stm class: Introduce an abstraction for System Trace Module devices

A System Trace Module (STM) is a device exporting data in System Trace
Protocol (STP) format as defined by MIPI STP standards. Examples of such
devices are Intel(R) Trace Hub and Coresight STM.

This abstraction provides a unified interface for software trace sources
to send their data over an STM device to a debug host. In order to do
that, such a trace source needs to be assigned a pair of master/channel
identifiers that all the data from this source will be tagged with. The
STP decoder on the debug host side will use these master/channel tags to
distinguish different trace streams from one another inside one STP
stream.

This abstraction provides a configfs-based policy management mechanism
for dynamic allocation of these master/channel pairs based on trace
source-supplied string identifier. It has the flexibility of being
defined at runtime and at the same time (provided that the policy
definition is aligned with the decoding end) consistency.

For userspace trace sources, this abstraction provides write()-based and
mmap()-based (if the underlying stm device allows this) output mechanism.

For kernel-side trace sources, we provide "stm_source" device class that
can be connected to an stm device at run time.

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

1 files changed, 1029 insertions, 0 deletions

diff --git a/drivers/hwtracing/stm/core.c b/drivers/hwtracing/stm/core.c
new file mode 100644
index 000000000000..b79c42c62557
--- /dev/null
+++ b/drivers/hwtracing/stm/core.c
@@ -0,0 +1,1029 @@
+/*
+ * System Trace Module (STM) infrastructure
+ * Copyright (c) 2014, Intel Corporation.
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms and conditions of the GNU General Public License,
+ * version 2, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+ * more details.
+ *
+ * STM class implements generic infrastructure for  System Trace Module devices
+ * as defined in MIPI STPv2 specification.
+ */
+
+#include <linux/uaccess.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/device.h>
+#include <linux/compat.h>
+#include <linux/kdev_t.h>
+#include <linux/srcu.h>
+#include <linux/slab.h>
+#include <linux/stm.h>
+#include <linux/fs.h>
+#include <linux/mm.h>
+#include "stm.h"
+
+#include <uapi/linux/stm.h>
+
+static unsigned int stm_core_up;
+
+/*
+ * The SRCU here makes sure that STM device doesn't disappear from under a
+ * stm_source_write() caller, which may want to have as little overhead as
+ * possible.
+ */
+static struct srcu_struct stm_source_srcu;
+
+static ssize_t masters_show(struct device *dev,
+			    struct device_attribute *attr,
+			    char *buf)
+{
+	struct stm_device *stm = to_stm_device(dev);
+	int ret;
+
+	ret = sprintf(buf, "%u %u\n", stm->data->sw_start, stm->data->sw_end);
+
+	return ret;
+}
+
+static DEVICE_ATTR_RO(masters);
+
+static ssize_t channels_show(struct device *dev,
+			     struct device_attribute *attr,
+			     char *buf)
+{
+	struct stm_device *stm = to_stm_device(dev);
+	int ret;
+
+	ret = sprintf(buf, "%u\n", stm->data->sw_nchannels);
+
+	return ret;
+}
+
+static DEVICE_ATTR_RO(channels);
+
+static struct attribute *stm_attrs[] = {
+	&dev_attr_masters.attr,
+	&dev_attr_channels.attr,
+	NULL,
+};
+
+ATTRIBUTE_GROUPS(stm);
+
+static struct class stm_class = {
+	.name		= "stm",
+	.dev_groups	= stm_groups,
+};
+
+static int stm_dev_match(struct device *dev, const void *data)
+{
+	const char *name = data;
+
+	return sysfs_streq(name, dev_name(dev));
+}
+
+/**
+ * stm_find_device() - find stm device by name
+ * @buf:	character buffer containing the name
+ *
+ * This is called when either policy gets assigned to an stm device or an
+ * stm_source device gets linked to an stm device.
+ *
+ * This grabs device's reference (get_device()) and module reference, both
+ * of which the calling path needs to make sure to drop with stm_put_device().
+ *
+ * Return:	stm device pointer or null if lookup failed.
+ */
+struct stm_device *stm_find_device(const char *buf)
+{
+	struct stm_device *stm;
+	struct device *dev;
+
+	if (!stm_core_up)
+		return NULL;
+
+	dev = class_find_device(&stm_class, NULL, buf, stm_dev_match);
+	if (!dev)
+		return NULL;
+
+	stm = to_stm_device(dev);
+	if (!try_module_get(stm->owner)) {
+		put_device(dev);
+		return NULL;
+	}
+
+	return stm;
+}
+
+/**
+ * stm_put_device() - drop references on the stm device
+ * @stm:	stm device, previously acquired by stm_find_device()
+ *
+ * This drops the module reference and device reference taken by
+ * stm_find_device().
+ */
+void stm_put_device(struct stm_device *stm)
+{
+	module_put(stm->owner);
+	put_device(&stm->dev);
+}
+
+/*
+ * Internally we only care about software-writable masters here, that is the
+ * ones in the range [stm_data->sw_start..stm_data..sw_end], however we need
+ * original master numbers to be visible externally, since they are the ones
+ * that will appear in the STP stream. Thus, the internal bookkeeping uses
+ * $master - stm_data->sw_start to reference master descriptors and such.
+ */
+
+#define __stm_master(_s, _m)				\
+	((_s)->masters[(_m) - (_s)->data->sw_start])
+
+static inline struct stp_master *
+stm_master(struct stm_device *stm, unsigned int idx)
+{
+	if (idx < stm->data->sw_start || idx > stm->data->sw_end)
+		return NULL;
+
+	return __stm_master(stm, idx);
+}
+
+static int stp_master_alloc(struct stm_device *stm, unsigned int idx)
+{
+	struct stp_master *master;
+	size_t size;
+
+	size = ALIGN(stm->data->sw_nchannels, 8) / 8;
+	size += sizeof(struct stp_master);
+	master = kzalloc(size, GFP_ATOMIC);
+	if (!master)
+		return -ENOMEM;
+
+	master->nr_free = stm->data->sw_nchannels;
+	__stm_master(stm, idx) = master;
+
+	return 0;
+}
+
+static void stp_master_free(struct stm_device *stm, unsigned int idx)
+{
+	struct stp_master *master = stm_master(stm, idx);
+
+	if (!master)
+		return;
+
+	__stm_master(stm, idx) = NULL;
+	kfree(master);
+}
+
+static void stm_output_claim(struct stm_device *stm, struct stm_output *output)
+{
+	struct stp_master *master = stm_master(stm, output->master);
+
+	if (WARN_ON_ONCE(master->nr_free < output->nr_chans))
+		return;
+
+	bitmap_allocate_region(&master->chan_map[0], output->channel,
+			       ilog2(output->nr_chans));
+
+	master->nr_free -= output->nr_chans;
+}
+
+static void
+stm_output_disclaim(struct stm_device *stm, struct stm_output *output)
+{
+	struct stp_master *master = stm_master(stm, output->master);
+
+	bitmap_release_region(&master->chan_map[0], output->channel,
+			      ilog2(output->nr_chans));
+
+	output->nr_chans = 0;
+	master->nr_free += output->nr_chans;
+}
+
+/*
+ * This is like bitmap_find_free_region(), except it can ignore @start bits
+ * at the beginning.
+ */
+static int find_free_channels(unsigned long *bitmap, unsigned int start,
+			      unsigned int end, unsigned int width)
+{
+	unsigned int pos;
+	int i;
+
+	for (pos = start; pos < end + 1; pos = ALIGN(pos, width)) {
+		pos = find_next_zero_bit(bitmap, end + 1, pos);
+		if (pos + width > end + 1)
+			break;
+
+		if (pos & (width - 1))
+			continue;
+
+		for (i = 1; i < width && !test_bit(pos + i, bitmap); i++)
+			;
+		if (i == width)
+			return pos;
+	}
+
+	return -1;
+}
+
+static unsigned int
+stm_find_master_chan(struct stm_device *stm, unsigned int width,
+		     unsigned int *mstart, unsigned int mend,
+		     unsigned int *cstart, unsigned int cend)
+{
+	struct stp_master *master;
+	unsigned int midx;
+	int pos, err;
+
+	for (midx = *mstart; midx <= mend; midx++) {
+		if (!stm_master(stm, midx)) {
+			err = stp_master_alloc(stm, midx);
+			if (err)
+				return err;
+		}
+
+		master = stm_master(stm, midx);
+
+		if (!master->nr_free)
+			continue;
+
+		pos = find_free_channels(master->chan_map, *cstart, cend,
+					 width);
+		if (pos < 0)
+			continue;
+
+		*mstart = midx;
+		*cstart = pos;
+		return 0;
+	}
+
+	return -ENOSPC;
+}
+
+static int stm_output_assign(struct stm_device *stm, unsigned int width,
+			     struct stp_policy_node *policy_node,
+			     struct stm_output *output)
+{
+	unsigned int midx, cidx, mend, cend;
+	int ret = -EINVAL;
+
+	if (width > stm->data->sw_nchannels)
+		return -EINVAL;
+
+	if (policy_node) {
+		stp_policy_node_get_ranges(policy_node,
+					   &midx, &mend, &cidx, &cend);
+	} else {
+		midx = stm->data->sw_start;
+		cidx = 0;
+		mend = stm->data->sw_end;
+		cend = stm->data->sw_nchannels - 1;
+	}
+
+	spin_lock(&stm->mc_lock);
+	/* output is already assigned -- shouldn't happen */
+	if (WARN_ON_ONCE(output->nr_chans))
+		goto unlock;
+
+	ret = stm_find_master_chan(stm, width, &midx, mend, &cidx, cend);
+	if (ret)
+		goto unlock;
+
+	output->master = midx;
+	output->channel = cidx;
+	output->nr_chans = width;
+	stm_output_claim(stm, output);
+	dev_dbg(&stm->dev, "assigned %u:%u (+%u)\n", midx, cidx, width);
+
+	ret = 0;
+unlock:
+	spin_unlock(&stm->mc_lock);
+
+	return ret;
+}
+
+static void stm_output_free(struct stm_device *stm, struct stm_output *output)
+{
+	spin_lock(&stm->mc_lock);
+	if (output->nr_chans)
+		stm_output_disclaim(stm, output);
+	spin_unlock(&stm->mc_lock);
+}
+
+static int major_match(struct device *dev, const void *data)
+{
+	unsigned int major = *(unsigned int *)data;
+
+	return MAJOR(dev->devt) == major;
+}
+
+static int stm_char_open(struct inode *inode, struct file *file)
+{
+	struct stm_file *stmf;
+	struct device *dev;
+	unsigned int major = imajor(inode);
+	int err = -ENODEV;
+
+	dev = class_find_device(&stm_class, NULL, &major, major_match);
+	if (!dev)
+		return -ENODEV;
+
+	stmf = kzalloc(sizeof(*stmf), GFP_KERNEL);
+	if (!stmf)
+		return -ENOMEM;
+
+	stmf->stm = to_stm_device(dev);
+
+	if (!try_module_get(stmf->stm->owner))
+		goto err_free;
+
+	file->private_data = stmf;
+
+	return nonseekable_open(inode, file);
+
+err_free:
+	kfree(stmf);
+
+	return err;
+}
+
+static int stm_char_release(struct inode *inode, struct file *file)
+{
+	struct stm_file *stmf = file->private_data;
+
+	stm_output_free(stmf->stm, &stmf->output);
+	stm_put_device(stmf->stm);
+	kfree(stmf);
+
+	return 0;
+}
+
+static int stm_file_assign(struct stm_file *stmf, char *id, unsigned int width)
+{
+	struct stm_device *stm = stmf->stm;
+	int ret;
+
+	stmf->policy_node = stp_policy_node_lookup(stm, id);
+
+	ret = stm_output_assign(stm, width, stmf->policy_node, &stmf->output);
+
+	if (stmf->policy_node)
+		stp_policy_node_put(stmf->policy_node);
+
+	return ret;
+}
+
+static void stm_write(struct stm_data *data, unsigned int master,
+		      unsigned int channel, const char *buf, size_t count)
+{
+	unsigned int flags = STP_PACKET_TIMESTAMPED;
+	const unsigned char *p = buf, nil = 0;
+	size_t pos;
+	ssize_t sz;
+
+	for (pos = 0, p = buf; count > pos; pos += sz, p += sz) {
+		sz = min_t(unsigned int, count - pos, 8);
+		sz = data->packet(data, master, channel, STP_PACKET_DATA, flags,
+				  sz, p);
+		flags = 0;
+	}
+
+	data->packet(data, master, channel, STP_PACKET_FLAG, 0, 0, &nil);
+}
+
+static ssize_t stm_char_write(struct file *file, const char __user *buf,
+			      size_t count, loff_t *ppos)
+{
+	struct stm_file *stmf = file->private_data;
+	struct stm_device *stm = stmf->stm;
+	char *kbuf;
+	int err;
+
+	/*
+	 * if no m/c have been assigned to this writer up to this
+	 * point, use "default" policy entry
+	 */
+	if (!stmf->output.nr_chans) {
+		err = stm_file_assign(stmf, "default", 1);
+		/*
+		 * EBUSY means that somebody else just assigned this
+		 * output, which is just fine for write()
+		 */
+		if (err && err != -EBUSY)
+			return err;
+	}
+
+	kbuf = kmalloc(count + 1, GFP_KERNEL);
+	if (!kbuf)
+		return -ENOMEM;
+
+	err = copy_from_user(kbuf, buf, count);
+	if (err) {
+		kfree(kbuf);
+		return -EFAULT;
+	}
+
+	stm_write(stm->data, stmf->output.master, stmf->output.channel, kbuf,
+		  count);
+
+	kfree(kbuf);
+
+	return count;
+}
+
+static int stm_char_mmap(struct file *file, struct vm_area_struct *vma)
+{
+	struct stm_file *stmf = file->private_data;
+	struct stm_device *stm = stmf->stm;
+	unsigned long size, phys;
+
+	if (!stm->data->mmio_addr)
+		return -EOPNOTSUPP;
+
+	if (vma->vm_pgoff)
+		return -EINVAL;
+
+	size = vma->vm_end - vma->vm_start;
+
+	if (stmf->output.nr_chans * stm->data->sw_mmiosz != size)
+		return -EINVAL;
+
+	phys = stm->data->mmio_addr(stm->data, stmf->output.master,
+				    stmf->output.channel,
+				    stmf->output.nr_chans);
+
+	if (!phys)
+		return -EINVAL;
+
+	vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
+	vma->vm_flags |= VM_IO | VM_DONTEXPAND | VM_DONTDUMP;
+	vm_iomap_memory(vma, phys, size);
+
+	return 0;
+}
+
+static int stm_char_policy_set_ioctl(struct stm_file *stmf, void __user *arg)
+{
+	struct stm_device *stm = stmf->stm;
+	struct stp_policy_id *id;
+	int ret = -EINVAL;
+	u32 size;
+
+	if (stmf->output.nr_chans)
+		return -EBUSY;
+
+	if (copy_from_user(&size, arg, sizeof(size)))
+		return -EFAULT;
+
+	if (size >= PATH_MAX + sizeof(*id))
+		return -EINVAL;
+
+	/*
+	 * size + 1 to make sure the .id string at the bottom is terminated,
+	 * which is also why memdup_user() is not useful here
+	 */
+	id = kzalloc(size + 1, GFP_KERNEL);
+	if (!id)
+		return -ENOMEM;
+
+	if (copy_from_user(id, arg, size)) {
+		ret = -EFAULT;
+		goto err_free;
+	}
+
+	if (id->__reserved_0 || id->__reserved_1)
+		goto err_free;
+
+	if (id->width < 1 ||
+	    id->width > PAGE_SIZE / stm->data->sw_mmiosz)
+		goto err_free;
+
+	ret = stm_file_assign(stmf, id->id, id->width);
+	if (ret)
+		goto err_free;
+
+	ret = 0;
+
+	if (stm->data->link)
+		ret = stm->data->link(stm->data, stmf->output.master,
+				      stmf->output.channel);
+
+	if (ret) {
+		stm_output_free(stmf->stm, &stmf->output);
+		stm_put_device(stmf->stm);
+	}
+
+err_free:
+	kfree(id);
+
+	return ret;
+}
+
+static int stm_char_policy_get_ioctl(struct stm_file *stmf, void __user *arg)
+{
+	struct stp_policy_id id = {
+		.size		= sizeof(id),
+		.master		= stmf->output.master,
+		.channel	= stmf->output.channel,
+		.width		= stmf->output.nr_chans,
+		.__reserved_0	= 0,
+		.__reserved_1	= 0,
+	};
+
+	return copy_to_user(arg, &id, id.size) ? -EFAULT : 0;
+}
+
+static long
+stm_char_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
+{
+	struct stm_file *stmf = file->private_data;
+	struct stm_data *stm_data = stmf->stm->data;
+	int err = -ENOTTY;
+	u64 options;
+
+	switch (cmd) {
+	case STP_POLICY_ID_SET:
+		err = stm_char_policy_set_ioctl(stmf, (void __user *)arg);
+		if (err)
+			return err;
+
+		return stm_char_policy_get_ioctl(stmf, (void __user *)arg);
+
+	case STP_POLICY_ID_GET:
+		return stm_char_policy_get_ioctl(stmf, (void __user *)arg);
+
+	case STP_SET_OPTIONS:
+		if (copy_from_user(&options, (u64 __user *)arg, sizeof(u64)))
+			return -EFAULT;
+
+		if (stm_data->set_options)
+			err = stm_data->set_options(stm_data,
+						    stmf->output.master,
+						    stmf->output.channel,
+						    stmf->output.nr_chans,
+						    options);
+
+		break;
+	default:
+		break;
+	}
+
+	return err;
+}
+
+#ifdef CONFIG_COMPAT
+static long
+stm_char_compat_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
+{
+	return stm_char_ioctl(file, cmd, (unsigned long)compat_ptr(arg));
+}
+#else
+#define stm_char_compat_ioctl	NULL
+#endif
+
+static const struct file_operations stm_fops = {
+	.open		= stm_char_open,
+	.release	= stm_char_release,
+	.write		= stm_char_write,
+	.mmap		= stm_char_mmap,
+	.unlocked_ioctl	= stm_char_ioctl,
+	.compat_ioctl	= stm_char_compat_ioctl,
+	.llseek		= no_llseek,
+};
+
+static void stm_device_release(struct device *dev)
+{
+	struct stm_device *stm = to_stm_device(dev);
+
+	kfree(stm);
+}
+
+int stm_register_device(struct device *parent, struct stm_data *stm_data,
+			struct module *owner)
+{
+	struct stm_device *stm;
+	unsigned int nmasters;
+	int err = -ENOMEM;
+
+	if (!stm_core_up)
+		return -EPROBE_DEFER;
+
+	if (!stm_data->packet || !stm_data->sw_nchannels)
+		return -EINVAL;
+
+	nmasters = stm_data->sw_end - stm_data->sw_start;
+	stm = kzalloc(sizeof(*stm) + nmasters * sizeof(void *), GFP_KERNEL);
+	if (!stm)
+		return -ENOMEM;
+
+	stm->major = register_chrdev(0, stm_data->name, &stm_fops);
+	if (stm->major < 0)
+		goto err_free;
+
+	device_initialize(&stm->dev);
+	stm->dev.devt = MKDEV(stm->major, 0);
+	stm->dev.class = &stm_class;
+	stm->dev.parent = parent;
+	stm->dev.release = stm_device_release;
+
+	err = kobject_set_name(&stm->dev.kobj, "%s", stm_data->name);
+	if (err)
+		goto err_device;
+
+	err = device_add(&stm->dev);
+	if (err)
+		goto err_device;
+
+	spin_lock_init(&stm->link_lock);
+	INIT_LIST_HEAD(&stm->link_list);
+
+	spin_lock_init(&stm->mc_lock);
+	mutex_init(&stm->policy_mutex);
+	stm->sw_nmasters = nmasters;
+	stm->owner = owner;
+	stm->data = stm_data;
+	stm_data->stm = stm;
+
+	return 0;
+
+err_device:
+	put_device(&stm->dev);
+err_free:
+	kfree(stm);
+
+	return err;
+}
+EXPORT_SYMBOL_GPL(stm_register_device);
+
+static void __stm_source_link_drop(struct stm_source_device *src,
+				   struct stm_device *stm);
+
+void stm_unregister_device(struct stm_data *stm_data)
+{
+	struct stm_device *stm = stm_data->stm;
+	struct stm_source_device *src, *iter;
+	int i;
+
+	spin_lock(&stm->link_lock);
+	list_for_each_entry_safe(src, iter, &stm->link_list, link_entry) {
+		__stm_source_link_drop(src, stm);
+	}
+	spin_unlock(&stm->link_lock);
+
+	synchronize_srcu(&stm_source_srcu);
+
+	unregister_chrdev(stm->major, stm_data->name);
+
+	mutex_lock(&stm->policy_mutex);
+	if (stm->policy)
+		stp_policy_unbind(stm->policy);
+	mutex_unlock(&stm->policy_mutex);
+
+	for (i = 0; i < stm->sw_nmasters; i++)
+		stp_master_free(stm, i);
+
+	device_unregister(&stm->dev);
+	stm_data->stm = NULL;
+}
+EXPORT_SYMBOL_GPL(stm_unregister_device);
+
+/**
+ * stm_source_link_add() - connect an stm_source device to an stm device
+ * @src:	stm_source device
+ * @stm:	stm device
+ *
+ * This function establishes a link from stm_source to an stm device so that
+ * the former can send out trace data to the latter.
+ *
+ * Return:	0 on success, -errno otherwise.
+ */
+static int stm_source_link_add(struct stm_source_device *src,
+			       struct stm_device *stm)
+{
+	char *id;
+	int err;
+
+	spin_lock(&stm->link_lock);
+	spin_lock(&src->link_lock);
+
+	/* src->link is dereferenced under stm_source_srcu but not the list */
+	rcu_assign_pointer(src->link, stm);
+	list_add_tail(&src->link_entry, &stm->link_list);
+
+	spin_unlock(&src->link_lock);
+	spin_unlock(&stm->link_lock);
+
+	id = kstrdup(src->data->name, GFP_KERNEL);
+	if (id) {
+		src->policy_node =
+			stp_policy_node_lookup(stm, id);
+
+		kfree(id);
+	}
+
+	err = stm_output_assign(stm, src->data->nr_chans,
+				src->policy_node, &src->output);
+
+	if (src->policy_node)
+		stp_policy_node_put(src->policy_node);
+
+	if (err)
+		goto fail_detach;
+
+	/* this is to notify the STM device that a new link has been made */
+	if (stm->data->link)
+		err = stm->data->link(stm->data, src->output.master,
+				      src->output.channel);
+
+	if (err)
+		goto fail_free_output;
+
+	/* this is to let the source carry out all necessary preparations */
+	if (src->data->link)
+		src->data->link(src->data);
+
+	return 0;
+
+fail_free_output:
+	stm_output_free(stm, &src->output);
+	stm_put_device(stm);
+
+fail_detach:
+	spin_lock(&stm->link_lock);
+	spin_lock(&src->link_lock);
+
+	rcu_assign_pointer(src->link, NULL);
+	list_del_init(&src->link_entry);
+
+	spin_unlock(&src->link_lock);
+	spin_unlock(&stm->link_lock);
+
+	return err;
+}
+
+/**
+ * __stm_source_link_drop() - detach stm_source from an stm device
+ * @src:	stm_source device
+ * @stm:	stm device
+ *
+ * If @stm is @src::link, disconnect them from one another and put the
+ * reference on the @stm device.
+ *
+ * Caller must hold stm::link_lock.
+ */
+static void __stm_source_link_drop(struct stm_source_device *src,
+				   struct stm_device *stm)
+{
+	spin_lock(&src->link_lock);
+	if (WARN_ON_ONCE(src->link != stm)) {
+		spin_unlock(&src->link_lock);
+		return;
+	}
+
+	stm_output_free(src->link, &src->output);
+	/* caller must hold stm::link_lock */
+	list_del_init(&src->link_entry);
+	/* matches stm_find_device() from stm_source_link_store() */
+	stm_put_device(src->link);
+	rcu_assign_pointer(src->link, NULL);
+
+	spin_unlock(&src->link_lock);
+}
+
+/**
+ * stm_source_link_drop() - detach stm_source from its stm device
+ * @src:	stm_source device
+ *
+ * Unlinking means disconnecting from source's STM device; after this
+ * writes will be unsuccessful until it is linked to a new STM device.
+ *
+ * This will happen on "stm_source_link" sysfs attribute write to undo
+ * the existing link (if any), or on linked STM device's de-registration.
+ */
+static void stm_source_link_drop(struct stm_source_device *src)
+{
+	struct stm_device *stm;
+	int idx;
+
+	idx = srcu_read_lock(&stm_source_srcu);
+	stm = srcu_dereference(src->link, &stm_source_srcu);
+
+	if (stm) {
+		if (src->data->unlink)
+			src->data->unlink(src->data);
+
+		spin_lock(&stm->link_lock);
+		__stm_source_link_drop(src, stm);
+		spin_unlock(&stm->link_lock);
+	}
+
+	srcu_read_unlock(&stm_source_srcu, idx);
+}
+
+static ssize_t stm_source_link_show(struct device *dev,
+				    struct device_attribute *attr,
+				    char *buf)
+{
+	struct stm_source_device *src = to_stm_source_device(dev);
+	struct stm_device *stm;
+	int idx, ret;
+
+	idx = srcu_read_lock(&stm_source_srcu);
+	stm = srcu_dereference(src->link, &stm_source_srcu);
+	ret = sprintf(buf, "%s\n",
+		      stm ? dev_name(&stm->dev) : "<none>");
+	srcu_read_unlock(&stm_source_srcu, idx);
+
+	return ret;
+}
+
+static ssize_t stm_source_link_store(struct device *dev,
+				     struct device_attribute *attr,
+				     const char *buf, size_t count)
+{
+	struct stm_source_device *src = to_stm_source_device(dev);
+	struct stm_device *link;
+	int err;
+
+	stm_source_link_drop(src);
+
+	link = stm_find_device(buf);
+	if (!link)
+		return -EINVAL;
+
+	err = stm_source_link_add(src, link);
+	if (err)
+		stm_put_device(link);
+
+	return err ? : count;
+}
+
+static DEVICE_ATTR_RW(stm_source_link);
+
+static struct attribute *stm_source_attrs[] = {
+	&dev_attr_stm_source_link.attr,
+	NULL,
+};
+
+ATTRIBUTE_GROUPS(stm_source);
+
+static struct class stm_source_class = {
+	.name		= "stm_source",
+	.dev_groups	= stm_source_groups,
+};
+
+static void stm_source_device_release(struct device *dev)
+{
+	struct stm_source_device *src = to_stm_source_device(dev);
+
+	kfree(src);
+}
+
+/**
+ * stm_source_register_device() - register an stm_source device
+ * @parent:	parent device
+ * @data:	device description structure
+ *
+ * This will create a device of stm_source class that can write
+ * data to an stm device once linked.
+ *
+ * Return:	0 on success, -errno otherwise.
+ */
+int stm_source_register_device(struct device *parent,
+			       struct stm_source_data *data)
+{
+	struct stm_source_device *src;
+	int err;
+
+	if (!stm_core_up)
+		return -EPROBE_DEFER;
+
+	src = kzalloc(sizeof(*src), GFP_KERNEL);
+	if (!src)
+		return -ENOMEM;
+
+	device_initialize(&src->dev);
+	src->dev.class = &stm_source_class;
+	src->dev.parent = parent;
+	src->dev.release = stm_source_device_release;
+
+	err = kobject_set_name(&src->dev.kobj, "%s", data->name);
+	if (err)
+		goto err;
+
+	err = device_add(&src->dev);
+	if (err)
+		goto err;
+
+	spin_lock_init(&src->link_lock);
+	INIT_LIST_HEAD(&src->link_entry);
+	src->data = data;
+	data->src = src;
+
+	return 0;
+
+err:
+	put_device(&src->dev);
+	kfree(src);
+
+	return err;
+}
+EXPORT_SYMBOL_GPL(stm_source_register_device);
+
+/**
+ * stm_source_unregister_device() - unregister an stm_source device
+ * @data:	device description that was used to register the device
+ *
+ * This will remove a previously created stm_source device from the system.
+ */
+void stm_source_unregister_device(struct stm_source_data *data)
+{
+	struct stm_source_device *src = data->src;
+
+	stm_source_link_drop(src);
+
+	device_destroy(&stm_source_class, src->dev.devt);
+}
+EXPORT_SYMBOL_GPL(stm_source_unregister_device);
+
+int stm_source_write(struct stm_source_data *data, unsigned int chan,
+		     const char *buf, size_t count)
+{
+	struct stm_source_device *src = data->src;
+	struct stm_device *stm;
+	int idx;
+
+	if (!src->output.nr_chans)
+		return -ENODEV;
+
+	if (chan >= src->output.nr_chans)
+		return -EINVAL;
+
+	idx = srcu_read_lock(&stm_source_srcu);
+
+	stm = srcu_dereference(src->link, &stm_source_srcu);
+	if (stm)
+		stm_write(stm->data, src->output.master,
+			  src->output.channel + chan,
+			  buf, count);
+	else
+		count = -ENODEV;
+
+	srcu_read_unlock(&stm_source_srcu, idx);
+
+	return count;
+}
+EXPORT_SYMBOL_GPL(stm_source_write);
+
+static int __init stm_core_init(void)
+{
+	int err;
+
+	err = class_register(&stm_class);
+	if (err)
+		return err;
+
+	err = class_register(&stm_source_class);
+	if (err)
+		goto err_stm;
+
+	err = stp_configfs_init();
+	if (err)
+		goto err_src;
+
+	init_srcu_struct(&stm_source_srcu);
+
+	stm_core_up++;
+
+	return 0;
+
+err_src:
+	class_unregister(&stm_source_class);
+err_stm:
+	class_unregister(&stm_class);
+
+	return err;
+}
+
+module_init(stm_core_init);
+
+static void __exit stm_core_exit(void)
+{
+	cleanup_srcu_struct(&stm_source_srcu);
+	class_unregister(&stm_source_class);
+	class_unregister(&stm_class);
+	stp_configfs_exit();
+}
+
+module_exit(stm_core_exit);
+
+MODULE_LICENSE("GPL v2");
+MODULE_DESCRIPTION("System Trace Module device class");
+MODULE_AUTHOR("Alexander Shishkin <alexander.shishkin@linux.intel.com>");