stm class: Factor out default framing protocol

The STP framing pattern that the stm class implicitly applies to the
data payload is, in fact, a protocol. This patch moves the relevant code
out of the stm core into its own driver module.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Tested-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hwtracing/stm/p_basic.c

@@ -0,0 +1,48 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Basic framing protocol for STM devices.
+ * Copyright (c) 2018, Intel Corporation.
+ */
+
+#include <linux/module.h>
+#include <linux/device.h>
+#include <linux/stm.h>
+#include "stm.h"
+
+static ssize_t basic_write(struct stm_data *data, struct stm_output *output,
+			   unsigned int chan, const char *buf, size_t count)
+{
+	unsigned int c = output->channel + chan;
+	unsigned int m = output->master;
+	const unsigned char nil = 0;
+	ssize_t sz;
+
+	sz = stm_data_write(data, m, c, true, buf, count);
+	if (sz > 0)
+		data->packet(data, m, c, STP_PACKET_FLAG, 0, 0, &nil);
+
+	return sz;
+}
+
+static const struct stm_protocol_driver basic_pdrv = {
+	.owner	= THIS_MODULE,
+	.name	= "p_basic",
+	.write	= basic_write,
+};
+
+static int basic_stm_init(void)
+{
+	return stm_register_protocol(&basic_pdrv);
+}
+
+static void basic_stm_exit(void)
+{
+	stm_unregister_protocol(&basic_pdrv);
+}
+
+module_init(basic_stm_init);
+module_exit(basic_stm_exit);
+
+MODULE_LICENSE("GPL v2");
+MODULE_DESCRIPTION("Basic STM framing protocol driver");
+MODULE_AUTHOR("Alexander Shishkin <alexander.shishkin@linux.intel.com>");