tcl/tools: Add function to measure the speed of ARM Cortex-M devices

Tested on an EFM32PG12 Starter Kit.

Change-Id: I2cbc36fe0d2ad2089bf8c1e7d2260daaae4ddbb4
Signed-off-by: Marc Schink <dev@zapb.de>
Reviewed-on: https://review.openocd.org/c/openocd/+/5353
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

tcl/tools/test_cpu_speed.tcl

@@ -0,0 +1,50 @@
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+# Description:
+#  Measure the CPU clock frequency of an ARM Cortex-M based device.
+#
+# Return:
+#  The CPU clock frequency in Hz. A negative value indicates that the loop
+#  counter was saturated.
+#
+# Note:
+#  You may need to adapt the number of cycles for your device.
+#
+add_help_text cortex_m_test_cpu_speed "Measure the CPU clock frequency of an ARM Cortex-M based device"
+add_usage_text cortex_m_test_cpu_speed {address [timeout [cycles_per_loop]]}
+proc cortex_m_test_cpu_speed { address { timeout 200 } { cycles_per_loop 4 } } {
+	set loop_counter_start 0xffffffff
+
+	halt
+
+	# Backup registers and memory.
+	set backup_regs [get_reg -force {pc r0 xPSR}]
+	set backup_mem [read_memory $address 16 3]
+
+	# We place the following code at the given address to measure the
+	# CPU clock frequency:
+	#
+	# 3801: subs r0, #1
+	# d1fd: bne #-2
+	# e7fe: b #-4
+	write_memory $address 16 {0x3801 0xd1fd 0xe7fe}
+
+	set_reg "pc $address r0 $loop_counter_start"
+	resume
+	sleep $timeout
+	halt
+
+	# Get the loop counter value from register r0.
+	set loop_counter_end [dict values [get_reg r0]]
+	set loop_counter_diff [expr {$loop_counter_start - $loop_counter_end}]
+
+	# Restore registers and memory.
+	set_reg $backup_regs
+	write_memory $address 16 $backup_mem
+
+	if { [expr {$loop_counter_end == 0}] } {
+		return -1
+	}
+
+	return [expr {double($loop_counter_diff) * $cycles_per_loop / $timeout * 1000}]
+}