From c21f241deb06647856f471f21b87a798ff5a671d Mon Sep 17 00:00:00 2001
From: 3djc <5167938+3djc@users.noreply.github.com>
Date: Tue, 27 Jan 2026 07:01:59 +0100
Subject: [PATCH] fix(h7): UF2 bootloader unable to flash (#6985)

Co-authored-by: 3djc <3djc@gh.com>
Co-authored-by: raphaelcoeffic <1050031+raphaelcoeffic@users.noreply.github.com>
---
 radio/src/drivers/uf2_ghostfat.cpp | 47 +++++++++++++++---------------
 radio/src/drivers/uf2_ghostfat.h   |  2 +-
 2 files changed, 25 insertions(+), 24 deletions(-)

diff --git a/radio/src/drivers/uf2_ghostfat.cpp b/radio/src/drivers/uf2_ghostfat.cpp
index 26459824b..912ffb424 100644
--- a/radio/src/drivers/uf2_ghostfat.cpp
+++ b/radio/src/drivers/uf2_ghostfat.cpp
@@ -35,17 +35,6 @@
 
 #include <string.h>
 
-#if defined(FLASHSIZE)
-  #define UF2_MAX_FW_SIZE FLASHSIZE
-#else
-  #define UF2_MAX_FW_SIZE (2 * 1024 * 1024)
-#endif
-
-#define UF2_MAX_BLOCKS (UF2_MAX_FW_SIZE / 256)
-
-#define UF2_ERASE_BLOCK_SIZE (4 * 1024)
-#define UF2_ERASE_BLOCKS (UF2_MAX_FW_SIZE / UF2_ERASE_BLOCK_SIZE)
-
 typedef struct {
     uint8_t JumpInstruction[3];
     uint8_t OEMInfo[8];
@@ -183,31 +172,43 @@ static_assert(ARRAY_SIZE(indexFile) < 512);
 
 
 #define NUM_FILES (ARRAY_SIZE(info))
-#define NUM_DIRENTRIES (NUM_FILES + 1) // Code adds volume label as first root directory entry
+#define NUM_DIRENTRIES (NUM_FILES + 1) /* Code adds volume label as first root directory entry */
 
 #ifndef BOOTLOADER_ADDRESS
 #define BOOTLOADER_ADDRESS FIRMWARE_ADDRESS
 #endif
 
+#define UF2_SIZE           (current_flash_size() * 2 + 512 * REBOOT_BLOCK)
+#define UF2_SECTORS        (UF2_SIZE / 512)
+#define UF2_FIRST_SECTOR   (NUM_FILES + 1) /* WARNING -- code presumes each non-UF2 file content fits in single sector */
+#define UF2_LAST_SECTOR    (UF2_FIRST_SECTOR + UF2_SECTORS - 1)
+
+#define RESERVED_SECTORS   1
+#define ROOT_DIR_SECTORS   4
+#define SECTORS_PER_FAT    ((NUM_FAT_BLOCKS * 2 + 511) / 512) /* 256 sectors per FAT */
+
+#define START_FAT0         RESERVED_SECTORS
+#define START_FAT1         (START_FAT0 + SECTORS_PER_FAT)        /* 1 + 256 */
+#define START_ROOTDIR      (START_FAT1 + SECTORS_PER_FAT)        /* 1 + 256 + 256 */
+#define START_CLUSTERS     (START_ROOTDIR + ROOT_DIR_SECTORS)    /* 1 + 256 + 256 + 4 */
+#define DATA_SECTORS       (NUM_FAT_BLOCKS - 2 - START_CLUSTERS) /* 65535 - 2 - (1 + 256 + 256 + 4) = 65016 */
+
 #if BOOTLOADER_ADDRESS == FIRMWARE_ADDRESS
 #define REBOOT_BLOCK 0
 #else
 #define REBOOT_BLOCK 1
 #endif
 
-#define UF2_SIZE           (current_flash_size() * 2 + 512 * REBOOT_BLOCK)
-#define UF2_SECTORS        (UF2_SIZE / 512)
-#define UF2_FIRST_SECTOR   (NUM_FILES + 1) // WARNING -- code presumes each non-UF2 file content fits in single sector
-#define UF2_LAST_SECTOR    (UF2_FIRST_SECTOR + UF2_SECTORS - 1)
+#define UF2_MAX_FW_SIZE (512 * (DATA_SECTORS / 2 - REBOOT_BLOCK) / 2) /* 8126 kB */
 
-#define RESERVED_SECTORS   1
-#define ROOT_DIR_SECTORS   4
-#define SECTORS_PER_FAT    ((NUM_FAT_BLOCKS * 2 + 511) / 512)
+#if defined(FLASHSIZE)
+static_assert(FLASHSIZE > UF2_MAX_FW_SIZE, "FLASHSIZE is smaller than UF2_MAX_FW_SIZE");
+#endif
 
-#define START_FAT0         RESERVED_SECTORS
-#define START_FAT1         (START_FAT0 + SECTORS_PER_FAT)
-#define START_ROOTDIR      (START_FAT1 + SECTORS_PER_FAT)
-#define START_CLUSTERS     (START_ROOTDIR + ROOT_DIR_SECTORS)
+#define UF2_MAX_BLOCKS (UF2_MAX_FW_SIZE / 256)
+
+#define UF2_ERASE_BLOCK_SIZE (4 * 1024)
+#define UF2_ERASE_BLOCKS (UF2_MAX_FW_SIZE / UF2_ERASE_BLOCK_SIZE)
 
 // all directory entries must fit in a single sector
 // because otherwise current code overflows buffer
diff --git a/radio/src/drivers/uf2_ghostfat.h b/radio/src/drivers/uf2_ghostfat.h
index 665296b47..601117642 100644
--- a/radio/src/drivers/uf2_ghostfat.h
+++ b/radio/src/drivers/uf2_ghostfat.h
@@ -23,7 +23,7 @@
 
 #include <stdint.h>
 
-#define UF2_NUM_BLOCKS 65536 // at least 32MB
+#define UF2_NUM_BLOCKS 65535
 #define UF2_INVALID_NUM_BLOCKS 0xFFFFFFFF
 
 typedef struct {