1. 概述
详细讲解请参考这篇博客：【genius_platform软件平台开发】第五十一讲：Linux系统内存映射mmap原理
主要就是内核内存区域的迎神，方便用户空间操控该内存，获取数据。
2. 应用层
// 为这个结构体变量分配内存，这个结构体主要的目的保存的是每一个缓冲帧的地址和大小
// 将多个(m_nBuffersCount)已申请到的缓冲帧映射到应用程序空间，用m_pBuffers指针记录
// 获取每个缓存的信息，并mmap到用户空间
m_pBuffers = (StuV4l2FrameBuffer *)calloc(info.count, sizeof(StuV4l2FrameBuffer));
CHECKI(m_pBuffers);
LOGMSG("CV4l2CaptureIr::initMmap calloc m_pBuffers=[%p] size=[%u]", m_pBuffers, info.count * sizeof(StuV4l2FrameBuffer));
// 映射所有的缓存
for (int nIdx = 0; nIdx < m_nBuffersCount; ++nIdx)
{
/*
* struct v4l2_buffer
* {
* __u32 index; // buffer 序号
* enum v4l2_buf_type type; // buffer 类型
* __u32 bytesused; // buffer 中已使用的字节数
* __u32 flags; // 区分是MMAP 还是USERPTR
* enum v4l2_field field;
* struct timeval timestamp; // 获取第一个字节时的系统时间
* struct v4l2_timecode timecode;
* __u32 sequence; // 队列中的序号
* enum v4l2_memory memory; // IO 方式，被应用程序设置
* union m
* {
* __u32 offset; // 缓冲帧地址，只对MMAP 有效
* unsigned long userptr;
* };
* __u32 length; // 缓冲帧长度
* __u32 input;
* __u32 reserved;
* };
* 原文链接：https://blog.csdn.net/u010661782/article/details/49020695
*/
struct v4l2_buffer info;
MEMSET(info);
info.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
info.memory = V4L2_MEMORY_MMAP;
info.index = nIdx;
// 将申请的内核缓冲区放入视频采集输入队列中排队
if (-1 == this->ioCtrl(m_nFd, VIDIOC_QBUF, &info))
{
LOGERROR("CV4l2CaptureIr::initMmap is error... VIDIOC_QBUF");
return ReturnCode_Error;
}
// 查询帧缓冲区在内核空间中的长度和偏移量
// 查询序号为n_buffers 的缓冲区，得到其起始物理地址和大小
// 获取到对应index的缓存信息，此处主要利用length信息及offset信息来完成后面的mmap操作
// 查询申请到的缓冲区的信息
if (-1 == this->ioCtrl(m_nFd, VIDIOC_QUERYBUF, &info))
{
LOGERROR("CV4l2CaptureIr::initMmap is error... DALI_IR_SENSOR_DEV_NAME=[%s] VIDIOC_QUERYBUF", DALI_IR_SENSOR_DEV_NAME);
return ReturnCode_Error;
}
// 转换成相对地址
m_pBuffers[nIdx].unLength = info.length;
/*
* 将这些帧缓冲区从内核空间映射到用户空间，便于应用程序读取 / 处理视频数据;
* void *mmap(void *addr, size_t length, int prot, int flags, int fd, off_t offset);
* addr 映射起始地址，一般为NULL ，让内核自动选择
* length 被映射内存块的长度
* prot 标志映射后能否被读写，其值为PROT_EXEC,PROT_READ,PROT_WRITE, PROT_NONE
* flags 确定此内存映射能否被其他进程共享，MAP_SHARED,MAP_PRIVATE
* fd,offset, 确定被映射的内存地址
* 返回成功映射后的地址，不成功返回MAP_FAILED ((void*)-1);
*
* int munmap(void *addr, size_t length);// 断开映射
* addr 为映射后的地址，length 为映射后的内存长度
*
* 2019-04-13 00:29:24.595 Message: CV4l2CaptureIr::initMmap nIdx=[0] pBuffer=[0x7fbda41000] unLength=[77440] m_nFd=[3] offset=[0]
* 2019-04-13 00:29:24.595 Message: CV4l2CaptureIr::initMmap nIdx=[1] pBuffer=[0x7fbda2e000] unLength=[77440] m_nFd=[3] offset=[77824]
* 2019-04-13 00:29:24.595 Message: CV4l2CaptureIr::initMmap nIdx=[2] pBuffer=[0x7fbda1b000] unLength=[77440] m_nFd=[3] offset=[155648]
* 2019-04-13 00:29:24.595 Message: CV4l2CaptureIr::initMmap nIdx=[3] pBuffer=[0x7fbda08000] unLength=[77440] m_nFd=[3] offset=[233472]
* 分析一下这个结果，帧数据大小为160*242*2 = 77440，而且是平面视频格式，offset是length经过也对齐后的，77440页对齐后大小为77824，
* mmap需要页对齐，所以这里将length进行一个页对齐的操作，通过offset找到对应的buffer及plane的值；
*
* 原文链接：https ://blog.csdn.net/u010661782/article/details/49020695
*/
// 应用程序通过内存映射，将帧缓冲区的地址映射到用户空间，返回映射后的地址
m_pBuffers[nIdx].pBuffer = mmap(NULL /*addr anywhere */, info.length, PROT_READ | PROT_WRITE /* required */, MAP_SHARED /* recommended */, m_nFd, info.m.offset);
// MAP_FAILED表示mmap没有成功映射，其返回的值
if (MAP_FAILED == m_pBuffers[nIdx].pBuffer)
{
LOGERROR("CV4l2CaptureIr::initMmap DALI_IR_SENSOR_DEV_NAME=[%s] VIDIOC_QUERYBUF", DALI_IR_SENSOR_DEV_NAME);
return ReturnCode_Error;
}
LOGMSG("CV4l2CaptureIr::initMmap nIdx=[%d] pBuffer=[%p] unLength=[%d] m_nFd=[%d] offset=[%u]", nIdx, m_pBuffers[nIdx].pBuffer, m_pBuffers[nIdx].unLength, m_nFd, info.m.offset);
}
3. 驱动层
3.1 vb2_mmap函数
int vb2_mmap(struct vb2_queue *q, struct vm_area_struct *vma)
{
unsigned long off = vma->vm_pgoff << PAGE_SHIFT;
struct vb2_buffer *vb;
unsigned int buffer = 0, plane = 0;
int ret;
unsigned long length;
// 检查memory类型
if (q->memory != VB2_MEMORY_MMAP) {
dprintk(1, "queue is not currently set up for mmap\n");
return -EINVAL;
}
/*
* Check memory area access mode.
*/
if (!(vma->vm_flags & VM_SHARED)) {
dprintk(1, "invalid vma flags, VM_SHARED needed\n");
return -EINVAL;
}
if (q->is_output) {
if (!(vma->vm_flags & VM_WRITE)) {
dprintk(1, "invalid vma flags, VM_WRITE needed\n");
return -EINVAL;
}
} else {
if (!(vma->vm_flags & VM_READ)) {
dprintk(1, "invalid vma flags, VM_READ needed\n");
return -EINVAL;
}
}
mutex_lock(&q->mmap_lock);
if (vb2_fileio_is_active(q)) {
dprintk(1, "mmap: file io in progress\n");
ret = -EBUSY;
goto unlock;
}
//找到与用户空间传递的偏移量对应的平面
/*
* Find the plane corresponding to the offset passed by userspace.
*/
ret = __find_plane_by_offset(q, off, &buffer, &plane);
if (ret)
goto unlock;
vb = q->bufs[buffer];
//MMAP需要页面对齐的缓冲区，我们需要对缓冲区长度使用在_vb2_buf_mem_alloc（）处对齐
/*
* MMAP requires page_aligned buffers.
* The buffer length was page_aligned at __vb2_buf_mem_alloc(),
* so, we need to do the same here.
*/
length = PAGE_ALIGN(vb->planes[plane].length);
if (length < (vma->vm_end - vma->vm_start)) {
dprintk(1,
"MMAP invalid, as it would overflow buffer length\n");
ret = -EINVAL;
goto unlock;
}
ret = call_memop(vb, mmap, vb->planes[plane].mem_priv, vma);
unlock:
mutex_unlock(&q->mmap_lock);
if (ret)
return ret;
dprintk(3, "buffer %d, plane %d successfully mapped\n", buffer, plane);
return 0;
}
EXPORT_SYMBOL_GPL(vb2_mmap);
#define log_memop(vb, op) \
dprintk(2, "call_memop(%p, %d, %s)%s\n", \
(vb)->vb2_queue, (vb)->index, #op, \
(vb)->vb2_queue->mem_ops->op ? "" : " (nop)")
#define call_memop(vb, op, args...) \
({ \
struct vb2_queue *_q = (vb)->vb2_queue; \
int err; \
\
log_memop(vb, op); \
err = _q->mem_ops->op ? _q->mem_ops->op(args) : 0; \
if (!err) \
(vb)->cnt_mem_ ## op++; \
err; \
})
