 4 */
+	0b111100111001111, /* 5 */
+	0b111100111101111, /* 6 */
+	0b111001001010010, /* 7 */
+	0b111101111101111, /* 8 */
+	0b111101111001111, /* 9 */
+	0b111101111101101, /* A */
+	0b100100111101111, /* b */
+	0b111100100100111, /* C */
+	0b001001111101111, /* d */
+	0b111100111100111, /* E */
+	0b111100111100100, /* F */
+};
+
+static void fill_rect(u32 base, u32 x, u32 y, u32 w, u32 h, u32 c)
+{
+	volatile u32 *fb = (volatile u32 *)(u64)base;
+	for (u32 yy = 0; yy < h; yy++) {
+		volatile u32 *row = fb + (u64)(y + yy) * DIAG_FB_WIDTH + x;
+		for (u32 xx = 0; xx < w; xx++)
+			row[xx] = c;
+	}
+}
 
 static void render_diag(u32 base, const u32 *d, int n)
 {
-	volatile u32 *fb = (volatile u32 *)(u64)base;
+	const u32 S = DIAG_DIGIT_SCALE;
 
 	for (int i = 0; i < n; i++) {
-		u32 y0 = 40 + (u32)i * 64;
-		if (y0 + 32 > 1920)
+		u32 y0 = 30 + (u32)i * 70;
+		if (y0 + 50 > 1920)
 			break;
-		for (u32 b = 0; b < 32; b++) {
-			u32 c = ((d[i] >> (31 - b)) & 1) ? 0x00FFFFFFu
-							 : 0x00000000u;
-			for (u32 yy = 0; yy < 32; yy++) {
-				volatile u32 *row = fb +
-					(u64)(y0 + yy) * DIAG_FB_WIDTH + b * 32;
-				for (u32 xx = 0; xx < 32; xx++)
-					row[xx] = c;
-			}
+		for (u32 j = 0; j < 8; j++) {
+			u32 nib = (d[i] >> (28 - 4 * j)) & 0xF;
+			u16 g = hexfont[nib];
+			u32 x0 = 40 + j * 4 * S;
+			for (u32 gy = 0; gy < 5; gy++)
+				for (u32 gx = 0; gx < 3; gx++)
+					if ((g >> (14 - gy * 3 - gx)) & 1)
+						fill_rect(base,
+							  x0 + gx * S,
+							  y0 + gy * S,
+							  S, S, 0x00FFFFFFu);
 		}
 	}
 }
@@ -363,9 +395,6 @@ static void fail_dump(u32 marker, const u32 *words, int n)
 	render_diag(l0, diag, 16);
 	halt();
 }
-
-/* ---- main checkpoint sequence ------------------------------------------- */
-
 static void checkpoint(const struct fbinfo *f, int r, int g, int b)
 {
 	fill_screen(f, mkcolor(f, r, g, b));