[nrf fromtree] bluetooth: mesh: fix BLOB IO flash write alignment for non-erase devices

The wr_chunk function had a fast path for devices without explicit erase
(e.g. nRF54L RRAM) that wrote chunk data directly without aligning the
offset or size to the flash write block size. On RRAM with
write-block-size=16, this caused -EINVAL from the flash driver for every
chunk write since chunk_size (161) and most chunk offsets are not
16-byte aligned.

The failed writes caused the BLOB server to never mark chunks as
received, resulting in block_status_rsp always reporting all chunks
missing. This led to infinite retransmission of block 0 and eventual
DFU timeout.

Fix by using a read-modify-write approach for non-erase devices:
read existing data at the aligned boundaries, overlay the chunk data,
and write back the full aligned buffer. This satisfies the driver's
alignment requirements while preserving neighboring bytes.

Signed-off-by: Aleksandr Khromykh <aleksandr.khromykh@nordicsemi.no>
(cherry picked from commit a9ad258)

Author: alxelax

subsys/bluetooth/mesh/blob_io_flash.c

@@ -142,17 +142,6 @@ static int wr_chunk(const struct bt_mesh_blob_io *io,
 
 	const struct flash_parameters *fparam = flash_get_parameters(fdev);
 
-	/*
-	 * If device has no erase requirement then write directly.
-	 * This is required since trick with padding using the erase value will
-	 * not work in this case.
-	 */
-	if (!(flash_params_get_erase_cap(fparam) & FLASH_ERASE_C_EXPLICIT)) {
-		return flash_area_write(flash->area,
-					flash->offset + block->offset + chunk->offset,
-					chunk->data, chunk->size);
-	}
-
 	/*
 	 * Allocate one additional write block for the case where a chunk will need
 	 * an extra write block on both sides to fit.
@@ -162,6 +151,28 @@ static int wr_chunk(const struct bt_mesh_blob_io *io,
 	uint32_t write_block_size = flash_area_align(flash->area);
 	off_t area_offset = flash->offset + block->offset + chunk->offset;
 	int start_pad = area_offset % write_block_size;
+	off_t aligned_offset = ROUND_DOWN(area_offset, write_block_size);
+	size_t write_size = ROUND_UP(start_pad + chunk->size, write_block_size);
+
+	if (!(flash_params_get_erase_cap(fparam) & FLASH_ERASE_C_EXPLICIT)) {
+		/*
+		 * For devices without explicit erase (e.g. RRAM), read existing
+		 * data at alignment boundaries to preserve bytes outside the
+		 * chunk region, then overlay the chunk data and write back.
+		 * The erase-value padding trick cannot be used here because
+		 * writes are destructive regardless of the value written.
+		 */
+		int err;
+
+		err = flash_area_read(flash->area, aligned_offset, buf, write_size);
+		if (err) {
+			return err;
+		}
+
+		memcpy(&buf[start_pad], chunk->data, chunk->size);
+
+		return flash_area_write(flash->area, aligned_offset, buf, write_size);
+	}
 
 	/*
 	 * Fill buffer with erase value, to make sure only the part of the
@@ -173,10 +184,7 @@ static int wr_chunk(const struct bt_mesh_blob_io *io,
 
 	memcpy(&buf[start_pad], chunk->data, chunk->size);
 
-	return flash_area_write(flash->area,
-				ROUND_DOWN(area_offset, write_block_size),
-				buf,
-				ROUND_UP(start_pad + chunk->size, write_block_size));
+	return flash_area_write(flash->area, aligned_offset, buf, write_size);
 }
 
 int bt_mesh_blob_io_flash_init(struct bt_mesh_blob_io_flash *flash,