Selection of memory access methods, aampostincrement detection (#508)

* Add flexible selection of memory access methods, detection of aampostincrement.

New configuration command introduced: "riscv set_mem_access".
It allows to specify which RISC-V memory access methods (progbuf,
sysbus and/or abstract access) should be tried and in which order
of priority.

Command "riscv set_prefer_sba" is left and works in backward
compatible way, but is marked as deprecated.

First time abstract memory access is executed, it is tried with
set aampostincrement bit. If the abstract command fails or the
address is not incremented correctly, aampostincrement will not
be used for any subsequent accesses.

Signed-off-by: Samuel Obuch <sobuch@codasip.com>

* remove unnecessary variable

* fix doc

3 files changed, 445 insertions, 100 deletions

diff --git a/src/target/riscv/riscv-013.c b/src/target/riscv/riscv-013.c
index 877f9b4..234b464 100644
--- a/src/target/riscv/riscv-013.c
+++ b/src/target/riscv/riscv-013.c
@@ -215,6 +215,8 @@ typedef struct {
 	bool abstract_read_fpr_supported;
 	bool abstract_write_fpr_supported;
 
+	yes_no_maybe_t has_aampostincrement;
+
 	/* When a function returns some error due to a failure indicated by the
 	 * target in cmderr, the caller can look here to see what that error was.
 	 * (Compare with errno.) */
@@ -1842,27 +1844,40 @@ static int riscv013_hart_count(struct target *target)
 	return dm->hart_count;
 }
 
+/* Try to find out the widest memory access size depending on the selected memory access methods. */
 static unsigned riscv013_data_bits(struct target *target)
 {
 	RISCV013_INFO(info);
-	/* TODO: Once there is a spec for discovering abstract commands, we can
-	 * take those into account as well.  For now we assume abstract commands
-	 * support XLEN-wide accesses. */
-	if (has_sufficient_progbuf(target, 3) && !riscv_prefer_sba)
-		return riscv_xlen(target);
+	RISCV_INFO(r);
 
-	if (get_field(info->sbcs, DM_SBCS_SBACCESS128))
-		return 128;
-	if (get_field(info->sbcs, DM_SBCS_SBACCESS64))
-		return 64;
-	if (get_field(info->sbcs, DM_SBCS_SBACCESS32))
-		return 32;
-	if (get_field(info->sbcs, DM_SBCS_SBACCESS16))
-		return 16;
-	if (get_field(info->sbcs, DM_SBCS_SBACCESS8))
-		return 8;
-
-	return riscv_xlen(target);
+	for (unsigned i = 0; i < RISCV_NUM_MEM_ACCESS_METHODS; i++) {
+		int method = r->mem_access_methods[i];
+
+		if (method == RISCV_MEM_ACCESS_PROGBUF) {
+			if (has_sufficient_progbuf(target, 3))
+				return riscv_xlen(target);
+		} else if (method == RISCV_MEM_ACCESS_SYSBUS) {
+			if (get_field(info->sbcs, DM_SBCS_SBACCESS128))
+				return 128;
+			if (get_field(info->sbcs, DM_SBCS_SBACCESS64))
+				return 64;
+			if (get_field(info->sbcs, DM_SBCS_SBACCESS32))
+				return 32;
+			if (get_field(info->sbcs, DM_SBCS_SBACCESS16))
+				return 16;
+			if (get_field(info->sbcs, DM_SBCS_SBACCESS8))
+				return 8;
+		} else if (method == RISCV_MEM_ACCESS_ABSTRACT) {
+			/* TODO: Once there is a spec for discovering abstract commands, we can
+			 * take those into account as well.  For now we assume abstract commands
+			 * support XLEN-wide accesses. */
+			return riscv_xlen(target);
+		} else if (method == RISCV_MEM_ACCESS_UNSPECIFIED)
+			/* No further mem access method to try. */
+			break;
+	}
+	LOG_ERROR("Unable to determine supported data bits on this target. Assuming 32 bits.");
+	return 32;
 }
 
 static int prep_for_vector_access(struct target *target, uint64_t *vtype,
@@ -2070,6 +2085,8 @@ static int init_target(struct command_context *cmd_ctx,
 	info->abstract_read_fpr_supported = true;
 	info->abstract_write_fpr_supported = true;
 
+	info->has_aampostincrement = YNM_MAYBE;
+
 	return ERROR_OK;
 }
 
@@ -2672,6 +2689,142 @@ static int batch_run(const struct target *target, struct riscv_batch *batch)
 	return riscv_batch_run(batch);
 }
 
+static void log_mem_access_result(struct target *target, bool success, int method, bool read)
+{
+	RISCV_INFO(r);
+	bool warn = false;
+	char msg[60];
+
+	/* Compose the message */
+	snprintf(msg, 60, "%s to %s memory via %s.",
+			success ? "Succeeded" : "Failed",
+			read ? "read" : "write",
+			(method == RISCV_MEM_ACCESS_PROGBUF) ? "program buffer" :
+			(method == RISCV_MEM_ACCESS_SYSBUS) ? "system bus" : "abstract access");
+
+	/* Determine the log message severity. Show warnings only once. */
+	if (!success) {
+		if (method == RISCV_MEM_ACCESS_PROGBUF) {
+			warn = r->mem_access_progbuf_warn;
+			r->mem_access_progbuf_warn = false;
+		}
+		if (method == RISCV_MEM_ACCESS_SYSBUS) {
+			warn = r->mem_access_sysbus_warn;
+			r->mem_access_sysbus_warn = false;
+		}
+		if (method == RISCV_MEM_ACCESS_ABSTRACT) {
+			warn = r->mem_access_abstract_warn;
+			r->mem_access_abstract_warn = false;
+		}
+	}
+
+	if (warn)
+		LOG_WARNING("%s", msg);
+	else
+		LOG_DEBUG("%s", msg);
+}
+
+static bool mem_should_skip_progbuf(struct target *target, target_addr_t address,
+		uint32_t size, bool read, char **skip_reason)
+{
+	assert(skip_reason);
+
+	if (!has_sufficient_progbuf(target, 3)) {
+		LOG_DEBUG("Skipping mem %s via progbuf - insufficient progbuf size.",
+				read ? "read" : "write");
+		*skip_reason = "skipped (insufficient progbuf)";
+		return true;
+	}
+	if (target->state != TARGET_HALTED) {
+		LOG_DEBUG("Skipping mem %s via progbuf - target not halted.",
+				read ? "read" : "write");
+		*skip_reason = "skipped (target not halted)";
+		return true;
+	}
+	if (riscv_xlen(target) < size * 8) {
+		LOG_DEBUG("Skipping mem %s via progbuf - XLEN (%d) is too short for %d-bit memory access.",
+				read ? "read" : "write", riscv_xlen(target), size * 8);
+		*skip_reason = "skipped (XLEN too short)";
+		return true;
+	}
+	if (size > 8) {
+		LOG_DEBUG("Skipping mem %s via progbuf - unsupported size.",
+				read ? "read" : "write");
+		*skip_reason = "skipped (unsupported size)";
+		return true;
+	}
+	if ((sizeof(address) * 8 > riscv_xlen(target)) && (address >> riscv_xlen(target))) {
+		LOG_DEBUG("Skipping mem %s via progbuf - progbuf only supports %u-bit address.",
+				read ? "read" : "write", riscv_xlen(target));
+		*skip_reason = "skipped (too large address)";
+		return true;
+	}
+
+	return false;
+}
+
+static bool mem_should_skip_sysbus(struct target *target, target_addr_t address,
+		uint32_t size, uint32_t increment, bool read, char **skip_reason)
+{
+	assert(skip_reason);
+
+	RISCV013_INFO(info);
+	if ((!get_field(info->sbcs, DM_SBCS_SBACCESS8) || size != 1) &&
+			(!get_field(info->sbcs, DM_SBCS_SBACCESS16) || size != 2) &&
+			(!get_field(info->sbcs, DM_SBCS_SBACCESS32) || size != 4) &&
+			(!get_field(info->sbcs, DM_SBCS_SBACCESS64) || size != 8) &&
+			(!get_field(info->sbcs, DM_SBCS_SBACCESS128) || size != 16)) {
+		LOG_DEBUG("Skipping mem %s via system bus - unsupported size.",
+				read ? "read" : "write");
+		*skip_reason = "skipped (unsupported size)";
+		return true;
+	}
+	unsigned sbasize = get_field(info->sbcs, DM_SBCS_SBASIZE);
+	if ((sizeof(address) * 8 > sbasize) && (address >> sbasize)) {
+		LOG_DEBUG("Skipping mem %s via system bus - sba only supports %u-bit address.",
+				read ? "read" : "write", sbasize);
+		*skip_reason = "skipped (too large address)";
+		return true;
+	}
+	if (read && increment != size && (get_field(info->sbcs, DM_SBCS_SBVERSION) == 0 || increment != 0)) {
+		LOG_DEBUG("Skipping mem read via system bus - "
+				"sba reads only support size==increment or also size==0 for sba v1.");
+		*skip_reason = "skipped (unsupported increment)";
+		return true;
+	}
+
+	return false;
+}
+
+static bool mem_should_skip_abstract(struct target *target, target_addr_t address,
+		uint32_t size, uint32_t increment, bool read, char **skip_reason)
+{
+	assert(skip_reason);
+
+	if (size > 8) {
+		/* TODO: Add 128b support if it's ever used. Involves modifying
+				 read/write_abstract_arg() to work on two 64b values. */
+		LOG_DEBUG("Skipping mem %s via abstract access - unsupported size: %d bits",
+				read ? "read" : "write", size * 8);
+		*skip_reason = "skipped (unsupported size)";
+		return true;
+	}
+	if ((sizeof(address) * 8 > riscv_xlen(target)) && (address >> riscv_xlen(target))) {
+		LOG_DEBUG("Skipping mem %s via abstract access - abstract access only supports %u-bit address.",
+				read ? "read" : "write", riscv_xlen(target));
+		*skip_reason = "skipped (too large address)";
+		return true;
+	}
+	if (read && size != increment) {
+		LOG_ERROR("Skipping mem read via abstract access - "
+				"abstract command reads only support size==increment.");
+		*skip_reason = "skipped (unsupported increment)";
+		return true;
+	}
+
+	return false;
+}
+
 /*
  * Performs a memory read using memory access abstract commands. The read sizes
  * supported are 1, 2, and 4 bytes despite the spec's support of 8 and 16 byte
@@ -2680,12 +2833,10 @@ static int batch_run(const struct target *target, struct riscv_batch *batch)
 static int read_memory_abstract(struct target *target, target_addr_t address,
 		uint32_t size, uint32_t count, uint8_t *buffer, uint32_t increment)
 {
-	if (size != increment) {
-		LOG_ERROR("abstract command reads only support size==increment");
-		return ERROR_NOT_IMPLEMENTED;
-	}
+	RISCV013_INFO(info);
 
 	int result = ERROR_OK;
+	bool use_aampostincrement = info->has_aampostincrement != YNM_NO;
 
 	LOG_DEBUG("reading %d words of %d bytes from 0x%" TARGET_PRIxADDR, count,
 			  size, address);
@@ -2694,25 +2845,19 @@ static int read_memory_abstract(struct target *target, target_addr_t address,
 
 	/* Convert the size (bytes) to width (bits) */
 	unsigned width = size << 3;
-	if (width > 64) {
-		/* TODO: Add 128b support if it's ever used. Involves modifying
-				 read/write_abstract_arg() to work on two 64b values. */
-		LOG_ERROR("Unsupported size: %d bits", size);
-		return ERROR_FAIL;
-	}
 
 	/* Create the command (physical address, postincrement, read) */
-	uint32_t command = access_memory_command(target, false, width, true, false);
+	uint32_t command = access_memory_command(target, false, width, use_aampostincrement, false);
 
 	/* Execute the reads */
 	uint8_t *p = buffer;
 	bool updateaddr = true;
-	unsigned width32 = (width + 31) / 32 * 32;
+	unsigned width32 = (width < 32) ? 32 : width;
 	for (uint32_t c = 0; c < count; c++) {
-		/* Only update the address initially and let postincrement update it */
+		/* Update the address if it is the first time or aampostincrement is not supported by the target. */
 		if (updateaddr) {
 			/* Set arg1 to the address: address + c * size */
-			result = write_abstract_arg(target, 1, address, riscv_xlen(target));
+			result = write_abstract_arg(target, 1, address + c * size, riscv_xlen(target));
 			if (result != ERROR_OK) {
 				LOG_ERROR("Failed to write arg1 during read_memory_abstract().");
 				return result;
@@ -2721,16 +2866,38 @@ static int read_memory_abstract(struct target *target, target_addr_t address,
 
 		/* Execute the command */
 		result = execute_abstract_command(target, command);
-		if (result != ERROR_OK) {
-			LOG_ERROR("Failed to execute command read_memory_abstract().");
-			return result;
+
+		if (info->has_aampostincrement == YNM_MAYBE) {
+			if (result == ERROR_OK) {
+				/* Safety: double-check that the address was really auto-incremented */
+				riscv_reg_t new_address = read_abstract_arg(target, 1, riscv_xlen(target));
+				if (new_address == address + size) {
+					LOG_DEBUG("aampostincrement is supported on this target.");
+					info->has_aampostincrement = YNM_YES;
+				} else {
+					LOG_WARNING("Buggy aampostincrement! Address not incremented correctly.");
+					info->has_aampostincrement = YNM_NO;
+				}
+			} else {
+				/* Try the same access but with postincrement disabled. */
+				command = access_memory_command(target, false, width, false, false);
+				result = execute_abstract_command(target, command);
+				if (result == ERROR_OK) {
+					LOG_DEBUG("aampostincrement is not supported on this target.");
+					info->has_aampostincrement = YNM_NO;
+				}
+			}
 		}
 
+		if (result != ERROR_OK)
+			return result;
+
 		/* Copy arg0 to buffer (rounded width up to nearest 32) */
 		riscv_reg_t value = read_abstract_arg(target, 0, width32);
 		write_to_buf(p, value, size);
 
-		updateaddr = false;
+		if (info->has_aampostincrement == YNM_YES)
+			updateaddr = false;
 		p += size;
 	}
 
@@ -2745,22 +2912,18 @@ static int read_memory_abstract(struct target *target, target_addr_t address,
 static int write_memory_abstract(struct target *target, target_addr_t address,
 		uint32_t size, uint32_t count, const uint8_t *buffer)
 {
+	RISCV013_INFO(info);
 	int result = ERROR_OK;
+	bool use_aampostincrement = info->has_aampostincrement != YNM_NO;
 
 	LOG_DEBUG("writing %d words of %d bytes from 0x%" TARGET_PRIxADDR, count,
 			  size, address);
 
 	/* Convert the size (bytes) to width (bits) */
 	unsigned width = size << 3;
-	if (width > 64) {
-		/* TODO: Add 128b support if it's ever used. Involves modifying
-				 read/write_abstract_arg() to work on two 64b values. */
-		LOG_ERROR("Unsupported size: %d bits", width);
-		return ERROR_FAIL;
-	}
 
 	/* Create the command (physical address, postincrement, write) */
-	uint32_t command = access_memory_command(target, false, width, true, true);
+	uint32_t command = access_memory_command(target, false, width, use_aampostincrement, true);
 
 	/* Execute the writes */
 	const uint8_t *p = buffer;
@@ -2774,10 +2937,10 @@ static int write_memory_abstract(struct target *target, target_addr_t address,
 			return result;
 		}
 
-		/* Only update the address initially and let postincrement update it */
+		/* Update the address if it is the first time or aampostincrement is not supported by the target. */
 		if (updateaddr) {
 			/* Set arg1 to the address: address + c * size */
-			result = write_abstract_arg(target, 1, address, riscv_xlen(target));
+			result = write_abstract_arg(target, 1, address + c * size, riscv_xlen(target));
 			if (result != ERROR_OK) {
 				LOG_ERROR("Failed to write arg1 during write_memory_abstract().");
 				return result;
@@ -2786,12 +2949,34 @@ static int write_memory_abstract(struct target *target, target_addr_t address,
 
 		/* Execute the command */
 		result = execute_abstract_command(target, command);
-		if (result != ERROR_OK) {
-			LOG_ERROR("Failed to execute command write_memory_abstract().");
-			return result;
+
+		if (info->has_aampostincrement == YNM_MAYBE) {
+			if (result == ERROR_OK) {
+				/* Safety: double-check that the address was really auto-incremented */
+				riscv_reg_t new_address = read_abstract_arg(target, 1, riscv_xlen(target));
+				if (new_address == address + size) {
+					LOG_DEBUG("aampostincrement is supported on this target.");
+					info->has_aampostincrement = YNM_YES;
+				} else {
+					LOG_WARNING("Buggy aampostincrement! Address not incremented correctly.");
+					info->has_aampostincrement = YNM_NO;
+				}
+			} else {
+				/* Try the same access but with postincrement disabled. */
+				command = access_memory_command(target, false, width, false, true);
+				result = execute_abstract_command(target, command);
+				if (result == ERROR_OK) {
+					LOG_DEBUG("aampostincrement is not supported on this target.");
+					info->has_aampostincrement = YNM_NO;
+				}
+			}
 		}
 
-		updateaddr = false;
+		if (result != ERROR_OK)
+			return result;
+
+		if (info->has_aampostincrement == YNM_YES)
+			updateaddr = false;
 		p += size;
 	}
 
@@ -3113,12 +3298,6 @@ static int read_memory_progbuf_one(struct target *target, target_addr_t address,
 static int read_memory_progbuf(struct target *target, target_addr_t address,
 		uint32_t size, uint32_t count, uint8_t *buffer, uint32_t increment)
 {
-	if (riscv_xlen(target) < size * 8) {
-		LOG_ERROR("XLEN (%d) is too short for %d-bit memory read.",
-				riscv_xlen(target), size * 8);
-		return ERROR_FAIL;
-	}
-
 	int result = ERROR_OK;
 
 	LOG_DEBUG("reading %d words of %d bytes from 0x%" TARGET_PRIxADDR, count,
@@ -3234,30 +3413,62 @@ static int read_memory(struct target *target, target_addr_t address,
 	if (count == 0)
 		return ERROR_OK;
 
-	RISCV013_INFO(info);
-	if (has_sufficient_progbuf(target, 3) && !riscv_prefer_sba)
-		return read_memory_progbuf(target, address, size, count, buffer,
-			increment);
-
-	if ((get_field(info->sbcs, DM_SBCS_SBACCESS8) && size == 1) ||
-			(get_field(info->sbcs, DM_SBCS_SBACCESS16) && size == 2) ||
-			(get_field(info->sbcs, DM_SBCS_SBACCESS32) && size == 4) ||
-			(get_field(info->sbcs, DM_SBCS_SBACCESS64) && size == 8) ||
-			(get_field(info->sbcs, DM_SBCS_SBACCESS128) && size == 16)) {
-		if (get_field(info->sbcs, DM_SBCS_SBVERSION) == 0)
-			return read_memory_bus_v0(target, address, size, count, buffer,
-				increment);
-		else if (get_field(info->sbcs, DM_SBCS_SBVERSION) == 1)
-			return read_memory_bus_v1(target, address, size, count, buffer,
-				increment);
+	if (size != 1 && size != 2 && size != 4 && size != 8 && size != 16) {
+		LOG_ERROR("BUG: Unsupported size for memory read: %d", size);
+		return ERROR_FAIL;
 	}
 
-	if (has_sufficient_progbuf(target, 3))
-		return read_memory_progbuf(target, address, size, count, buffer,
-			increment);
+	int ret = ERROR_FAIL;
+	RISCV_INFO(r);
+	RISCV013_INFO(info);
+
+	char *progbuf_result = "disabled";
+	char *sysbus_result = "disabled";
+	char *abstract_result = "disabled";
+
+	for (unsigned i = 0; i < RISCV_NUM_MEM_ACCESS_METHODS; i++) {
+		int method = r->mem_access_methods[i];
+
+		if (method == RISCV_MEM_ACCESS_PROGBUF) {
+			if (mem_should_skip_progbuf(target, address, size, true, &progbuf_result))
+				continue;
+
+			ret = read_memory_progbuf(target, address, size, count, buffer, increment);
+
+			if (ret != ERROR_OK)
+				progbuf_result = "failed";
+		} else if (method == RISCV_MEM_ACCESS_SYSBUS) {
+			if (mem_should_skip_sysbus(target, address, size, increment, true, &sysbus_result))
+				continue;
+
+			if (get_field(info->sbcs, DM_SBCS_SBVERSION) == 0)
+				ret = read_memory_bus_v0(target, address, size, count, buffer, increment);
+			else if (get_field(info->sbcs, DM_SBCS_SBVERSION) == 1)
+				ret = read_memory_bus_v1(target, address, size, count, buffer, increment);
+
+			if (ret != ERROR_OK)
+				sysbus_result = "failed";
+		} else if (method == RISCV_MEM_ACCESS_ABSTRACT) {
+			if (mem_should_skip_abstract(target, address, size, increment, true, &abstract_result))
+				continue;
+
+			ret = read_memory_abstract(target, address, size, count, buffer, increment);
 
-	return read_memory_abstract(target, address, size, count, buffer,
-		increment);
+			if (ret != ERROR_OK)
+				abstract_result = "failed";
+		} else if (method == RISCV_MEM_ACCESS_UNSPECIFIED)
+			/* No further mem access method to try. */
+			break;
+
+		log_mem_access_result(target, ret == ERROR_OK, method, true);
+
+		if (ret == ERROR_OK)
+			return ret;
+	}
+
+	LOG_ERROR("Target %s: Failed to read memory (addr=0x%" PRIx64 ")", target_name(target), address);
+	LOG_ERROR("  progbuf=%s, sysbus=%s, abstract=%s", progbuf_result, sysbus_result, abstract_result);
+	return ret;
 }
 
 static int write_memory_bus_v0(struct target *target, target_addr_t address,
@@ -3473,12 +3684,6 @@ static int write_memory_progbuf(struct target *target, target_addr_t address,
 {
 	RISCV013_INFO(info);
 
-	if (riscv_xlen(target) < size * 8) {
-		LOG_ERROR("XLEN (%d) is too short for %d-bit memory write.",
-				riscv_xlen(target), size * 8);
-		return ERROR_FAIL;
-	}
-
 	LOG_DEBUG("writing %d words of %d bytes to 0x%08lx", count, size, (long)address);
 
 	select_dmi(target);
@@ -3663,26 +3868,62 @@ error:
 static int write_memory(struct target *target, target_addr_t address,
 		uint32_t size, uint32_t count, const uint8_t *buffer)
 {
+	if (size != 1 && size != 2 && size != 4 && size != 8 && size != 16) {
+		LOG_ERROR("BUG: Unsupported size for memory write: %d", size);
+		return ERROR_FAIL;
+	}
+
+	int ret = ERROR_FAIL;
+	RISCV_INFO(r);
 	RISCV013_INFO(info);
 
-	if (has_sufficient_progbuf(target, 3) && !riscv_prefer_sba)
-		return write_memory_progbuf(target, address, size, count, buffer);
+	char *progbuf_result = "disabled";
+	char *sysbus_result = "disabled";
+	char *abstract_result = "disabled";
 
-	if ((get_field(info->sbcs, DM_SBCS_SBACCESS8) && size == 1) ||
-			(get_field(info->sbcs, DM_SBCS_SBACCESS16) && size == 2) ||
-			(get_field(info->sbcs, DM_SBCS_SBACCESS32) && size == 4) ||
-			(get_field(info->sbcs, DM_SBCS_SBACCESS64) && size == 8) ||
-			(get_field(info->sbcs, DM_SBCS_SBACCESS128) && size == 16)) {
-		if (get_field(info->sbcs, DM_SBCS_SBVERSION) == 0)
-			return write_memory_bus_v0(target, address, size, count, buffer);
-		else if (get_field(info->sbcs, DM_SBCS_SBVERSION) == 1)
-			return write_memory_bus_v1(target, address, size, count, buffer);
-	}
+	for (unsigned i = 0; i < RISCV_NUM_MEM_ACCESS_METHODS; i++) {
+		int method = r->mem_access_methods[i];
 
-	if (has_sufficient_progbuf(target, 3))
-		return write_memory_progbuf(target, address, size, count, buffer);
+		if (method == RISCV_MEM_ACCESS_PROGBUF) {
+			if (mem_should_skip_progbuf(target, address, size, false, &progbuf_result))
+				continue;
+
+			ret = write_memory_progbuf(target, address, size, count, buffer);
+
+			if (ret != ERROR_OK)
+				progbuf_result = "failed";
+		} else if (method == RISCV_MEM_ACCESS_SYSBUS) {
+			if (mem_should_skip_sysbus(target, address, size, 0, false, &sysbus_result))
+				continue;
+
+			if (get_field(info->sbcs, DM_SBCS_SBVERSION) == 0)
+				ret = write_memory_bus_v0(target, address, size, count, buffer);
+			else if (get_field(info->sbcs, DM_SBCS_SBVERSION) == 1)
+				ret = write_memory_bus_v1(target, address, size, count, buffer);
+
+			if (ret != ERROR_OK)
+				sysbus_result = "failed";
+		} else if (method == RISCV_MEM_ACCESS_ABSTRACT) {
+			if (mem_should_skip_abstract(target, address, size, 0, false, &abstract_result))
+				continue;
+
+			ret = write_memory_abstract(target, address, size, count, buffer);
+
+			if (ret != ERROR_OK)
+				abstract_result = "failed";
+		} else if (method == RISCV_MEM_ACCESS_UNSPECIFIED)
+			/* No further mem access method to try. */
+			break;
+
+		log_mem_access_result(target, ret == ERROR_OK, method, false);
+
+		if (ret == ERROR_OK)
+			return ret;
+	}
 
-	return write_memory_abstract(target, address, size, count, buffer);
+	LOG_ERROR("Target %s: Failed to write memory (addr=0x%" PRIx64 ")", target_name(target), address);
+	LOG_ERROR("  progbuf=%s, sysbus=%s, abstract=%s", progbuf_result, sysbus_result, abstract_result);
+	return ret;
 }
 
 static int arch_state(struct target *target)
diff --git a/src/target/riscv/riscv.c b/src/target/riscv/riscv.c
index ab5729d..dd9564e 100644
--- a/src/target/riscv/riscv.c
+++ b/src/target/riscv/riscv.c
@@ -252,7 +252,6 @@ int riscv_command_timeout_sec = DEFAULT_COMMAND_TIMEOUT_SEC;
 /* Wall-clock timeout after reset. Settable via RISC-V Target commands.*/
 int riscv_reset_timeout_sec = DEFAULT_RESET_TIMEOUT_SEC;
 
-bool riscv_prefer_sba;
 bool riscv_enable_virt2phys = true;
 bool riscv_ebreakm = true;
 bool riscv_ebreaks = true;
@@ -2287,11 +2286,84 @@ COMMAND_HANDLER(riscv_test_compliance) {
 
 COMMAND_HANDLER(riscv_set_prefer_sba)
 {
+	struct target *target = get_current_target(CMD_CTX);
+	RISCV_INFO(r);
+	bool prefer_sba;
+	LOG_WARNING("`riscv set_prefer_sba` is deprecated. Please use `riscv set_mem_access` instead.");
 	if (CMD_ARGC != 1) {
 		LOG_ERROR("Command takes exactly 1 parameter");
 		return ERROR_COMMAND_SYNTAX_ERROR;
 	}
-	COMMAND_PARSE_ON_OFF(CMD_ARGV[0], riscv_prefer_sba);
+	COMMAND_PARSE_ON_OFF(CMD_ARGV[0], prefer_sba);
+	if (prefer_sba) {
+		/* Use system bus with highest priority */
+		r->mem_access_methods[0] = RISCV_MEM_ACCESS_SYSBUS;
+		r->mem_access_methods[1] = RISCV_MEM_ACCESS_PROGBUF;
+		r->mem_access_methods[2] = RISCV_MEM_ACCESS_ABSTRACT;
+	} else {
+		/* Use progbuf with highest priority */
+		r->mem_access_methods[0] = RISCV_MEM_ACCESS_PROGBUF;
+		r->mem_access_methods[1] = RISCV_MEM_ACCESS_SYSBUS;
+		r->mem_access_methods[2] = RISCV_MEM_ACCESS_ABSTRACT;
+	}
+
+	/* Reset warning flags */
+	r->mem_access_progbuf_warn = true;
+	r->mem_access_sysbus_warn = true;
+	r->mem_access_abstract_warn = true;
+
+	return ERROR_OK;
+}
+
+COMMAND_HANDLER(riscv_set_mem_access)
+{
+	struct target *target = get_current_target(CMD_CTX);
+	RISCV_INFO(r);
+	int progbuf_cnt = 0;
+	int sysbus_cnt = 0;
+	int abstract_cnt = 0;
+
+	if (CMD_ARGC < 1 || CMD_ARGC > RISCV_NUM_MEM_ACCESS_METHODS) {
+		LOG_ERROR("Command takes 1 to %d parameters", RISCV_NUM_MEM_ACCESS_METHODS);
+		return ERROR_COMMAND_SYNTAX_ERROR;
+	}
+
+	/* Check argument validity */
+	for (unsigned i = 0; i < CMD_ARGC; i++) {
+		if (strcmp("progbuf", CMD_ARGV[i]) == 0)
+			progbuf_cnt++;
+		else if (strcmp("sysbus", CMD_ARGV[i]) == 0)
+			sysbus_cnt++;
+		else if (strcmp("abstract", CMD_ARGV[i]) == 0)
+			abstract_cnt++;
+		else {
+			LOG_ERROR("Unknown argument '%s'. "
+				"Must be one of: 'progbuf', 'sysbus' or 'abstract'.", CMD_ARGV[i]);
+			return ERROR_COMMAND_SYNTAX_ERROR;
+		}
+	}
+	if (progbuf_cnt > 1 || sysbus_cnt > 1 || abstract_cnt > 1) {
+		LOG_ERROR("Syntax error - duplicate arguments to `riscv set_mem_access`.");
+		return ERROR_COMMAND_SYNTAX_ERROR;
+	}
+
+	/* Args are valid, store them */
+	for (unsigned i = 0; i < RISCV_NUM_MEM_ACCESS_METHODS; i++)
+		r->mem_access_methods[i] = RISCV_MEM_ACCESS_UNSPECIFIED;
+	for (unsigned i = 0; i < CMD_ARGC; i++) {
+		if (strcmp("progbuf", CMD_ARGV[i]) == 0)
+			r->mem_access_methods[i] = RISCV_MEM_ACCESS_PROGBUF;
+		else if (strcmp("sysbus", CMD_ARGV[i]) == 0)
+			r->mem_access_methods[i] = RISCV_MEM_ACCESS_SYSBUS;
+		else if (strcmp("abstract", CMD_ARGV[i]) == 0)
+			r->mem_access_methods[i] = RISCV_MEM_ACCESS_ABSTRACT;
+	}
+
+	/* Reset warning flags */
+	r->mem_access_progbuf_warn = true;
+	r->mem_access_sysbus_warn = true;
+	r->mem_access_abstract_warn = true;
+
 	return ERROR_OK;
 }
 
@@ -2742,6 +2814,14 @@ static const struct command_registration riscv_exec_command_handlers[] = {
 			"When off (default), prefer to use the Program Buffer to access memory."
 	},
 	{
+		.name = "set_mem_access",
+		.handler = riscv_set_mem_access,
+		.mode = COMMAND_ANY,
+		.usage = "method1 [method2] [method3]",
+		.help = "Set which memory access methods shall be used and in which order "
+			"of priority. Method can be one of: 'progbuf', 'sysbus' or 'abstract'."
+	},
+	{
 		.name = "set_enable_virtual",
 		.handler = riscv_set_enable_virtual,
 		.mode = COMMAND_ANY,
@@ -2987,6 +3067,14 @@ void riscv_info_init(struct target *target, riscv_info_t *r)
 
 	for (size_t h = 0; h < RISCV_MAX_HARTS; ++h)
 		r->xlen[h] = -1;
+
+	r->mem_access_methods[0] = RISCV_MEM_ACCESS_PROGBUF;
+	r->mem_access_methods[1] = RISCV_MEM_ACCESS_SYSBUS;
+	r->mem_access_methods[2] = RISCV_MEM_ACCESS_ABSTRACT;
+
+	r->mem_access_progbuf_warn = true;
+	r->mem_access_sysbus_warn = true;
+	r->mem_access_abstract_warn = true;
 }
 
 static int riscv_resume_go_all_harts(struct target *target)
diff --git a/src/target/riscv/riscv.h b/src/target/riscv/riscv.h
index 5e171d9..458a0d8 100644
--- a/src/target/riscv/riscv.h
+++ b/src/target/riscv/riscv.h
@@ -26,6 +26,8 @@ struct riscv_program;
 
 # define PG_MAX_LEVEL 4
 
+#define RISCV_NUM_MEM_ACCESS_METHODS  3
+
 extern struct target_type riscv011_target;
 extern struct target_type riscv013_target;
 
@@ -36,6 +38,13 @@ typedef uint64_t riscv_reg_t;
 typedef uint32_t riscv_insn_t;
 typedef uint64_t riscv_addr_t;
 
+enum riscv_mem_access_method {
+	RISCV_MEM_ACCESS_UNSPECIFIED,
+	RISCV_MEM_ACCESS_PROGBUF,
+	RISCV_MEM_ACCESS_SYSBUS,
+	RISCV_MEM_ACCESS_ABSTRACT
+};
+
 enum riscv_halt_reason {
 	RISCV_HALT_INTERRUPT,
 	RISCV_HALT_BREAKPOINT,
@@ -179,6 +188,15 @@ typedef struct {
 	/* Set when trigger registers are changed by the user. This indicates we eed
 	 * to beware that we may hit a trigger that we didn't realize had been set. */
 	bool manual_hwbp_set;
+
+	/* Memory access methods to use, ordered by priority, highest to lowest. */
+	int mem_access_methods[RISCV_NUM_MEM_ACCESS_METHODS];
+
+	/* Different memory regions may need different methods but single configuration is applied
+	 * for all. Following flags are used to warn only once about failing memory access method. */
+	bool mem_access_progbuf_warn;
+	bool mem_access_sysbus_warn;
+	bool mem_access_abstract_warn;
 } riscv_info_t;
 
 typedef struct {
@@ -205,8 +223,6 @@ extern int riscv_command_timeout_sec;
 /* Wall-clock timeout after reset. Settable via RISC-V Target commands.*/
 extern int riscv_reset_timeout_sec;
 
-extern bool riscv_prefer_sba;
-
 extern bool riscv_enable_virtual;
 extern bool riscv_ebreakm;
 extern bool riscv_ebreaks;