pbio/sys/storage: Limit program size to slot division.

Technically, only the sum of all slots is limited since they are stored
consecutively. However, we send only one static value to Pybricks Code, so it
doesn't know about the dynamic limit. Having a fixed limit ensures that we
can give the appropriate error message for large programs instead of failing
with a Bluetooth/USB error code when storage is full.

This limits programs on SPIKE Prime to 52,300 bytes, which should cover most
use cases. We can relax this limit once we make upgrades to Pybricks Code.

Author: laurensvalk

lib/pbio/sys/storage.c

@@ -73,10 +73,11 @@ static uint32_t pbsys_storage_get_used_program_data_size(void) {
  * @returns             Maximum program size in bytes.
  */
 uint32_t pbsys_storage_get_maximum_program_size(void) {
-    // FIXME: This is the total size of *all* slots. This is not a
-    // good indicator of the free space for multi-slot hubs. We need to inform
-    // the host dynamically about the available size of the current slot.
-    return pbdrv_block_device_get_writable_size() - sizeof(pbsys_storage_data_map_t);
+    // FIXME: This is overly conservative since only the sum of of all program
+    // slots is the limiting factor. However, then we would need to inform the
+    // host dynamically about the available size of the current slot. Until
+    // we have that, it is safer to use an absolute limit.
+    return ((pbdrv_block_device_get_writable_size() - sizeof(pbsys_storage_data_map_t)) / PBSYS_CONFIG_STORAGE_NUM_SLOTS / sizeof(uint32_t)) * sizeof(uint32_t);
 }
 
 /**
@@ -266,7 +267,7 @@ pbio_error_t pbsys_storage_set_program_size(uint32_t new_size) {
 }
 
 /**
- * Writes program data to user RAM.
+ * Writes program data to user RAM, from the offset of the incoming slot.
  *
  * Should be combined with at least one call to ::pbsys_storage_set_program_size.
  *
@@ -280,10 +281,7 @@ pbio_error_t pbsys_storage_set_program_size(uint32_t new_size) {
  *                          Otherwise ::PBIO_SUCCESS.
  */
 pbio_error_t pbsys_storage_set_program_data(uint32_t offset, const void *data, uint32_t size) {
-    // REVISIT: This protects against writing beyond the limit, but we should
-    // be informing the host about this ahead of time instead of failing during
-    // runtime.
-    if (map->slot_info[incoming_slot].offset + offset + size > pbsys_storage_get_maximum_program_size()) {
+    if (offset + size > pbsys_storage_get_maximum_program_size()) {
         return PBIO_ERROR_INVALID_ARG;
     }