Chatter

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##Series articles
Analysis of Qualcomm msm-V4L2-Camera driver 1- first knowledge
Qualcomm msm-V4L2-Camera driver analysis 2-framework details
Analysis of Qualcomm msm-V4L2-Camera driver 3-session
Analysis of Qualcomm msm-V4L2-Camera driver 4-stream
Analysis of Qualcomm msm-V4L2-Camera driver 5-buffer

1, Design idea of session

When chatting on wechat, if we want to communicate with people, we will create a session. If multiple people chat alone, there will be multiple sessions;
Each session is responsible for managing all resources, such as expression packs, pictures, chat records, etc.

Qualcomm's thinking is the same:

• Divide all functions of camera into different modules, and let the module decide its own affairs (high cohesion and low coupling). The module needs a unified interface and format.
• There is a port in the module, which connects the module through the port and hangs the module on the bus.
• The connection of each port is a stream. These streams are managed by pipeline.
• Each time a camera is started, a session is created to manage everything in the camera.
• For each session, the module is shared. It can be the hardware resources of camera or other resources (such as some software algorithms).

2, How is a session created

2.1. Data structure of session

msm_session

struct msm_session {
	struct list_head list;

	/* session index */
	unsigned int session_id;

	/* event queue sent by imaging server */
	struct msm_event event_q;

	/* ACK by imaging server. Object type of
	 * struct msm_command_ack per open,
	 * assumption is application can send
	 * command on every opened video node
	 */
	struct msm_queue_head command_ack_q;

	/* real streams(either data or metadate) owned by one
	 * session struct msm_stream
	 */
	struct msm_queue_head stream_q;
	struct mutex lock;
	struct mutex lock_q;
	struct mutex close_lock;
	rwlock_t	stream_rwlock;
	struct kgsl_pwr_limit *sysfs_pwr_limit;
}

msm_event

/** msm_event:
 *
 *  event sent by imaging server
 **/
struct msm_event {
	struct video_device *vdev;
	atomic_t on_heap;
};

• struct list_head list: a linked list used to manage all sessions
• unsigned int session_id: session id
• msm_event event_q: video_device instance
• struct msm_queue_head command_ack_q: cmd queue
• struct msm_queue_head stream_q: Stream queue

The most important thing here is msm_event event_q. It's essentially a video_device instance.
In addition, cmd queue and stream queue are more important.

2.2 session and video_device relationship

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2.3 video_device,v4l2_device and v4l2_subdev relationship

2.4 creation of session

• 1. Initialization of global session queue

struct msm_queue_head {
	struct list_head list;
	spinlock_t lock;
	int len;
	int max;
};
static struct msm_queue_head *msm_session_q;
static int msm_probe(struct platform_device *pdev)
{
···
    //Request memory
	msm_session_q = kzalloc(sizeof(*msm_session_q), GFP_KERNEL);
    //initialization
	msm_init_queue(msm_session_q);
···
}

In msm_probe, MSM_ session_ The Q queue will request memory and then initialize.

• 2. Creation of session

int msm_create_session(unsigned int session_id, struct video_device *vdev)
{
	struct msm_session *session = NULL;
    //Judge msm_session_q is the queue empty
	if (!msm_session_q) {
		pr_err("%s : session queue not available Line %d\n",
				__func__, __LINE__);
		return -ENODEV;
	}
    //According to session_id to find whether the current session is in the global session queue
	session = msm_queue_find(msm_session_q, struct msm_session,
		list, __msm_queue_find_session, &session_id);
    //If the session already exists, return directly
	if (session) {
		pr_err("%s: Session exist session_id=%d\n",
				__func__, session_id);
		return -EINVAL;
	}
    //Request memory space
	session = kzalloc(sizeof(*session), GFP_KERNEL);
    //Assignment session_id
	session->session_id = session_id;
    //Assign video_device device
	session->event_q.vdev = vdev;
    //Initialize cmd queue
	msm_init_queue(&session->command_ack_q);
    //Initial stream queue
	msm_init_queue(&session->stream_q);
    //Queue: add the new session to the global queue msm_session_q
	msm_enqueue(msm_session_q, &session->list);
	mutex_init(&session->lock);
	mutex_init(&session->lock_q);
	mutex_init(&session->close_lock);
	rwlock_init(&session->stream_rwlock);

	return 0;
}

• Video is maintained in a session_ Device, create cmd queue and stream queue at the same time, and finally add the session to the global queue msm_session_q for management, as shown in the figure below:

The session creation process was described in the previous article:
rc = msm_create_session(pvdev->vdev->num, pvdev->vdev);
The first parameter here is session_ id = pvdev->vdev->num;
That is, it corresponds to 1 and 2 in our video1/video2;
session_id = 1 or 2

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