lorawan: services: frag decoder: add lowmem implementation

that utilizes the fact that the ldpc recovery matrix is triangular and only
stores the half with non-zero values.

This implementation is hopefully going to make forward error correction
usable on lower memory devices.

Signed-off-by: Lucas Romero <luqasn@gmail.com>

subsys/lorawan/services/CMakeLists.txt

@@ -15,6 +15,11 @@ if(CONFIG_LORAWAN_SERVICES)
     frag_transport.c
   )
 
+  zephyr_library_sources_ifdef(
+    CONFIG_LORAWAN_FRAG_TRANSPORT_DECODER_LOWMEM
+          frag_decoder_lowmem.c
+  )
+
   zephyr_library_sources_ifdef(
     CONFIG_LORAWAN_REMOTE_MULTICAST
     multicast.c

subsys/lorawan/services/Kconfig

@@ -75,6 +75,11 @@ config LORAWAN_FRAG_TRANSPORT_DECODER_SEMTECH
 	help
 	  The default decoder implementation from LoRaMAC-node.
 
+config LORAWAN_FRAG_TRANSPORT_DECODER_LOWMEM
+	bool "Low Memory Footprint Algorithm"
+	help
+	  Alternate implementation with reduced RAM footprint.
+
 endchoice # LORAWAN_FRAG_TRANSPORT_DECODER
 
 DT_CHOSEN_Z_CODE_PARTITION := zephyr,code-partition

subsys/lorawan/services/frag_decoder_lowmem.c

@@ -0,0 +1,390 @@
+/*
+ * Copyright (c) 2024 A Labs GmbH
+ * Copyright (c) 2024 tado GmbH
+ * Copyright (c) 2022 Jiapeng Li
+ *
+ * Based on: https://github.com/JiapengLi/LoRaWANFragmentedDataBlockTransportAlgorithm
+ * Original algorithm: http://www.inference.org.uk/mackay/gallager/papers/ldpc.pdf
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include "frag_decoder_lowmem.h"
+#include "frag_flash.h"
+
+#include <zephyr/logging/log.h>
+#include <zephyr/sys/util.h>
+#include <zephyr/sys/bitarray.h>
+
+LOG_MODULE_REGISTER(lorawan_frag_dec, CONFIG_LORAWAN_SERVICES_LOG_LEVEL);
+
+SYS_BITARRAY_DEFINE_STATIC(lost_frames, FRAG_MAX_NB);
+SYS_BITARRAY_DEFINE_STATIC(lost_frames_matrix, (FRAG_TOLERANCE * (FRAG_TOLERANCE + 1) / 2));
+SYS_BITARRAY_DEFINE_STATIC(matched_lost_frm_bm0, FRAG_TOLERANCE);
+SYS_BITARRAY_DEFINE_STATIC(matched_lost_frm_bm1, FRAG_TOLERANCE);
+SYS_BITARRAY_DEFINE_STATIC(matrix_line_bm, FRAG_MAX_NB);
+
+
+static inline size_t matrix_location_to_index(size_t x, size_t y, size_t m)
+{
+	/* We only store the top half of the matrix because it is triangular,
+	 * but that means when mapping the coordinates into the flat representation
+	 * we need to account for that
+	 */
+	return (y + 1) * (m + m - y) / 2 - (m - x);
+}
+
+static bool triangular_matrix_get_entry(struct sys_bitarray *m2tbm, size_t x, size_t y, size_t m)
+{
+	/* We are dealing with triangular matrices, so we don't expect actions in the lower half */
+	__ASSERT(x >= y, "x: %d, y: %d, m: %d", x, y, m);
+	size_t bit;
+	int ret;
+
+	ret = sys_bitarray_test_bit(m2tbm, matrix_location_to_index(x, y, m), &bit);
+	__ASSERT_NO_MSG(ret == 0);
+
+	return bit != 0;
+}
+
+static void triangular_matrix_set_entry(struct sys_bitarray *m2tbm, size_t x, size_t y, size_t m)
+{
+	/* We are dealing with triangular matrices, so we don't expect actions in the lower half */
+	__ASSERT(x >= y, "x: %d, y: %d, m: %d", x, y, m);
+	int ret;
+
+	ret = sys_bitarray_set_bit(m2tbm, matrix_location_to_index(x, y, m));
+	__ASSERT_NO_MSG(ret == 0);
+}
+
+static void triangular_matrix_clear_entry(struct sys_bitarray *m2tbm, size_t x, size_t y, size_t m)
+{
+	/* We are dealing with triangular matrices, so we don't expect actions in the lower half */
+	__ASSERT(x >= y, "x: %d, y: %d, m: %d", x, y, m);
+
+	int ret;
+
+	ret = sys_bitarray_clear_bit(m2tbm, matrix_location_to_index(x, y, m));
+	__ASSERT_NO_MSG(ret == 0);
+}
+
+static inline bool bit_get(struct sys_bitarray *bitmap, size_t index)
+{
+	int bit, ret;
+
+	ret = sys_bitarray_test_bit(bitmap, index, &bit);
+	__ASSERT_NO_MSG(ret == 0);
+	return bit != 0;
+}
+
+static inline void bit_set(struct sys_bitarray *bitmap, size_t index)
+{
+	int ret;
+
+	ret = sys_bitarray_set_bit(bitmap, index);
+	__ASSERT_NO_MSG(ret == 0);
+}
+
+static inline void bit_clear(struct sys_bitarray *bitmap, size_t index)
+{
+	int ret;
+
+	ret = sys_bitarray_clear_bit(bitmap, index);
+	__ASSERT_NO_MSG(ret == 0);
+}
+
+static inline size_t bit_count_ones(struct sys_bitarray *bitmap, size_t index)
+{
+	size_t count;
+	int ret;
+
+	ret = sys_bitarray_popcount_region(bitmap, index + 1, 0, &count);
+	__ASSERT_NO_MSG(ret == 0);
+	return count;
+}
+
+static inline void bit_xor(struct sys_bitarray *des, struct sys_bitarray *src, size_t size)
+{
+	int ret;
+
+	ret = sys_bitarray_xor(des, src, size, 0);
+	__ASSERT_NO_MSG(ret == 0);
+}
+
+static inline void bit_clear_all(struct sys_bitarray *bitmap, size_t size)
+{
+	int ret;
+
+	ret = sys_bitarray_clear_region(bitmap, size, 0);
+	__ASSERT_NO_MSG(ret == 0);
+}
+
+/**
+ * Generate a 23bit Pseudorandom Binary Sequence (PRBS)
+ *
+ * @param previous Previous value in the sequence
+ *
+ * @returns Next value in the pseudorandom sequence
+ */
+static int32_t prbs23(int32_t previous)
+{
+	int32_t b0 = previous & 1;
+	int32_t b1 = (previous & 32) / 32;
+
+	return (previous / 2) + ((b0 ^ b1) << 22);
+}
+
+/**
+ * Generate vector for coded fragment n of the MxN parity matrix
+ *
+ * @param m Total number of uncoded fragments (M)
+ * @param n Coded fragment number (starting at 1 and not 0)
+ * @param vec Output vector (buffer size must be greater than m)
+ */
+static void frag_dec_parity_matrix_vector(size_t m, size_t n, struct sys_bitarray *vec)
+{
+	size_t mm, r;
+	int32_t x;
+	int ret;
+
+	ret = sys_bitarray_clear_region(vec, m, 0);
+	__ASSERT_NO_MSG(ret == 0);
+
+	/*
+	 * Powers of 2 must be treated differently to make sure matrix content is close
+	 * to random. Powers of 2 tend to generate patterns.
+	 */
+	if (is_power_of_two(m)) {
+		mm = m + 1;
+	} else {
+		mm = m;
+	}
+
+	x = 1 + (1001 * n);
+
+	for (size_t nb_coeff = 0; nb_coeff < (m / 2); nb_coeff++) {
+		r = (1 << 16);
+		while (r >= m) {
+			x = prbs23(x);
+			r = x % mm;
+		}
+		ret = sys_bitarray_set_bit(vec, r);
+		__ASSERT_NO_MSG(ret == 0);
+	}
+}
+
+void frag_dec_init(struct frag_decoder *decoder, size_t nb_frag, size_t frag_size)
+{
+	decoder->nb_frag = nb_frag;
+	decoder->frag_size = frag_size;
+	/* Set all frames lost, from 0 to nb_frag-1 */
+	decoder->lost_frame_count = decoder->nb_frag;
+	sys_bitarray_set_region(&lost_frames, decoder->nb_frag, 0);
+
+	sys_bitarray_clear_region(&lost_frames_matrix, (FRAG_TOLERANCE * (FRAG_TOLERANCE + 1) / 2),
+				  0);
+
+	decoder->filled_lost_frame_count = 0;
+	decoder->status = FRAG_DEC_STA_UNCODED;
+}
+
+void frag_dec_frame_received(struct frag_decoder *decoder, uint16_t index)
+{
+	int ret, was_set;
+
+	ret = sys_bitarray_test_and_clear_bit(&lost_frames, index, &was_set);
+	__ASSERT_NO_MSG(ret == 0);
+
+	if (was_set != 0) {
+		decoder->lost_frame_count--;
+	}
+}
+
+static void frag_dec_write_vector(struct sys_bitarray *matrix, uint16_t line_index,
+				  struct sys_bitarray *vector, size_t len)
+{
+	for (size_t i = line_index; i < len; i++) {
+		if (bit_get(vector, i)) {
+			triangular_matrix_set_entry(matrix, i, line_index, len);
+		} else {
+			triangular_matrix_clear_entry(matrix, i, line_index, len);
+		}
+	}
+}
+
+static void frag_dec_read_vector(struct sys_bitarray *matrix, uint16_t line_index,
+				 struct sys_bitarray *vector, size_t len)
+{
+	for (size_t i = 0; i < len; i++) {
+		if (i >= line_index && triangular_matrix_get_entry(matrix, i, line_index, len)) {
+			bit_set(vector, i);
+		} else {
+			bit_clear(vector, i);
+		}
+	}
+}
+
+int frag_dec(struct frag_decoder *decoder, uint16_t frag_counter, const uint8_t *buf, size_t len)
+{
+	int ret;
+	int i, j;
+	size_t unmatched_frame_count;
+	size_t lost_frame_index, frame_index;
+	static uint8_t row_data_buf[FRAG_MAX_SIZE];
+	static uint8_t xor_row_data_buf[FRAG_MAX_SIZE];
+
+	if (decoder->status == FRAG_DEC_STA_DONE) {
+		return decoder->lost_frame_count;
+	}
+
+	if (len != decoder->frag_size) {
+		return FRAG_DEC_ERR_INVALID_FRAME;
+	}
+
+	__ASSERT_NO_MSG(frag_counter > 0);
+
+	if (frag_counter <= decoder->nb_frag && decoder->status == FRAG_DEC_STA_UNCODED) {
+		/* Mark new received frame */
+		frag_dec_frame_received(decoder, frag_counter - 1);
+		/* Save data to flash */
+		frag_flash_write((frag_counter - 1) * decoder->frag_size, (uint8_t *)buf,
+				 decoder->frag_size);
+		/* If no frame was lost, we are already done */
+		if (decoder->lost_frame_count == 0) {
+			decoder->status = FRAG_DEC_STA_DONE;
+			return decoder->lost_frame_count;
+		}
+		return FRAG_DEC_ONGOING;
+	}
+	/* At least one frame was lost, start recovering frames */
+	decoder->status = FRAG_DEC_STA_CODED;
+
+	/* Clear all temporary bm and buf */
+	bit_clear_all(&matched_lost_frm_bm0, decoder->lost_frame_count);
+	bit_clear_all(&matched_lost_frm_bm1, decoder->lost_frame_count);
+
+	/* Copy data buffer because we need to manipulate it */
+	memcpy(xor_row_data_buf, buf, decoder->frag_size);
+
+	if (decoder->lost_frame_count > FRAG_TOLERANCE) {
+		return FRAG_DEC_ERR_TOO_MANY_FRAMES_LOST;
+	}
+
+	unmatched_frame_count = 0;
+	/* Build parity matrix vector for current line */
+	frag_dec_parity_matrix_vector(decoder->nb_frag, frag_counter - decoder->nb_frag,
+				      &matrix_line_bm);
+	for (i = 0; i < decoder->nb_frag; i++) {
+		if (!bit_get(&matrix_line_bm, i)) {
+			/* This frame is not part of the recovery matrix for the current fragment */
+			continue;
+		}
+		if (bit_get(&lost_frames, i)) {
+			/* No match for this coded frame in the uncoded frames.
+			 * Check which lost frame we are processing by checking how many have been
+			 * lost between the start and the current coded fragment.
+			 */
+			bit_set(&matched_lost_frm_bm0, bit_count_ones(&lost_frames, i) - 1);
+			unmatched_frame_count++;
+		} else {
+			/* Restore frame by XORing with already received frame */
+			/* Load previously received data into buffer */
+			frag_flash_read(i * decoder->frag_size, row_data_buf, decoder->frag_size);
+			/* XOR previously received data with data for current frame */
+			mem_xor_n(xor_row_data_buf, xor_row_data_buf, row_data_buf,
+				  decoder->frag_size);
+		}
+	}
+	if (unmatched_frame_count == 0) {
+		return FRAG_DEC_ONGOING;
+	}
+
+	/* &matched_lost_frm_bm0 now contains new coded frame which excludes all received
+	 * frames content start to diagonal &matched_lost_frm_bm0
+	 */
+	do {
+		ret = sys_bitarray_find_nth_set(&matched_lost_frm_bm0, 1, decoder->lost_frame_count,
+						0, &lost_frame_index);
+		if (ret == 1) {
+			/* Not found */
+			break;
+		}
+		if (ret != 0) {
+			return FRAG_DEC_ERR;
+		}
+		/* We know which one is the next lost frame, try to find it in the lost frame bitmap
+		 */
+		ret = sys_bitarray_find_nth_set(&lost_frames, lost_frame_index + 1,
+						decoder->nb_frag, 0, &frame_index);
+		if (ret == 1) {
+			/* Not found */
+			break;
+		}
+		if (ret != 0) {
+			return FRAG_DEC_ERR;
+		}
+		/* If current frame contains new information, save it */
+		if (!triangular_matrix_get_entry(&lost_frames_matrix, lost_frame_index,
+						 lost_frame_index, decoder->lost_frame_count)) {
+			frag_dec_write_vector(&lost_frames_matrix, lost_frame_index,
+					      &matched_lost_frm_bm0, decoder->lost_frame_count);
+			frag_flash_write(frame_index * decoder->frag_size, xor_row_data_buf,
+					 decoder->frag_size);
+			decoder->filled_lost_frame_count++;
+			break;
+		}
+
+		frag_dec_read_vector(&lost_frames_matrix, lost_frame_index, &matched_lost_frm_bm1,
+				     decoder->lost_frame_count);
+		bit_xor(&matched_lost_frm_bm0, &matched_lost_frm_bm1, decoder->lost_frame_count);
+		frag_flash_read(frame_index * decoder->frag_size, row_data_buf, decoder->frag_size);
+		mem_xor_n(xor_row_data_buf, xor_row_data_buf, row_data_buf, decoder->frag_size);
+	} while (!sys_bitarray_is_region_cleared(&matched_lost_frm_bm0, decoder->lost_frame_count,
+						 0));
+
+	if (decoder->filled_lost_frame_count != decoder->lost_frame_count) {
+		return FRAG_DEC_ONGOING;
+	}
+
+	if (decoder->lost_frame_count < 2) {
+		decoder->status = FRAG_DEC_STA_DONE;
+		return decoder->lost_frame_count;
+	}
+
+	/* All frame content is received, now to reconstruct the whole frame */
+	for (i = (decoder->lost_frame_count - 2); i >= 0; i--) {
+		ret = sys_bitarray_find_nth_set(&lost_frames, i + 1, decoder->nb_frag, 0,
+						&frame_index);
+		if (ret != 0) {
+			return FRAG_DEC_ERR;
+		}
+		frag_flash_read(frame_index * decoder->frag_size, xor_row_data_buf,
+				decoder->frag_size);
+		frag_dec_read_vector(&lost_frames_matrix, i, &matched_lost_frm_bm1,
+				     decoder->lost_frame_count);
+		for (j = (decoder->lost_frame_count - 1); j > i; j--) {
+			if (!bit_get(&matched_lost_frm_bm1, j)) {
+				continue;
+			}
+			ret = sys_bitarray_find_nth_set(&lost_frames, j + 1, decoder->nb_frag, 0,
+							&lost_frame_index);
+			if (ret != 0) {
+				return FRAG_DEC_ERR;
+			}
+			frag_dec_read_vector(&lost_frames_matrix, j, &matched_lost_frm_bm0,
+					     decoder->lost_frame_count);
+			bit_xor(&matched_lost_frm_bm1, &matched_lost_frm_bm0,
+				decoder->lost_frame_count);
+			frag_flash_read(lost_frame_index * decoder->frag_size, row_data_buf,
+					decoder->frag_size);
+			mem_xor_n(xor_row_data_buf, xor_row_data_buf, row_data_buf,
+				  decoder->frag_size);
+			frag_dec_write_vector(&lost_frames_matrix, i, &matched_lost_frm_bm1,
+					      decoder->lost_frame_count);
+		}
+		frag_flash_write(frame_index * decoder->frag_size, xor_row_data_buf,
+				 decoder->frag_size);
+	}
+	decoder->status = FRAG_DEC_STA_DONE;
+	return decoder->lost_frame_count;
+}

subsys/lorawan/services/frag_decoder_lowmem.h

@@ -0,0 +1,56 @@
+/*
+ * Copyright (c) 2024 A Labs GmbH
+ * Copyright (c) 2024 tado GmbH
+ * Copyright (c) 2022 Jiapeng Li
+ *
+ * Based on: https://github.com/JiapengLi/LoRaWANFragmentedDataBlockTransportAlgorithm
+ * Original algorithm: http://www.inference.org.uk/mackay/gallager/papers/ldpc.pdf
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#ifndef FRAG_DEC_H_
+#define FRAG_DEC_H_
+
+#include <stdint.h>
+#include <stddef.h>
+
+#define FRAG_MAX_NB                                                                                \
+	(CONFIG_LORAWAN_FRAG_TRANSPORT_IMAGE_SIZE / CONFIG_LORAWAN_FRAG_TRANSPORT_MIN_FRAG_SIZE +  \
+	 1U)
+#define FRAG_MAX_SIZE  (CONFIG_LORAWAN_FRAG_TRANSPORT_MAX_FRAG_SIZE)
+#define FRAG_TOLERANCE (FRAG_MAX_NB * CONFIG_LORAWAN_FRAG_TRANSPORT_MAX_REDUNDANCY / 100U)
+
+#define FRAG_DEC_ONGOING                  (-1)
+#define FRAG_DEC_ERR_INVALID_FRAME        (-2)
+#define FRAG_DEC_ERR_TOO_MANY_FRAMES_LOST (-3)
+#define FRAG_DEC_ERR                      (-4)
+
+enum frag_decoder_status {
+	/** Processing uncoded fragments */
+	FRAG_DEC_STA_UNCODED,
+	/** Processing coded fragments and restoring data with the help of other fragments */
+	FRAG_DEC_STA_CODED,
+	/** All fragments received and/or restored */
+	FRAG_DEC_STA_DONE,
+};
+
+struct frag_decoder {
+	/** Current decoder status */
+	enum frag_decoder_status status;
+
+	/** Number of fragments */
+	uint16_t nb_frag;
+	/** Size of individual fragment */
+	uint8_t frag_size;
+
+	/** Number of frames lost in this session */
+	uint16_t lost_frame_count;
+	/** Number of frames recovered in this session */
+	uint16_t filled_lost_frame_count;
+};
+
+void frag_dec_init(struct frag_decoder *decoder, size_t nb_frag, size_t frag_size);
+int frag_dec(struct frag_decoder *decoder, uint16_t frag_counter, const uint8_t *buf, size_t len);
+
+#endif /* FRAG_DEC_H_ */

subsys/lorawan/services/frag_transport.c

@@ -15,6 +15,8 @@
 #include <LoRaMac.h>
 #ifdef CONFIG_LORAWAN_FRAG_TRANSPORT_DECODER_SEMTECH
 #include <FragDecoder.h>
+#elif defined(CONFIG_LORAWAN_FRAG_TRANSPORT_DECODER_LOWMEM)
+#include "frag_decoder_lowmem.h"
 #endif
 
 #include <zephyr/lorawan/lorawan.h>
@@ -85,6 +87,8 @@ struct frag_transport_context {
 #ifdef CONFIG_LORAWAN_FRAG_TRANSPORT_DECODER_SEMTECH
 	/* variables required for FragDecoder.h */
 	FragDecoderCallbacks_t decoder_callbacks;
+#elif defined(CONFIG_LORAWAN_FRAG_TRANSPORT_DECODER_LOWMEM)
+	struct frag_decoder decoder;
 #endif
 };
 
@@ -219,6 +223,8 @@ static void frag_transport_package_callback(uint8_t port, bool data_pending, int
 
 				ctx.decoder_callbacks.FragDecoderWrite = frag_flash_write;
 				ctx.decoder_callbacks.FragDecoderRead = frag_flash_read;
+#elif defined(CONFIG_LORAWAN_FRAG_TRANSPORT_DECODER_LOWMEM)
+				frag_dec_init(&ctx.decoder, ctx.nb_frag, ctx.frag_size);
 #endif
 				frag_flash_init(ctx.frag_size);
 				ctx.is_active = true;
@@ -269,6 +275,9 @@ static void frag_transport_package_callback(uint8_t port, bool data_pending, int
 #ifdef CONFIG_LORAWAN_FRAG_TRANSPORT_DECODER_SEMTECH
 			decoder_process_status = FragDecoderProcess(
 				frag_counter, (uint8_t *)&rx_buf[rx_pos]);
+#elif defined(CONFIG_LORAWAN_FRAG_TRANSPORT_DECODER_LOWMEM)
+			decoder_process_status = frag_dec(
+				&ctx.decoder, frag_counter, &rx_buf[rx_pos], ctx.frag_size);
 #endif
 
 			LOG_INF("DataFragment %u of %u (%u lost), session: %u, decoder result: %d",