前言

笔者计划通过虚拟摄像头系列文章、让读者完整掌握在android系统中实现过程和应用方法。以作备忘之用、再则让有需求朋友可以参考。

第一步 移植 v4l2loopback 驱动至 android的 linux 内核

在kernel/driver/ 创建 virtual_camera 文件夹,内容结构如下:

robot@ubuntu:~/android_build/vendor/nxp-opensource/kernel_imx/drivers/virtual_camera$ tree -L 1
.
|-- Makefile
|-- v4l2loopback.c
|-- v4l2loopback.h
`-- v4l2loopback_formats.h
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编写 Makefile 文件,编译 v4l2loopback.ko 模块驱动涉及文件内容如下:

include Kbuild
ifeq ($(KBUILD_MODULES),)

#KERNELRELEASE  ?= `uname -r`
#KERNEL_DIR ?= /lib/modules/$(KERNELRELEASE)/build
ARCH=arm64
KERNEL_DIR=/home/robot/android_build/vendor/nxp-opensource/kernel_imx
CROSS_COMPILE=/home/robot/android_build/prebuilts/gcc/linux-x86/aarch64/aarch64-linux-android-4.9/bin/aarch64-linux-android-

PWD     := $(shell pwd)

PREFIX ?= /usr/local
BINDIR  = $(PREFIX)/bin
MANDIR  = $(PREFIX)/share/man
MAN1DIR = $(MANDIR)/man1
INSTALL = install
INSTALL_PROGRAM = $(INSTALL) -p -m 755
INSTALL_DIR     = $(INSTALL) -p -m 755 -d
INSTALL_DATA    = $(INSTALL) -m 644

MODULE_OPTIONS = devices=2

##########################################
# note on build targets
#
# module-assistant makes some assumptions about targets, namely
#  <modulename>: must be present and build the module 
#                <modulename>.ko is not enough
# install: must be present (and should only install the module)
#
# we therefore make <modulename> a .PHONY alias to <modulename>.ko
# and remove utils-installation from 'install'
# call 'make install-all' if you want to install everything
##########################################


.PHONY: all install clean distclean
.PHONY: install-all install-utils install-man
.PHONY: modprobe v4l2loopback

# we don't control the .ko file dependencies, as it is done by kernel
# makefiles. therefore v4l2loopback.ko is a phony target actually
.PHONY: v4l2loopback.ko utils

all: v4l2loopback.ko utils

v4l2loopback: v4l2loopback.ko
v4l2loopback.ko:
    @echo "Building v4l2-loopback driver..."
    $(MAKE) -C $(KERNEL_DIR) M=$(PWD) modules

install-all: install install-utils install-man
install:
    $(MAKE) -C $(KERNEL_DIR) M=$(PWD) modules_install
    @echo ""
    @echo "SUCCESS (if you got 'SSL errors' above, you can safely ignore them)"
    @echo ""

install-utils: utils/v4l2loopback-ctl
    $(INSTALL_DIR) "$(DESTDIR)$(BINDIR)"
    $(INSTALL_PROGRAM) $< "$(DESTDIR)$(BINDIR)"

install-man: man/v4l2loopback-ctl.1
    $(INSTALL_DIR) "$(DESTDIR)$(MAN1DIR)"
    $(INSTALL_DATA) $< "$(DESTDIR)$(MAN1DIR)"

clean:
    rm -f *~
    rm -f Module.symvers Module.markers modules.order
    $(MAKE) -C $(KERNEL_DIR) M=$(PWD) clean
    $(MAKE) -C utils clean

distclean: clean
    rm -f man/v4l2loopback-ctl.1

modprobe: v4l2loopback.ko
    chmod a+r v4l2loopback.ko
    sudo modprobe videodev
    -sudo rmmod v4l2loopback
    sudo insmod ./v4l2loopback.ko $(MODULE_OPTIONS)

man/v4l2loopback-ctl.1: utils/v4l2loopback-ctl
    help2man -N --name "control v4l2 loopback devices" \
        --no-discard-stderr --help-option=-h --version-option=-v \
        $^ > $@

utils: utils/v4l2loopback-ctl
utils/v4l2loopback-ctl: utils/v4l2loopback-ctl.c
    $(MAKE) -C utils

.clang-format:
    curl "https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/plain/.clang-format" > $@

.PHONY: clang-format
clang-format: .clang-format
    clang-format -i *.c *.h utils/*.c

endif # !kbuild
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此Makefile使用源码中文件，仅修改交叉编译工具内容。

第二步 在 kernel/driver/Makefile 中添加增加的 v4l2loopback 设备:

增加新增设备

obj-$(CONFIG_MULTIPLEXER)       += mux/
obj-y                           += virtual_camera/
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编译输出内容如下：

robot@ubuntu:~/android_build/out/target/product/mek_8q/obj/KERNEL_OBJ$ ls drivers/virtual_camera/
built-in.o  modules.builtin  modules.order  v4l2loopback.ko  v4l2loopback.mod.c  v4l2loopback.mod.o  v4l2loopback.o
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再次编译android源码后，v4l2loopback.ko 内核模块就添加打包至kernel.img的镜像中。

第三步 加载并启动 v4l2loopback.ko 模块

@device/fsl/imx8q/mek_8q/BoardConfig.mk 文件中包含 v4l2loopback.ko

BOARD_VENDOR_KERNEL_MODULES += \
                            $(KERNEL_OUT)/drivers/net/wireless/qcacld-2.0/wlan.ko \
                            $(KERNEL_OUT)/drivers/virtual_camera/v4l2loopback.ko
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@device/fsl/imx8q/mek_8q/init.rc 文件中增加insmod v4l2loopback.ko

on boot
    start audioserver
    start mediadrm
    start media
    start drm
    chmod 0777 /vendor/lib/modules/v4l2loopback.ko
    insmod /vendor/lib/modules/v4l2loopback.ko
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第四步 验证 v4l2loopback.ko 模块是否加载成功

未加载 v4l2loopback.ko ,设备dev目录的video如下:

mek_8q:/ # rmmod vendor/lib/modules/v4l2loopback.ko
mek_8q:/ # ls /dev
FreescaleAccelerometer block           event-log-tags gpiochip2 hwbinder iio:device0  memory_bandwidth   ppp    shm    tty10 tty18 tty25 tty32 tty4  tty47 tty54 tty61  ttyLP3   uinput        vga_arbiter video4
FreescaleGyroscope     console         fd             gpiochip3 i2c-1    input        mxc_asrc           pps0   snd    tty11 tty19 tty26 tty33 tty40 tty48 tty55 tty62  ttyS0    urandom       vhci        vndbinder
FreescaleMagnetometer  cpu_dma_latency fscklogs       gpiochip4 i2c-2    ion          mxc_hifi4          ptmx   socket tty12 tty2  tty27 tty34 tty41 tty49 tty56 tty63  ttyS1    usb-ffs       video0      watchdog
__properties__         cpuctl          full           gpiochip5 i2c-3    isoc         mxc_mem            ptp0   stune  tty13 tty20 tty28 tty35 tty42 tty5  tty57 tty7   ttyS2    usb_accessory video1      watchdog0
ashmem                 cpuset          fuse           gpiochip6 i2c-4    kmsg         mxc_sim            pts    sync   tty14 tty21 tty29 tty36 tty43 tty50 tty58 tty8   ttyS3    vcs           video12     xt_qtaguid
async                  ctrl            galcore        gpiochip7 i2c-5    kvm          network_latency    random tty    tty15 tty22 tty3  tty37 tty44 tty51 tty59 tty9   tun      vcs1          video13     zero
autofs                 device-mapper   gpiochip0      graphics  i2c-6    loop-control network_throughput rfkill tty0   tty16 tty23 tty30 tty38 tty45 tty52 tty6  ttyLP0 ubi_ctrl vcsa          video2
binder                 dri             gpiochip1      hw_random i2c-7    memcg        null               rtc0   tty1   tty17 tty24 tty31 tty39 tty46 tty53 tty60 ttyLP1 uhid     vcsa1         video3
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加载 v4l2loopback.ko ,设备dev目录的video如下:

mek_8q:/ # insmod vendor/lib/modules/v4l2loopback.ko
mek_8q:/ # ls /dev/
FreescaleAccelerometer block           event-log-tags gpiochip2 hwbinder iio:device0  memory_bandwidth   ppp    shm    tty10 tty18 tty25 tty32 tty4  tty47 tty54 tty61  ttyLP3   uinput        vcsa1       video3
FreescaleGyroscope     console         fd             gpiochip3 i2c-1    input        mxc_asrc           pps0   snd    tty11 tty19 tty26 tty33 tty40 tty48 tty55 tty62  ttyS0    urandom       vga_arbiter video4
FreescaleMagnetometer  cpu_dma_latency fscklogs       gpiochip4 i2c-2    ion          mxc_hifi4          ptmx   socket tty12 tty2  tty27 tty34 tty41 tty49 tty56 tty63  ttyS1    usb-ffs       vhci        video5
__properties__         cpuctl          full           gpiochip5 i2c-3    isoc         mxc_mem            ptp0   stune  tty13 tty20 tty28 tty35 tty42 tty5  tty57 tty7   ttyS2    usb_accessory video0      vndbinder
ashmem                 cpuset          fuse           gpiochip6 i2c-4    kmsg         mxc_sim            pts    sync   tty14 tty21 tty29 tty36 tty43 tty50 tty58 tty8   ttyS3    v4l2loopback  video1      watchdog
async                  ctrl            galcore        gpiochip7 i2c-5    kvm          network_latency    random tty    tty15 tty22 tty3  tty37 tty44 tty51 tty59 tty9   tun      vcs           video12     watchdog0
autofs                 device-mapper   gpiochip0      graphics  i2c-6    loop-control network_throughput rfkill tty0   tty16 tty23 tty30 tty38 tty45 tty52 tty6  ttyLP0 ubi_ctrl vcs1          video13     xt_qtaguid
binder                 dri             gpiochip1      hw_random i2c-7    memcg        null               rtc0   tty1   tty17 tty24 tty31 tty39 tty46 tty53 tty60 ttyLP1 uhid     vcsa          video2      zero
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设备 video5 就是 v4l2loopback.ko 模块驱动的设备,至此 v4l2loopback.ko 模块移植成功。

接下来我们将把此设备配置为前置或后置摄像头、并配置sensor相关属性信息，以此通过android用户空间程序调用此虚拟摄像头实现拍照。