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Subject: Re: [PATCH 4/7] platform/surface: aggregator_cdev: Add support for
 forwarding events to user-space
To:     Maximilian Luz <luzmaximilian@gmail.com>
Cc:     Mark Gross <mgross@linux.intel.com>,
        Jonathan Corbet <corbet@lwn.net>,
        platform-driver-x86@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20210603234526.2503590-1-luzmaximilian@gmail.com>
 <20210603234526.2503590-5-luzmaximilian@gmail.com>
From:   Hans de Goede <hdegoede@redhat.com>
Message-ID: <a1a43f55-378f-d5e7-c439-d5397abdf838@redhat.com>
Date:   Fri, 4 Jun 2021 13:32:48 +0200
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Hi,

I've one review remark inline below.

On 6/4/21 1:45 AM, Maximilian Luz wrote:
> Currently, debugging unknown events requires writing a custom driver.
> This is somewhat difficult, slow to adapt, and not entirely
> user-friendly for quickly trying to figure out things on devices of some
> third-party user. We can do better. We already have a user-space
> interface intended for debugging SAM EC requests, so let's add support
> for receiving events to that.
> 
> This commit provides support for receiving events by reading from the
> controller file. It additionally introduces two new IOCTLs to control
> which event categories will be forwarded. Specifically, a user-space
> client can specify which target categories it wants to receive events
> from by registering the corresponding notifier(s) via the IOCTLs and
> after that, read the received events by reading from the controller
> device.
> 
> Signed-off-by: Maximilian Luz <luzmaximilian@gmail.com>
> ---
>  .../userspace-api/ioctl/ioctl-number.rst      |   2 +-
>  .../surface/surface_aggregator_cdev.c         | 457 +++++++++++++++++-
>  include/uapi/linux/surface_aggregator/cdev.h  |  41 +-
>  3 files changed, 474 insertions(+), 26 deletions(-)
> 
> diff --git a/Documentation/userspace-api/ioctl/ioctl-number.rst b/Documentation/userspace-api/ioctl/ioctl-number.rst
> index 9bfc2b510c64..1409e40e6345 100644
> --- a/Documentation/userspace-api/ioctl/ioctl-number.rst
> +++ b/Documentation/userspace-api/ioctl/ioctl-number.rst
> @@ -325,7 +325,7 @@ Code  Seq#    Include File                                           Comments
>  0xA3  90-9F  linux/dtlk.h
>  0xA4  00-1F  uapi/linux/tee.h                                        Generic TEE subsystem
>  0xA4  00-1F  uapi/asm/sgx.h                                          <mailto:linux-sgx@vger.kernel.org>
> -0xA5  01     linux/surface_aggregator/cdev.h                         Microsoft Surface Platform System Aggregator
> +0xA5  01-05  linux/surface_aggregator/cdev.h                         Microsoft Surface Platform System Aggregator
>                                                                       <mailto:luzmaximilian@gmail.com>
>  0xA5  20-2F  linux/surface_aggregator/dtx.h                          Microsoft Surface DTX driver
>                                                                       <mailto:luzmaximilian@gmail.com>
> diff --git a/drivers/platform/surface/surface_aggregator_cdev.c b/drivers/platform/surface/surface_aggregator_cdev.c
> index 79e28fab7e40..807930144039 100644
> --- a/drivers/platform/surface/surface_aggregator_cdev.c
> +++ b/drivers/platform/surface/surface_aggregator_cdev.c
> @@ -3,29 +3,69 @@
>   * Provides user-space access to the SSAM EC via the /dev/surface/aggregator
>   * misc device. Intended for debugging and development.
>   *
> - * Copyright (C) 2020 Maximilian Luz <luzmaximilian@gmail.com>
> + * Copyright (C) 2020-2021 Maximilian Luz <luzmaximilian@gmail.com>
>   */
>  
>  #include <linux/fs.h>
> +#include <linux/ioctl.h>
>  #include <linux/kernel.h>
> +#include <linux/kfifo.h>
>  #include <linux/kref.h>
>  #include <linux/miscdevice.h>
>  #include <linux/module.h>
>  #include <linux/platform_device.h>
> +#include <linux/poll.h>
>  #include <linux/rwsem.h>
>  #include <linux/slab.h>
>  #include <linux/uaccess.h>
> +#include <linux/vmalloc.h>
>  
>  #include <linux/surface_aggregator/cdev.h>
>  #include <linux/surface_aggregator/controller.h>
> +#include <linux/surface_aggregator/serial_hub.h>
>  
>  #define SSAM_CDEV_DEVICE_NAME	"surface_aggregator_cdev"
>  
> +
> +/* -- Main structures. ------------------------------------------------------ */
> +
> +enum ssam_cdev_device_state {
> +	SSAM_CDEV_DEVICE_SHUTDOWN_BIT = BIT(0),
> +};
> +
>  struct ssam_cdev {
>  	struct kref kref;
>  	struct rw_semaphore lock;
> +
> +	struct device *dev;
>  	struct ssam_controller *ctrl;
>  	struct miscdevice mdev;
> +	unsigned long flags;
> +
> +	struct rw_semaphore client_lock;  /* Guards client list. */
> +	struct list_head client_list;
> +};
> +
> +struct ssam_cdev_client;
> +
> +struct ssam_cdev_notifier {
> +	struct ssam_cdev_client *client;
> +	struct ssam_event_notifier nf;
> +};
> +
> +struct ssam_cdev_client {
> +	struct ssam_cdev *cdev;
> +	struct list_head node;
> +
> +	struct mutex notifier_lock;	/* Guards notifier access for registration */
> +	struct ssam_cdev_notifier *notifier[SSH_NUM_EVENTS];
> +
> +	struct mutex read_lock;		/* Guards FIFO buffer read access */
> +	struct mutex write_lock;	/* Guards FIFO buffer write access */
> +	DECLARE_KFIFO(buffer, u8, 4096);
> +
> +	wait_queue_head_t waitq;
> +	struct fasync_struct *fasync;
>  };
>  
>  static void __ssam_cdev_release(struct kref *kref)
> @@ -47,24 +87,169 @@ static void ssam_cdev_put(struct ssam_cdev *cdev)
>  		kref_put(&cdev->kref, __ssam_cdev_release);
>  }
>  
> -static int ssam_cdev_device_open(struct inode *inode, struct file *filp)
> +
> +/* -- Notifier handling. ---------------------------------------------------- */
> +
> +static u32 ssam_cdev_notifier(struct ssam_event_notifier *nf, const struct ssam_event *in)
>  {
> -	struct miscdevice *mdev = filp->private_data;
> -	struct ssam_cdev *cdev = container_of(mdev, struct ssam_cdev, mdev);
> +	struct ssam_cdev_notifier *cdev_nf = container_of(nf, struct ssam_cdev_notifier, nf);
> +	struct ssam_cdev_client *client = cdev_nf->client;
> +	struct ssam_cdev_event event;
> +	size_t n = struct_size(&event, data, in->length);
> +
> +	/* Translate event. */
> +	event.target_category = in->target_category;
> +	event.target_id = in->target_id;
> +	event.command_id = in->command_id;
> +	event.instance_id = in->instance_id;
> +	event.length = in->length;
> +
> +	mutex_lock(&client->write_lock);
> +
> +	/* Make sure we have enough space. */
> +	if (kfifo_avail(&client->buffer) < n) {
> +		dev_warn(client->cdev->dev,
> +			 "buffer full, dropping event (tc: %#04x, tid: %#04x, cid: %#04x, iid: %#04x)\n",
> +			 in->target_category, in->target_id, in->command_id, in->instance_id);
> +		mutex_unlock(&client->write_lock);
> +		return 0;
> +	}
>  
> -	filp->private_data = ssam_cdev_get(cdev);
> -	return stream_open(inode, filp);
> +	/* Copy event header and payload. */
> +	kfifo_in(&client->buffer, (const u8 *)&event, struct_size(&event, data, 0));
> +	kfifo_in(&client->buffer, &in->data[0], in->length);
> +
> +	mutex_unlock(&client->write_lock);
> +
> +	/* Notify waiting readers. */
> +	kill_fasync(&client->fasync, SIGIO, POLL_IN);
> +	wake_up_interruptible(&client->waitq);
> +
> +	/*
> +	 * Don't mark events as handled, this is the job of a proper driver and
> +	 * not the debugging interface.
> +	 */
> +	return 0;
>  }
>  
> -static int ssam_cdev_device_release(struct inode *inode, struct file *filp)
> +static int ssam_cdev_notifier_register(struct ssam_cdev_client *client, u8 tc, int priority)
>  {
> -	ssam_cdev_put(filp->private_data);
> -	return 0;
> +	const u16 rqid = ssh_tc_to_rqid(tc);
> +	const u16 event = ssh_rqid_to_event(rqid);
> +	struct ssam_cdev_notifier *nf;
> +	int status;
> +
> +	/* Validate notifier target category. */
> +	if (!ssh_rqid_is_event(rqid))
> +		return -EINVAL;
> +
> +	mutex_lock(&client->notifier_lock);
> +
> +	/* Check if the notifier has already been registered. */
> +	if (client->notifier[event]) {
> +		mutex_unlock(&client->notifier_lock);
> +		return -EEXIST;
> +	}
> +
> +	/* Allocate new notifier. */
> +	nf = kzalloc(sizeof(*nf), GFP_KERNEL);
> +	if (!nf) {
> +		mutex_unlock(&client->notifier_lock);
> +		return -ENOMEM;
> +	}
> +
> +	/*
> +	 * Create a dummy notifier with the minimal required fields for
> +	 * observer registration. Note that we can skip fully specifying event
> +	 * and registry here as we do not need any matching and use silent
> +	 * registration, which does not enable the corresponding event.
> +	 */
> +	nf->client = client;
> +	nf->nf.base.fn = ssam_cdev_notifier;
> +	nf->nf.base.priority = priority;
> +	nf->nf.event.id.target_category = tc;
> +	nf->nf.event.mask = 0;	/* Do not do any matching. */
> +	nf->nf.flags = SSAM_EVENT_NOTIFIER_OBSERVER;
> +
> +	/* Register notifier. */
> +	status = ssam_notifier_register(client->cdev->ctrl, &nf->nf);
> +	if (status)
> +		kfree(nf);
> +	else
> +		client->notifier[event] = nf;
> +
> +	mutex_unlock(&client->notifier_lock);
> +	return status;
>  }
>  
> -static long ssam_cdev_request(struct ssam_cdev *cdev, unsigned long arg)
> +static int ssam_cdev_notifier_unregister(struct ssam_cdev_client *client, u8 tc)
> +{
> +	const u16 rqid = ssh_tc_to_rqid(tc);
> +	const u16 event = ssh_rqid_to_event(rqid);
> +	int status;
> +
> +	/* Validate notifier target category. */
> +	if (!ssh_rqid_is_event(rqid))
> +		return -EINVAL;
> +
> +	mutex_lock(&client->notifier_lock);
> +
> +	/* Check if the notifier is currently registered. */
> +	if (!client->notifier[event]) {
> +		mutex_unlock(&client->notifier_lock);
> +		return -ENOENT;
> +	}
> +
> +	/* Unregister and free notifier. */
> +	status = ssam_notifier_unregister(client->cdev->ctrl, &client->notifier[event]->nf);
> +	kfree(client->notifier[event]);
> +	client->notifier[event] = NULL;
> +
> +	mutex_unlock(&client->notifier_lock);
> +	return status;
> +}
> +
> +static void ssam_cdev_notifier_unregister_all(struct ssam_cdev_client *client)
> +{
> +	int i;
> +
> +	down_read(&client->cdev->lock);
> +
> +	/*
> +	 * This function may be used during shutdown, thus we need to test for
> +	 * cdev->ctrl instead of the SSAM_CDEV_DEVICE_SHUTDOWN_BIT bit.
> +	 */
> +	if (client->cdev->ctrl) {
> +		for (i = 0; i < SSH_NUM_EVENTS; i++)
> +			ssam_cdev_notifier_unregister(client, i + 1);
> +
> +	} else {
> +		int count = 0;
> +
> +		/*
> +		 * Device has been shut down. Any notifier remaining is a bug,
> +		 * so warn about that as this would otherwise hardly be
> +		 * noticeable. Nevertheless, free them as well.
> +		 */
> +		mutex_lock(&client->notifier_lock);
> +		for (i = 0; i < SSH_NUM_EVENTS; i++) {
> +			count += !!(client->notifier[i]);
> +			kfree(client->notifier[i]);
> +			client->notifier[i] = NULL;
> +		}
> +		mutex_unlock(&client->notifier_lock);
> +
> +		WARN_ON(count > 0);
> +	}
> +
> +	up_read(&client->cdev->lock);
> +}
> +
> +
> +/* -- IOCTL functions. ------------------------------------------------------ */
> +
> +static long ssam_cdev_request(struct ssam_cdev_client *client, struct ssam_cdev_request __user *r)
>  {
> -	struct ssam_cdev_request __user *r;
>  	struct ssam_cdev_request rqst;
>  	struct ssam_request spec = {};
>  	struct ssam_response rsp = {};
> @@ -72,7 +257,6 @@ static long ssam_cdev_request(struct ssam_cdev *cdev, unsigned long arg)
>  	void __user *rspdata;
>  	int status = 0, ret = 0, tmp;
>  
> -	r = (struct ssam_cdev_request __user *)arg;
>  	ret = copy_struct_from_user(&rqst, sizeof(rqst), r, sizeof(*r));
>  	if (ret)
>  		goto out;
> @@ -152,7 +336,7 @@ static long ssam_cdev_request(struct ssam_cdev *cdev, unsigned long arg)
>  	}
>  
>  	/* Perform request. */
> -	status = ssam_request_sync(cdev->ctrl, &spec, &rsp);
> +	status = ssam_request_sync(client->cdev->ctrl, &spec, &rsp);
>  	if (status)
>  		goto out;
>  
> @@ -177,48 +361,244 @@ static long ssam_cdev_request(struct ssam_cdev *cdev, unsigned long arg)
>  	return ret;
>  }
>  
> -static long __ssam_cdev_device_ioctl(struct ssam_cdev *cdev, unsigned int cmd,
> +static long ssam_cdev_notif_register(struct ssam_cdev_client *client,
> +				     const struct ssam_cdev_notifier_desc __user *d)
> +{
> +	struct ssam_cdev_notifier_desc desc;
> +	long ret;
> +
> +	ret = copy_struct_from_user(&desc, sizeof(desc), d, sizeof(*d));
> +	if (ret)
> +		return ret;
> +
> +	return ssam_cdev_notifier_register(client, desc.target_category, desc.priority);
> +}
> +
> +static long ssam_cdev_notif_unregister(struct ssam_cdev_client *client,
> +				       const struct ssam_cdev_notifier_desc __user *d)
> +{
> +	struct ssam_cdev_notifier_desc desc;
> +	long ret;
> +
> +	ret = copy_struct_from_user(&desc, sizeof(desc), d, sizeof(*d));
> +	if (ret)
> +		return ret;
> +
> +	return ssam_cdev_notifier_unregister(client, desc.target_category);
> +}
> +
> +
> +/* -- File operations. ------------------------------------------------------ */
> +
> +static int ssam_cdev_device_open(struct inode *inode, struct file *filp)
> +{
> +	struct miscdevice *mdev = filp->private_data;
> +	struct ssam_cdev_client *client;
> +	struct ssam_cdev *cdev = container_of(mdev, struct ssam_cdev, mdev);
> +
> +	/* Initialize client */
> +	client = vzalloc(sizeof(*client));
> +	if (!client)
> +		return -ENOMEM;
> +
> +	client->cdev = ssam_cdev_get(cdev);
> +
> +	INIT_LIST_HEAD(&client->node);
> +
> +	mutex_init(&client->notifier_lock);
> +
> +	mutex_init(&client->read_lock);
> +	mutex_init(&client->write_lock);
> +	INIT_KFIFO(client->buffer);
> +	init_waitqueue_head(&client->waitq);
> +
> +	filp->private_data = client;
> +
> +	/* Attach client. */
> +	down_write(&cdev->client_lock);
> +
> +	if (test_bit(SSAM_CDEV_DEVICE_SHUTDOWN_BIT, &cdev->flags)) {
> +		up_write(&cdev->client_lock);
> +		ssam_cdev_put(client->cdev);

You are missing the mutex_destroy() calls here which you are
doing in ssam_cdev_device_release().

Or maybe move the mutex_init calls below this check
(before the up_write()) since I don't think the client can
be accessed by any code until the up_write is done?

Regards,

Hans


> +		vfree(client);
> +		return -ENODEV;
> +	}
> +	list_add_tail(&client->node, &cdev->client_list);
> +
> +	up_write(&cdev->client_lock);
> +
> +	stream_open(inode, filp);
> +	return 0;
> +}
> +
> +static int ssam_cdev_device_release(struct inode *inode, struct file *filp)
> +{
> +	struct ssam_cdev_client *client = filp->private_data;
> +
> +	/* Force-unregister all remaining notifiers of this client. */
> +	ssam_cdev_notifier_unregister_all(client);
> +
> +	/* Detach client. */
> +	down_write(&client->cdev->client_lock);
> +	list_del(&client->node);
> +	up_write(&client->cdev->client_lock);
> +
> +	/* Free client. */
> +	mutex_destroy(&client->write_lock);
> +	mutex_destroy(&client->read_lock);
> +
> +	mutex_destroy(&client->notifier_lock);
> +
> +	ssam_cdev_put(client->cdev);
> +	vfree(client);
> +
> +	return 0;
> +}
> +
> +static long __ssam_cdev_device_ioctl(struct ssam_cdev_client *client, unsigned int cmd,
>  				     unsigned long arg)
>  {
>  	switch (cmd) {
>  	case SSAM_CDEV_REQUEST:
> -		return ssam_cdev_request(cdev, arg);
> +		return ssam_cdev_request(client, (struct ssam_cdev_request __user *)arg);
> +
> +	case SSAM_CDEV_NOTIF_REGISTER:
> +		return ssam_cdev_notif_register(client,
> +						(struct ssam_cdev_notifier_desc __user *)arg);
> +
> +	case SSAM_CDEV_NOTIF_UNREGISTER:
> +		return ssam_cdev_notif_unregister(client,
> +						  (struct ssam_cdev_notifier_desc __user *)arg);
>  
>  	default:
>  		return -ENOTTY;
>  	}
>  }
>  
> -static long ssam_cdev_device_ioctl(struct file *file, unsigned int cmd,
> -				   unsigned long arg)
> +static long ssam_cdev_device_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
>  {
> -	struct ssam_cdev *cdev = file->private_data;
> +	struct ssam_cdev_client *client = file->private_data;
>  	long status;
>  
>  	/* Ensure that controller is valid for as long as we need it. */
> +	if (down_read_killable(&client->cdev->lock))
> +		return -ERESTARTSYS;
> +
> +	if (test_bit(SSAM_CDEV_DEVICE_SHUTDOWN_BIT, &client->cdev->flags)) {
> +		up_read(&client->cdev->lock);
> +		return -ENODEV;
> +	}
> +
> +	status = __ssam_cdev_device_ioctl(client, cmd, arg);
> +
> +	up_read(&client->cdev->lock);
> +	return status;
> +}
> +
> +static ssize_t ssam_cdev_read(struct file *file, char __user *buf, size_t count, loff_t *offs)
> +{
> +	struct ssam_cdev_client *client = file->private_data;
> +	struct ssam_cdev *cdev = client->cdev;
> +	unsigned int copied;
> +	int status = 0;
> +
>  	if (down_read_killable(&cdev->lock))
>  		return -ERESTARTSYS;
>  
> -	if (!cdev->ctrl) {
> +	/* Make sure we're not shut down. */
> +	if (test_bit(SSAM_CDEV_DEVICE_SHUTDOWN_BIT, &cdev->flags)) {
>  		up_read(&cdev->lock);
>  		return -ENODEV;
>  	}
>  
> -	status = __ssam_cdev_device_ioctl(cdev, cmd, arg);
> +	do {
> +		/* Check availability, wait if necessary. */
> +		if (kfifo_is_empty(&client->buffer)) {
> +			up_read(&cdev->lock);
> +
> +			if (file->f_flags & O_NONBLOCK)
> +				return -EAGAIN;
> +
> +			status = wait_event_interruptible(client->waitq,
> +							  !kfifo_is_empty(&client->buffer) ||
> +							  test_bit(SSAM_CDEV_DEVICE_SHUTDOWN_BIT,
> +								   &cdev->flags));
> +			if (status < 0)
> +				return status;
> +
> +			if (down_read_killable(&cdev->lock))
> +				return -ERESTARTSYS;
> +
> +			/* Need to check that we're not shut down again. */
> +			if (test_bit(SSAM_CDEV_DEVICE_SHUTDOWN_BIT, &cdev->flags)) {
> +				up_read(&cdev->lock);
> +				return -ENODEV;
> +			}
> +		}
> +
> +		/* Try to read from FIFO. */
> +		if (mutex_lock_interruptible(&client->read_lock)) {
> +			up_read(&cdev->lock);
> +			return -ERESTARTSYS;
> +		}
> +
> +		status = kfifo_to_user(&client->buffer, buf, count, &copied);
> +		mutex_unlock(&client->read_lock);
> +
> +		if (status < 0) {
> +			up_read(&cdev->lock);
> +			return status;
> +		}
> +
> +		/* We might not have gotten anything, check this here. */
> +		if (copied == 0 && (file->f_flags & O_NONBLOCK)) {
> +			up_read(&cdev->lock);
> +			return -EAGAIN;
> +		}
> +	} while (copied == 0);
>  
>  	up_read(&cdev->lock);
> -	return status;
> +	return copied;
> +}
> +
> +static __poll_t ssam_cdev_poll(struct file *file, struct poll_table_struct *pt)
> +{
> +	struct ssam_cdev_client *client = file->private_data;
> +	__poll_t events = 0;
> +
> +	if (test_bit(SSAM_CDEV_DEVICE_SHUTDOWN_BIT, &client->cdev->flags))
> +		return EPOLLHUP | EPOLLERR;
> +
> +	poll_wait(file, &client->waitq, pt);
> +
> +	if (!kfifo_is_empty(&client->buffer))
> +		events |= EPOLLIN | EPOLLRDNORM;
> +
> +	return events;
> +}
> +
> +static int ssam_cdev_fasync(int fd, struct file *file, int on)
> +{
> +	struct ssam_cdev_client *client = file->private_data;
> +
> +	return fasync_helper(fd, file, on, &client->fasync);
>  }
>  
>  static const struct file_operations ssam_controller_fops = {
>  	.owner          = THIS_MODULE,
>  	.open           = ssam_cdev_device_open,
>  	.release        = ssam_cdev_device_release,
> +	.read           = ssam_cdev_read,
> +	.poll           = ssam_cdev_poll,
> +	.fasync         = ssam_cdev_fasync,
>  	.unlocked_ioctl = ssam_cdev_device_ioctl,
>  	.compat_ioctl   = ssam_cdev_device_ioctl,
> -	.llseek         = noop_llseek,
> +	.llseek         = no_llseek,
>  };
>  
> +
> +/* -- Device and driver setup ----------------------------------------------- */
> +
>  static int ssam_dbg_device_probe(struct platform_device *pdev)
>  {
>  	struct ssam_controller *ctrl;
> @@ -236,6 +616,7 @@ static int ssam_dbg_device_probe(struct platform_device *pdev)
>  	kref_init(&cdev->kref);
>  	init_rwsem(&cdev->lock);
>  	cdev->ctrl = ctrl;
> +	cdev->dev = &pdev->dev;
>  
>  	cdev->mdev.parent   = &pdev->dev;
>  	cdev->mdev.minor    = MISC_DYNAMIC_MINOR;
> @@ -243,6 +624,9 @@ static int ssam_dbg_device_probe(struct platform_device *pdev)
>  	cdev->mdev.nodename = "surface/aggregator";
>  	cdev->mdev.fops     = &ssam_controller_fops;
>  
> +	init_rwsem(&cdev->client_lock);
> +	INIT_LIST_HEAD(&cdev->client_list);
> +
>  	status = misc_register(&cdev->mdev);
>  	if (status) {
>  		kfree(cdev);
> @@ -256,8 +640,32 @@ static int ssam_dbg_device_probe(struct platform_device *pdev)
>  static int ssam_dbg_device_remove(struct platform_device *pdev)
>  {
>  	struct ssam_cdev *cdev = platform_get_drvdata(pdev);
> +	struct ssam_cdev_client *client;
>  
> -	misc_deregister(&cdev->mdev);
> +	/*
> +	 * Mark device as shut-down. Prevent new clients from being added and
> +	 * new operations from being executed.
> +	 */
> +	set_bit(SSAM_CDEV_DEVICE_SHUTDOWN_BIT, &cdev->flags);
> +
> +	down_write(&cdev->client_lock);
> +
> +	/* Remove all notifiers registered by us. */
> +	list_for_each_entry(client, &cdev->client_list, node) {
> +		ssam_cdev_notifier_unregister_all(client);
> +	}
> +
> +	/* Wake up async clients. */
> +	list_for_each_entry(client, &cdev->client_list, node) {
> +		kill_fasync(&client->fasync, SIGIO, POLL_HUP);
> +	}
> +
> +	/* Wake up blocking clients. */
> +	list_for_each_entry(client, &cdev->client_list, node) {
> +		wake_up_interruptible(&client->waitq);
> +	}
> +
> +	up_write(&cdev->client_lock);
>  
>  	/*
>  	 * The controller is only guaranteed to be valid for as long as the
> @@ -266,8 +674,11 @@ static int ssam_dbg_device_remove(struct platform_device *pdev)
>  	 */
>  	down_write(&cdev->lock);
>  	cdev->ctrl = NULL;
> +	cdev->dev = NULL;
>  	up_write(&cdev->lock);
>  
> +	misc_deregister(&cdev->mdev);
> +
>  	ssam_cdev_put(cdev);
>  	return 0;
>  }
> diff --git a/include/uapi/linux/surface_aggregator/cdev.h b/include/uapi/linux/surface_aggregator/cdev.h
> index fbcce04abfe9..4f393fafc235 100644
> --- a/include/uapi/linux/surface_aggregator/cdev.h
> +++ b/include/uapi/linux/surface_aggregator/cdev.h
> @@ -6,7 +6,7 @@
>   * device. This device provides direct user-space access to the SSAM EC.
>   * Intended for debugging and development.
>   *
> - * Copyright (C) 2020 Maximilian Luz <luzmaximilian@gmail.com>
> + * Copyright (C) 2020-2021 Maximilian Luz <luzmaximilian@gmail.com>
>   */
>  
>  #ifndef _UAPI_LINUX_SURFACE_AGGREGATOR_CDEV_H
> @@ -73,6 +73,43 @@ struct ssam_cdev_request {
>  	} response;
>  } __attribute__((__packed__));
>  
> -#define SSAM_CDEV_REQUEST	_IOWR(0xA5, 1, struct ssam_cdev_request)
> +/**
> + * struct ssam_cdev_notifier_desc - Notifier descriptor.
> + * @priority:        Priority value determining the order in which notifier
> + *                   callbacks will be called. A higher value means higher
> + *                   priority, i.e. the associated callback will be executed
> + *                   earlier than other (lower priority) callbacks.
> + * @target_category: The event target category for which this notifier should
> + *                   receive events.
> + *
> + * Specifies the notifier that should be registered or unregistered,
> + * specifically with which priority and for which target category of events.
> + */
> +struct ssam_cdev_notifier_desc {
> +	__s32 priority;
> +	__u8 target_category;
> +} __attribute__((__packed__));
> +
> +/**
> + * struct ssam_cdev_event - SSAM event sent by the EC.
> + * @target_category: Target category of the event source. See &enum ssam_ssh_tc.
> + * @target_id:       Target ID of the event source.
> + * @command_id:      Command ID of the event.
> + * @instance_id:     Instance ID of the event source.
> + * @length:          Length of the event payload in bytes.
> + * @data:            Event payload data.
> + */
> +struct ssam_cdev_event {
> +	__u8 target_category;
> +	__u8 target_id;
> +	__u8 command_id;
> +	__u8 instance_id;
> +	__u16 length;
> +	__u8 data[];
> +} __attribute__((__packed__));
> +
> +#define SSAM_CDEV_REQUEST		_IOWR(0xA5, 1, struct ssam_cdev_request)
> +#define SSAM_CDEV_NOTIF_REGISTER	_IOW(0xA5, 2, struct ssam_cdev_notifier_desc)
> +#define SSAM_CDEV_NOTIF_UNREGISTER	_IOW(0xA5, 3, struct ssam_cdev_notifier_desc)
>  
>  #endif /* _UAPI_LINUX_SURFACE_AGGREGATOR_CDEV_H */
>