本文详细介绍了V4L2 CONTROLS机制的基本使用方法，包括控制变量的创建、关联和设置过程。通过实例代码展示了如何在驱动中实现V4L2控制变量，并解释了不同类型的控制变量（如声音、菜单、整数菜单等）的创建方法。文章还提供了一个实际应用示例——BTTV驱动，帮助开发者深入理解并实践V4L2 CONTROLS的使用。
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这是一篇关于V4L2 CONTROLS的基本使用方法介绍。
内核中相关的文件是：v4l2-ctrls.h v4l2-ctrls.c
V4L2 CONTROLS机制主要提供了设置硬件的方法，包含两个主要的对象：
struct v4l2_ctrl
struct v4l2_ctrl_handler
struct v4l2_ctrl代表一个控制变量和它的值，例如：声音。
struct v4l2_ctrl_handler是用来管理v4l2_ctrl的。
使用v4l2 control的步骤：
1.将v4l2_ctrl_handler增加到驱动顶层结构体中：
struct foo_dev {
...
struct v4l2_ctrl_handler ctrl_handler;
...
};
struct foo_dev *foo;
2.初始化v4l2_ctrl_handler:
v4l2_ctrl_handler_init(&foo->ctrl_handler, nr_of_controls);
第二个参数nr_of_controls是你要使用的控制变量的个数。
3.关联v4l2_ctrl_handler和v4l2_device:
struct foo_dev {
...
struct v4l2_device v4l2_dev;
...
struct v4l2_ctrl_handler ctrl_handler;
...
};
foo->v4l2_dev.ctrl_handler = &foo->ctrl_handler;
然后从v4l2_ioctl_ops中移除所有的控制函数：vidioc_queryctrl, vidioc_querymenu, vidioc_g_ctrl, vidioc_s_ctrl,vidioc_g_ext_ctrls, vidioc_try_ext_ctrls and vidioc_s_ext_ctrls。
对于子设备，我们这样做：
struct foo_dev {
...
struct v4l2_subdev sd;
...
struct v4l2_ctrl_handler ctrl_handler;
...
};
foo->sd.ctrl_handler = &foo->ctrl_handler;
然后对v4l2_subdev_core_ops结构增加函数支持：
.queryctrl = v4l2_subdev_queryctrl,
.querymenu = v4l2_subdev_querymenu,
.g_ctrl = v4l2_subdev_g_ctrl,
.s_ctrl = v4l2_subdev_s_ctrl,
.g_ext_ctrls = v4l2_subdev_g_ext_ctrls,
.try_ext_ctrls = v4l2_subdev_try_ext_ctrls,
.s_ext_ctrls = v4l2_subdev_s_ext_ctrls,
4.在驱动退出的时候清除v4l2_ctrl_handler:
v4l2_ctrl_handler_free(&foo->ctrl_handler);
5.增加控制变量：
（1）struct v4l2_ctrl *v4l2_ctrl_new_std(struct v4l2_ctrl_handler *hdl,
const struct v4l2_ctrl_ops *ops,
u32 id, s32 min, s32 max, u32 step, s32 def);
这个函数用来增加非菜单式的控制变量，如声音等。min是最小值,max是最大值,step是步进长度，def是默认值。这个函数适用于在某一范围内均匀变化的控制变量。
例：v4l2_ctrl_new_std(&foo->ctrl_handler, &foo_ctrl_ops,V4L2_CID_VOLUM, 0, 255, 1, 128);
（2）struct v4l2_ctrl *v4l2_ctrl_new_std_menu(struct v4l2_ctrl_handler *hdl,
const struct v4l2_ctrl_ops *ops,
u32 id, s32 max, s32 skip_mask, s32 def);
这个函数用来增加菜单式的控制变量，max是菜单最大值,skip_mask是屏蔽标志，比如它的值是0x4,就么就屏蔽菜单值为4的菜单选项，def是默认的菜单值。这个函数适用于从0开始，相邻变量值增加1的控制变量。
例：
enum{
BW_6M,
BW_7M,
BW_8M
}BW;
v4l2_ctrl_new_std_menu(&foo->ctrl_handler,&foo_ctrl_ops,V4L2_CID_BW,BW_8M
,0,BW_6M);
（3）struct v4l2_ctrl *v4l2_ctrl_new_int_menu(struct v4l2_ctrl_handler *hdl,
const struct v4l2_ctrl_ops *ops,
u32 id, s32 max, s32 def, const s64 *qmenu_int);
这个函数用来增加整数型菜单的控制变量，max是最大的菜单索引号，def是默认的菜单索引号，qmenu_int是菜单的数组指针。这个函数适用于变量值是不连续的无规则的整数的控制变量。
例：
static const s64 audio_sample_qmenu[] = {22100,44100,48000,96000};
v4l2_ctrl_new_int_menu(&foo->ctrl_handler,&foo_ctrl_ops,V4L2_CID_AUDIOSAMPLERATE,ARRAY_SIZE(audio_sample_qmenu)-1,1,audio_sample_qmenu);
（4）struct v4l2_ctrl *v4l2_ctrl_new_std_menu_items(
struct v4l2_ctrl_handler *hdl,
const struct v4l2_ctrl_ops *ops, u32 id, s32 max,
s32 skip_mask, s32 def, const char * const *qmenu);
这个函数用来增加一个驱动指定的菜单数组给控制变量，max是菜单最大索引号，skip_mask是需要跳过的菜单的索引号，def是默认菜单索引号，qmenu是菜单数组。
例：
static const char * const test_pattern[] = {
"Disabled",
"Vertical Bars",
"Solid Black",
"Solid White",
};
v4l2_ctrl_new_std_menu_items(&foo->ctrl_handler, &foo_ctrl_ops,
V4L2_CID_TEST_PATTERN, ARRAY_SIZE(test_pattern) - 1, 0,
0, test_pattern);
设置变量的值一般是在初始化了v4l2_ctrl_handler之后进行，在最后需要检查一下有没有错误 ：
if (foo->ctrl_handler.error) {
int err = foo->ctrl_handler.error;
v4l2_ctrl_handler_free(&foo->ctrl_handler);
return err;
}
6.（可选，推荐使用）使用控制变量的默认值初始化所有硬件：
v4l2_ctrl_handler_setup(&foo->ctrl_handler);
这个函数会调用s_ctrl设置每个控制变量到硬件或其他地方。
7.最后，实现v4l2_ctrl_ops结构体：
static const struct v4l2_ctrl_ops foo_ctrl_ops = {
.s_ctrl = foo_s_ctrl,
};
static int foo_s_ctrl(struct v4l2_ctrl *ctrl)
{
struct foo *state = container_of(ctrl->handler, struct foo, ctrl_handler);
switch (ctrl->id) {
case V4L2_CID_BRIGHTNESS:
write_reg(0x123, ctrl->val);
break;
case V4L2_CID_CONTRAST:
write_reg(0x456, ctrl->val);
break;
}
return 0;
}
上面就是V4L2框架提供的基本的控制变量的使用方法，下面是bttv驱动的实例代码：
struct bttv {
......
/* controls */
struct v4l2_ctrl_handler ctrl_handler;
struct v4l2_ctrl_handler radio_ctrl_handler;
......
};
static int bttv_probe(struct pci_dev *dev, const struct pci_device_id *pci_id)
{
......
hdl = &btv->ctrl_handler;
v4l2_ctrl_handler_init(hdl, 20);
btv->c.v4l2_dev.ctrl_handler = hdl;
v4l2_ctrl_handler_init(&btv->radio_ctrl_handler, 6);
......
v4l2_ctrl_new_std(hdl, &bttv_ctrl_ops,
V4L2_CID_BRIGHTNESS, 0, 0xff00, 0x100, 32768);
v4l2_ctrl_new_std(hdl, &bttv_ctrl_ops,
V4L2_CID_CONTRAST, 0, 0xff80, 0x80, 0x6c00);
v4l2_ctrl_new_std(hdl, &bttv_ctrl_ops,
V4L2_CID_SATURATION, 0, 0xff80, 0x80, 32768);
......
v4l2_ctrl_handler_setup(hdl);
......
}
static const struct v4l2_ctrl_ops bttv_ctrl_ops = {
.s_ctrl = bttv_s_ctrl,
};
static int bttv_s_ctrl(struct v4l2_ctrl *c)
{
struct bttv *btv = container_of(c->handler, struct bttv, ctrl_handler);
int val;
switch (c->id) {
case V4L2_CID_BRIGHTNESS:
bt848_bright(btv, c->val);
break;
case V4L2_CID_HUE:
bt848_hue(btv, c->val);
break;
......
}
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