linux驱动编写（摄像头驱动）

    对于现代嵌入式设备，特别是手机来说，摄像头是很重要的一个设备。很多同学买手机，一看颜值，第二就看摄像头拍照如何。所以，从某个角度来说，摄像头是各个厂家主打的应用功能。那么，linux是如何支持摄像头的，我们可以来看一下？

1、代码目录地址

drivers/media

2、v4l2框架

目前linux上的camera都是按照v4l2框架来设计，它的地址位于drivers/media/v4l2-core

3、查看三星soc是如何支持camera的，可以查看drviers/media/platform/Kconfig

config VIDEO_S3C_CAMIF
  tristate "Samsung S3C24XX/S3C64XX SoC Camera Interface driver"
  depends on VIDEO_V4L2 && I2C && VIDEO_V4L2_SUBDEV_API
  depends on PM
  depends on ARCH_S3C64XX || PLAT_S3C24XX || COMPILE_TEST
  depends on HAS_DMA
  select VIDEOBUF2_DMA_CONTIG
  ---help---
    This is a v4l2 driver for s3c24xx and s3c64xx SoC series camera
    host interface (CAMIF).

    To compile this driver as a module, choose M here: the module
    will be called s3c-camif.

4、从Kconfig确认除了VIDEO_V4L2和VIDEO_V4L2_SUBDEV_API之外，它只依赖自己的目录

obj-$(CONFIG_VIDEO_S3C_CAMIF)   += s3c-camif/

5、继续查看s3c-camif目录的代码结构

# Makefile for s3c244x/s3c64xx CAMIF driver

s3c-camif-objs := camif-core.o camif-capture.o camif-regs.o

obj-$(CONFIG_VIDEO_S3C_CAMIF) += s3c-camif.o

6、Makefile说的很清楚，它主要依赖camif-*.c三个文件

通过阅读，确认camif-core.c是注册文件

camif-regs.c是寄存器读写文件

camif-capture.c是主要的操作文件、接口文件

7、确认v4l2的函数操作接口

static const struct v4l2_ioctl_ops s3c_camif_ioctl_ops = {
  .vidioc_querycap    = s3c_camif_vidioc_querycap,
  .vidioc_enum_input    = s3c_camif_vidioc_enum_input,
  .vidioc_g_input     = s3c_camif_vidioc_g_input,
  .vidioc_s_input     = s3c_camif_vidioc_s_input,
  .vidioc_enum_fmt_vid_cap  = s3c_camif_vidioc_enum_fmt,
  .vidioc_try_fmt_vid_cap   = s3c_camif_vidioc_try_fmt,
  .vidioc_s_fmt_vid_cap   = s3c_camif_vidioc_s_fmt,
  .vidioc_g_fmt_vid_cap   = s3c_camif_vidioc_g_fmt,
  .vidioc_g_selection   = s3c_camif_g_selection,
  .vidioc_s_selection   = s3c_camif_s_selection,
  .vidioc_reqbufs     = s3c_camif_reqbufs,
  .vidioc_querybuf    = s3c_camif_querybuf,
  .vidioc_prepare_buf   = s3c_camif_prepare_buf,
  .vidioc_create_bufs   = s3c_camif_create_bufs,
  .vidioc_qbuf      = s3c_camif_qbuf,
  .vidioc_dqbuf     = s3c_camif_dqbuf,
  .vidioc_streamon    = s3c_camif_streamon,
  .vidioc_streamoff   = s3c_camif_streamoff,
  .vidioc_subscribe_event   = v4l2_ctrl_subscribe_event,
  .vidioc_unsubscribe_event = v4l2_event_unsubscribe,
  .vidioc_log_status    = v4l2_ctrl_log_status,
};

and

static const struct v4l2_file_operations s3c_camif_fops = {
         .owner          = THIS_MODULE,
         .open           = s3c_camif_open,
         .release        = s3c_camif_close,
         .poll           = s3c_camif_poll,
         .unlocked_ioctl = video_ioctl2,
         .mmap           = s3c_camif_mmap,
 };

static const struct vb2_ops s3c_camif_qops = {
  .queue_setup   = queue_setup,
  .buf_prepare   = buffer_prepare,
  .buf_queue   = buffer_queue,
  .wait_prepare  = vb2_ops_wait_prepare,
  .wait_finish   = vb2_ops_wait_finish,
  .start_streaming = start_streaming,
  .stop_streaming  = stop_streaming,
};

const struct vb2_mem_ops vb2_dma_contig_memops = {
  .alloc    = vb2_dc_alloc,
  .put    = vb2_dc_put,
  .get_dmabuf = vb2_dc_get_dmabuf,
  .cookie   = vb2_dc_cookie,
  .vaddr    = vb2_dc_vaddr,
  .mmap   = vb2_dc_mmap,
  .get_userptr  = vb2_dc_get_userptr,
  .put_userptr  = vb2_dc_put_userptr,
  .prepare  = vb2_dc_prepare,
  .finish   = vb2_dc_finish,
  .map_dmabuf = vb2_dc_map_dmabuf,
  .unmap_dmabuf = vb2_dc_unmap_dmabuf,
  .attach_dmabuf  = vb2_dc_attach_dmabuf,
  .detach_dmabuf  = vb2_dc_detach_dmabuf,
  .num_users  = vb2_dc_num_users,
};

8、和sd一样，接口比较简单，但是要深入了解camera，v4l2是绕不过去的。

这一点和sd卡、usb、asoc是一样的。