logger: add a SWO backend

This commit adds Serial Wire Output (SWO) logger backend. SWO is an
extension of Serial Wire Debug (SWD) port developed by ARM.

Signed-off-by: Piotr Mienkowski <piotr.mienkowski@gmail.com>

soc/arm/Kconfig

@@ -20,3 +20,9 @@ config CPU_HAS_ARM_SAU
 	select CPU_HAS_TEE
 	help
 	  MCU implements the ARM Security Attribution Unit (SAU).
+
+config HAS_SWO
+	bool
+	# Omit prompt to signify "hidden" option
+	help
+	  When enabled, indicates that SoC has an SWO output

subsys/logging/CMakeLists.txt

@@ -33,3 +33,8 @@ zephyr_sources_ifdef(
   CONFIG_LOG_BACKEND_RTT
   log_backend_rtt.c
 )
+
+zephyr_sources_ifdef(
+  CONFIG_LOG_BACKEND_SWO
+  log_backend_swo.c
+)

subsys/logging/Kconfig

@@ -302,6 +302,31 @@ config LOG_BACKEND_UART
 	help
 	  When enabled backend is using UART to output logs.
 
+config LOG_BACKEND_SWO
+	bool "Enable Serial Wire Output (SWO) backend"
+	depends on HAS_SWO
+	help
+	  When enabled, backend will use SWO for logging.
+
+if LOG_BACKEND_SWO
+
+config LOG_BACKEND_SWO_FREQ_HZ
+	int "Set SWO output frequency"
+	default 0
+	help
+	  Set SWO output frequency. Value 0 will select maximum frequency
+	  supported by the given MCU. Not all debug probes support high
+	  frequency SWO operation. In this case the frequency has to be set
+	  manually.
+
+	  SWO value defined by this option will be configured at boot. Most SWO
+	  viewer programs will configure SWO frequency when attached to the
+	  debug probe. Such configuration will persist only until the device
+	  reset. To ensure flawless operation the frequency configured here and
+	  by the SWO viewer program has to match.
+
+endif # LOG_BACKEND_SWO
+
 config LOG_BACKEND_RTT
 	bool "Enable Segger J-Link RTT backend"
 	depends on USE_SEGGER_RTT
@@ -448,14 +473,16 @@ endif # LOG_BACKEND_NET
 
 config LOG_BACKEND_SHOW_COLOR
 	bool "Enable colors in the backend"
-	depends on LOG_BACKEND_UART || LOG_BACKEND_NATIVE_POSIX || LOG_BACKEND_RTT
+	depends on LOG_BACKEND_UART || LOG_BACKEND_NATIVE_POSIX || LOG_BACKEND_RTT \
+	           || LOG_BACKEND_SWO
 	default y
 	help
 	  When enabled selected backend prints errors in red and warning in yellow.
 
 config LOG_BACKEND_FORMAT_TIMESTAMP
 	bool "Enable timestamp formatting in the backend"
-	depends on LOG_BACKEND_UART || LOG_BACKEND_NATIVE_POSIX || LOG_BACKEND_RTT
+	depends on LOG_BACKEND_UART || LOG_BACKEND_NATIVE_POSIX || LOG_BACKEND_RTT \
+	           || LOG_BACKEND_SWO
 	default y
 	help
 	  When enabled timestamp is formatted to hh:mm:ss:ms,us.

subsys/logging/log_backend_swo.c

@@ -0,0 +1,119 @@
+/*
+ * Copyright (c) 2018 Piotr Mienkowski
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/** @file
+ * @brief Serial Wire Output (SWO) backend implementation.
+ *
+ * SWO/SWV has been developed by ARM. The following code works only on ARM
+ * architecture.
+ *
+ * An SWO viewer program will typically set-up the SWO port including its
+ * frequency when connected to the debug probe. Such configuration can persist
+ * only until the MCU reset. The SWO backend initialization function will
+ * re-configure the SWO port upon boot and set the frequency as specified by
+ * the LOG_BACKEND_SWO_FREQ_HZ Kconfig option. To ensure flawless operation
+ * this frequency should much the one set by the SWO viewer program.
+ *
+ * The initialization code assumes that SWO core frequency is equal to HCLK
+ * as defined by SYS_CLOCK_HW_CYCLES_PER_SEC Kconfig option. This may require
+ * additional, vendor specific configuration.
+ */
+
+#include <logging/log_backend.h>
+#include <logging/log_core.h>
+#include <logging/log_msg.h>
+#include <logging/log_output.h>
+#include <soc.h>
+
+/** The stimulus port from which SWO data is received and displayed */
+#define ITM_PORT_LOGGER       0
+
+/* Set TPIU prescaler for the current debug trace clock frequency. */
+#if CONFIG_LOG_BACKEND_SWO_FREQ_HZ == 0
+#define SWO_FREQ_DIV  1
+#else
+#define SWO_FREQ (CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC \
+		  + (CONFIG_LOG_BACKEND_SWO_FREQ_HZ / 2))
+#define SWO_FREQ_DIV  (SWO_FREQ / CONFIG_LOG_BACKEND_SWO_FREQ_HZ)
+#if SWO_FREQ_DIV > 0xFFFF
+#error CONFIG_LOG_BACKEND_SWO_FREQ_HZ is too low. SWO clock divider is 16-bit. \
+	Minimum supported SWO clock frequency is \
+	CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC/2^16.
+#endif
+#endif
+
+static u8_t buf[1];
+
+static int char_out(u8_t *data, size_t length, void *ctx)
+{
+	ARG_UNUSED(ctx);
+
+	for (size_t i = 0; i < length; i++) {
+		ITM_SendChar(data[i]);
+	}
+
+	return length;
+}
+
+LOG_OUTPUT_DEFINE(log_output, char_out, buf, sizeof(buf));
+
+static void log_backend_swo_put(const struct log_backend *const backend,
+		struct log_msg *msg)
+{
+	log_msg_get(msg);
+
+	u32_t flags = LOG_OUTPUT_FLAG_LEVEL | LOG_OUTPUT_FLAG_TIMESTAMP;
+
+	if (IS_ENABLED(CONFIG_LOG_BACKEND_SHOW_COLOR)) {
+		flags |= LOG_OUTPUT_FLAG_COLORS;
+	}
+
+	if (IS_ENABLED(CONFIG_LOG_BACKEND_FORMAT_TIMESTAMP)) {
+		flags |= LOG_OUTPUT_FLAG_FORMAT_TIMESTAMP;
+	}
+
+	log_output_msg_process(&log_output, msg, flags);
+
+	log_msg_put(msg);
+}
+
+static void log_backend_swo_init(void)
+{
+	/* Enable DWT and ITM units */
+	CoreDebug->DEMCR |= CoreDebug_DEMCR_TRCENA_Msk;
+	/* Enable access to ITM registers */
+	ITM->LAR  = 0xC5ACCE55;
+	/* Disable stimulus ports ITM_STIM0-ITM_STIM31 */
+	ITM->TER  = 0x0;
+	/* Disable ITM */
+	ITM->TCR  = 0x0;
+	/* Select NRZ (UART) encoding protocol */
+	TPI->SPPR = 2;
+	/* Set SWO baud rate prescaler value: SWO_clk = ref_clock/(ACPR + 1) */
+	TPI->ACPR = SWO_FREQ_DIV - 1;
+	/* Enable unprivileged access to ITM stimulus ports */
+	ITM->TPR  = 0x0;
+	/* Configure Debug Watchpoint and Trace */
+	DWT->CTRL = 0x400003FE;
+	/* Configure Formatter and Flush Control Register */
+	TPI->FFCR = 0x00000100;
+	/* Enable ITM, set TraceBusID=1, no local timestamp generation */
+	ITM->TCR  = 0x0001000D;
+	/* Enable stimulus port used by the logger */
+	ITM->TER  = 1 << ITM_PORT_LOGGER;
+}
+
+static void log_backend_swo_panic(struct log_backend const *const backend)
+{
+}
+
+const struct log_backend_api log_backend_swo_api = {
+	.put = log_backend_swo_put,
+	.panic = log_backend_swo_panic,
+	.init = log_backend_swo_init,
+};
+
+LOG_BACKEND_DEFINE(log_backend_swo, log_backend_swo_api, true);