Update nuke.c to auto-detect flash size (#653)

* Update nuke.c to auto-detect flash size

Fixes #642


Thanks to @Gadgetoid for the work "Making A Thing" - Support [PiMoRoNi by buying stuff](https://shop.pimoroni.com/)!

* Add some validation of flash size

If flash_do_cmd returns a bad size for the flash or fails,
use a default flash size of 16MB

---------

Co-authored-by: Peter Harper <peter.harper@raspberrypi.com>

Author: tyeth

flash/nuke/CMakeLists.txt

@@ -7,6 +7,13 @@ target_link_libraries(flash_nuke
         hardware_flash
         )
 
+# We use flash command 0x9f to dynamically get the flash size
+# If that fails, assume 16Mb
+math(EXPR FLASH_SIZE_OVERRIDE "16 * 1024 * 1024" OUTPUT_FORMAT DECIMAL)
+target_compile_definitions(flash_nuke PRIVATE
+        PICO_FLASH_SIZE_BYTES=${FLASH_SIZE_OVERRIDE}
+        )
+
 # It doesn't make sense to run this program from flash. Always build a
 # RAM-only binary.
 pico_set_binary_type(flash_nuke no_flash)

flash/nuke/nuke.c

@@ -25,14 +25,18 @@
 #include "hardware/flash.h"
 #include "pico/bootrom.h"
 
+#define MIN_FLASH_SIZE (1024 * 1024 * 2)
+
 int main() {
-    uint flash_size_bytes;
-#ifndef PICO_FLASH_SIZE_BYTES
-#warning PICO_FLASH_SIZE_BYTES not set, assuming 16M
-    flash_size_bytes = 16 * 1024 * 1024;
-#else
-    flash_size_bytes = PICO_FLASH_SIZE_BYTES;
-#endif
+    uint flash_size_bytes = 0;
+    uint8_t txbuf[4];
+    uint8_t rxbuf[4];
+    txbuf[0] = 0x9f;
+    flash_do_cmd(txbuf, rxbuf, 4);
+    flash_size_bytes = 1u << rxbuf[3];
+    if (flash_size_bytes < MIN_FLASH_SIZE || flash_size_bytes > PICO_FLASH_SIZE_BYTES) {
+        flash_size_bytes = PICO_FLASH_SIZE_BYTES;
+    }
     flash_range_erase(0, flash_size_bytes);
     // Leave an eyecatcher pattern in the first page of flash so picotool can
     // more easily check the size: