分类： 树莓派

使用`VNC Viewer`连接`树莓派4B`远程桌面提示错误“`Cannot currently show the dekstop`”,如下图： 继续阅读使用VNC Viewer连接树莓派4B远程桌面提示错误“Cannot currently show the dekstop”

If you're building software for the Raspberry Pi (like I sometimes do), it can be a pain to have to constantly keep Pi hardware around and spotting Pi-specific problems can be difficult until too late.

One option (and the one I most like) is to emulate a Raspberry Pi locally before ever hitting the device. Why?

• Works anywhere you can install QEMU
• No hardware setup needed (no more scratching around for a power supply)
• Faster feedback cycle compared to hardware
• I can use Pi software (like Raspbian) in a virtual context
• I can prep my "virtual Pi" with all the tools I need regardless of my physical Pi's use case

Given I'm next-to-useless at Python, that last one is pretty important as it allows me to install every Python debugging and testing tool known to man on my virtual Pi while my end-product hardware stays comparatively pristine.

Getting started

First, you'll need a few prerequisites:

QEMU (more specifically qemu-system-arm)

You can find all the packages for your chosen platform on the QEMU website and is installable across Linux, macOS and even Windows.

Raspbian

Simply download the copy of Raspbian you need from the official site. Personally, I used the 2018-11-13 version of Raspbian Lite, since I don't need an X server.

Kernel

Since the standard RPi kernel can't be booted out of the box on QEMU, we'll need a custom kernel. We'll cover that in the next step.

Preparing

Get your kernel

First, you'll need to download a kernel. Personally, I (along with most people) use the dhruvvyas90/qemu-rpi-kernel repository's kernels. Either clone the repo:

$ git clone https://github.com/dhruvvyas90/qemu-rpi-kernel.git

or download a kernel directly:

$ wget https://github.com/dhruvvyas90/qemu-rpi-kernel/raw/master/kernel-qemu-4.4.34-jessie

or download a snapshot from my website directly:

$ wget https://www.mobibrw.com/wp-content/uploads/2019/03/qemu-rpi-kernel.zip

For the rest of these steps I'm going to be using the kernel-qemu-4.4.34-jessiekernel, so update the commands as needed if you're using another version.

Filesystem image

This step is optional, but recommended

When you download the Raspbian image it will be in the raw format, a plain disk image (generally with an .img extension).

A more efficient option is to convert this to a qcow2 image first. Use the qemu-imgcommand to do this:

$ qemu-img convert -f raw -O qcow2 2018-11-13-raspbian-stretch-lite.img raspbian-stretch-lite.qcow

Now we can also easily expand the image:

$ qemu-img resize raspbian-stretch-lite.qcow +6G

You can check on your image using the qemu-img info command

Starting

You've got everything you need now: a kernel, a disk image, and QEMU!

Actually running the virtual Pi is done using the qemu-system-arm command and it can be quite complicated. The full command is this (don't worry it's explained below):

$ sudo qemu-system-arm \
-kernel ./kernel-qemu-4.4.34-jessie \
-append "root=/dev/sda2 panic=1 rootfstype=ext4 rw" \
-hda raspbian-stretch-lite.qcow \
-cpu arm1176 -m 256 \
-M versatilepb \
-no-reboot \
-serial stdio \
-net nic -net user

如果需要指定上网方式的话，执行如下命令：

$ sudo qemu-system-arm \
-kernel ./kernel-qemu-4.4.34-jessie \
-append "root=/dev/sda2 panic=1 rootfstype=ext4 rw" \
-hda raspbian-stretch-lite.qcow \
-cpu arm1176 -m 256 \
-M versatilepb \
-no-reboot \
-serial stdio \
-net nic -net user \
-net tap,ifname=vnet0,script=no,downscript=no

So, in order:

• sudo qemu-system-arm: you need to run QEMU as root
• -kernel: this is the path to the QEMU kernel we downloaded in the previous step
• -append: here we are providing the boot args direct to the kernel, telling it where to find it's root filesytem and what type it is
• -hda: here we're attaching the disk image itself
• -cpu/-m: this sets the CPU type and RAM limit to match a Raspberry Pi
• -M: this sets the machine we are emulating. versatilepb is the 'ARM Versatile/PB' machine
• -no-reboot: just tells QEMU to exit rather than rebooting the machine
• -serial: redirects the machine's virtual serial port to our host's stdio
• -net: this configures the machine's network stack to attach a NIC, use the user-mode stack, connect the host's vnet0 TAP device to the new NIC and don't use config scripts.

If it's all gone well, you should now have a QEMU window pop up and you should see the familiar Raspberry Pi boot screen show up.

Now, go get yourself a drink to celebrate, because it might take a little while.

Networking

Now, that's all well and good, but without networking, we may as well be back on hardware. When the machine started, it will have attached a NIC and connected it to the host's vnet0 TAP device. If we configure that device with an IP and add it to a bridge on our host, you should be able to reliably access it like any other virtual machine.

(on host) Find a bridge and address

This will vary by host, but on my Fedora machine, for example, there is a pre-configured virbr0 bridge interface with an address in the 192.168.122.0/24 space:

virbr0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.122.1  netmask 255.255.255.0  broadcast 192.168.122.255
        ether 00:00:00:1e:77:43  txqueuelen 1000  (Ethernet)

I'm going to use this bridge and just pick a static address for my Pi: 192.168.122.200

Reusing an existing (pre-configured) bridge means you won't need to sort your own routing

(in guest) Configure interface

NOTE: I'm assuming Stretch here.

Open /etc/dhcpcd.conf in your new virtual Pi and configure the eth0 interface with a static address in your bridge's subnet. For example, for my bridge:

# in /etc/dhcpcd.conf
interface eth0
static ip_address=192.168.122.200/24
static routers=192.168.122.254
static domain_name_servers=8.8.8.8 8.8.4.4

You may need to reboot for this to take effect

(in host) Add TAP to bridge

Finally, add the machine's TAP interface to your chosen bridge with the brctl command:

$ sudo brctl addif virbr0 vnet0

Now, on your host, you should be able to ping 192.168.122.200 (or your Pi's address).

Set up SSH

Now, in your machine, you can run sudo raspi-config and enable the SSH server (in the "Interfacing Options" menu at time of writing).

Make sure you change the password from default while you're there!

Finally, on your host, run ssh-copy-id pi@192.168.122.200 to copy your SSH key into the Pi's pi user and you can now SSH directly into your Pi without a password prompt.

参考链接

---

Using QEMU to emulate a Raspberry Pi

最近在参照 树莓派实时系统下脚本语言的选择(应当使用Lua而不是Python) 调用调试lua-zmq的时候，发现使用PAIR模式进行线程之间通信，长时间运行后会出现死锁的情况。

当我们需要跟踪问题的时候，使用apt-get安装的版本缺乏必要的调试信息。

我们可以手工安装调试信息包，如下：

$ sudo apt install libzmq-dbg

# 系统默认的lua 5.1的符号信息使用如下命令安装
$ sudo apt-get install liblua5.1-0-dbg

# libreadline提供了多个版本（5,6,7），根据需要下载对应的版本
$ sudo apt-get install libreadline6-dbg

# sudo apt-get install search libtinfo* 根据返回的结果安装
$ sudo apt-get install libtinfo5-dbg

我们也可以从源代码重新编译一份，如下：

# 打开源代码的源
$ sudo sed -i "s/^#deb-src/deb-src/g" /etc/apt/sources.list

# 安装必要的编译工具
$ sudo apt-get install build-essential devscripts lintian

$ cd ~

# 下载源代码，此处不可sudo,否则后续操作会出现权限问题
$ apt-get source libzmq-dev

$ cd zeromq*/

# 安装依赖项
$ sudo apt-get install libpgm-dev

$ sudo apt-get install uuid-dev

$ debuild -us -uc -b

$ cd ..

$ sudo dpkg -i libzmq-dev*.deb

$ sudo dpkg -i libzmq-dbg*.deb

$ sudo ldconfig

以上是针对Debian安装包来进行的处理，然而对于luarocks安装的插件来说，默认luarocks是不能编译调试版本的，此时就需要我们手工编译了。我们以lua-zmq为例子，参考如下：

$ sudo apt-get -y install cmake

$ git clone git://github.com/Neopallium/lua-zmq.git

$ git checkout v1.1

$ cd lua-zmq ; mkdir build ; cd build

$ cmake -DCMAKE_BUILD_TYPE=Debug ..

$ make

$ sudo make install

$ sudo ldconfig

最近在树莓派上需要安装libgtk2.0-dev，执行如下命令：

$ sudo apt-get install libgtk2.0-dev

出错信息如下：

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 libgtk2.0-dev : Depends: libpango1.0-dev (>= 1.20) but it is not going to be installed
                 Depends: libcairo2-dev (>= 1.6.4-6.1) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

根据出错信息，明显是软件源中出现了安装包缺失的问题。这个现象是不应该出现的。网上查询了不少地方，最终找到解决方法：

$ sudo vim /etc/apt/sources.list.d/raspi.list

可以看到如下内容：

#deb http://archive.raspberrypi.org/debian/ stretch main ui staging
# Uncomment line below then 'apt-get update' to enable 'apt-get source'
#deb-src http://archive.raspberrypi.org/debian/ stretch main ui

默认里面的内容都是被注释掉的，我们需要做的就是把这个源打开即可。
也可以直接执行如下命令来开启：

#先备份配置文件
$ sudo cp /etc/apt/sources.list.d/raspi.list /etc/apt/sources.list.d/raspi.list.bak

$ sudo sed -i "s/^\#deb/deb/g" /etc/apt/sources.list.d/raspi.list

参考链接

---

Unable to install libgtk2.0-dev on Raspberry Pi 2

最新在使用的Raspberry Pi Zero W V1.3在使用目前（2018.09.26)的系统的时候发现无法正常输出内容到屏幕上面，屏幕一直黑屏无信号。

原因在于Raspberry Pi Zero W在启动的时候没有正确检测到屏幕信号，导致没有正常输出。

解决方法是打开启动配置文件/boot/config.txt, 找到如下内容：

..................

# uncomment if hdmi display is not detected and composite is being output
#hdmi_force_hotplug=1

.................

然后去掉注释，修改为如下：

# uncomment if hdmi display is not detected and composite is being output
hdmi_force_hotplug=1

最近在使用树莓派与其他设备通过SPI接口进行通信，使用一个GPIO管脚触发读取数据的信号,为了简化开发，使用了Python。

在实际运行过程中，发现当长时间运行的是，会出现中断管脚信号丢失的情况，在参考 Ubuntu 16.04 (x64)下从源代码为Raspberry Pi Zero W编译实时内核  更换为实时内核之后，短时间运行已经可以正常，但是在十几个小时之后，依然出现了中断丢失的现象。

这个现象初步评估为Python的GC动作时间过长导致的中断信号丢失。Python本身并不是为实时系统设计的，因此在GC进行垃圾回收的时候，是没有实时性的考虑的，因此在严格要求实时性的系统环境下，不是非常的合适。更何况很多的IO操作默认都是阻塞的，更加容易导致实时性问题。

由于树莓派本身也是支持Lua脚本的，默认安装的Lua引擎默认是5.1.4。Lua本身在游戏中使用较多，而游戏本身对于实时性的要求是很高的。

尤其是Lua 5.1开始使用最新的GC已经能很好的解决实时性问题。

关于Lua的GC相关信息，参考如下的文章：

继续阅读树莓派实时系统下脚本语言的选择(应当使用Lua而不是Python)

首先参考 Ubuntu 16.04 (x64)树莓派B+ (Raspberry Pi B+)源代码编译 保证能够成功编译标准内核的源代码，然后切换到实时内核分支，并执行如下编译命令：

$ export PATH=$PATH:~/rpi/rpi-tools/arm-bcm2708/gcc-linaro-arm-linux-gnueabihf-raspbian-x64/bin

$ cd ~/rpi/rpi-linux/
 
$ git checkout rpi-4.14.y-rt

$ git reset --hard

$ KERNEL=kernel
 
$ make clean

$ make mrproper

$ rm -rf .config

#调整内核切换频率，增加实时性
$ sed -i '$a\CONFIG_HZ_1000=y' arch/arm/configs/bcmrpi_defconfig

#Raspberry Pi Zero W的CPU是BCM2835
$ make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- bcmrpi_defconfig

$ make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- zImage modules dtbs -j8
 
$ mkdir rt_kernel
 
$ make modules_install ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- INSTALL_MOD_PATH=./rt_kernel -j8

$ make dtbs_install ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- INSTALL_DTBS_PATH=./rt_kernel -j8

$ ./scripts/mkknlimg ./arch/arm/boot/zImage ./rt_kernel/kernel.img

安装编译好的内核

Ubuntu下面，SD卡会自动挂载,默认挂载到了/media/目录下面，如果是使用NOOBS安装的话，系统目录是固定的，执行如下命令拷贝到目标SD卡上面

$ cd ~/rpi/rpi-linux

#备份需要修改的文件
$ mv /media/`whoami`/boot/kernel.img /media/`whoami`/boot/kernel_old.img

$ mv /media/`whoami`/boot/overlays /media/`whoami`/boot/overlays.old

#拷贝内核
$ cp rt_kernel/kernel.img /media/`whoami`/boot/kernel.img

#拷贝硬件配置
$ cp rt_kernel/bcm2835*.dtb /media/`whoami`/boot/

#拷贝overlays
$ cp -r rt_kernel/overlays /media/`whoami`/boot/

#拷贝内核模块
$ sudo cp -r rt_kernel/lib/modules/* /media/`whoami`/rootfs/lib/modules/

#卸载设备
$ sudo umount -A -R -a /media/`whoami`/boot

借助编译环境，单独编译内核模块

有时，我们需要单独编译新下载的内核驱动，这个时候，就可以使用如下的方式进行单独内核模块的编译。

下面，我们以ASIX AX88772系列的USB有线网卡驱动的编译为例：

$ export PATH=$PATH:~/rpi/rpi-tools/arm-bcm2708/gcc-linaro-arm-linux-gnueabihf-raspbian-x64/bin

$ cd ~/rpi/rpi-linux

# wget https://www.mobibrw.com/wp-content/uploads/2018/09/AX88772C_772B_772A_760_772_178_LINUX_DRIVER_v4.22.0_Source.tar.bz2

$ wget http://www.asix.com.tw/FrootAttach/driver/AX88772C_772B_772A_760_772_178_LINUX_DRIVER_v4.22.0_Source.tar.bz2

$ tar xvf AX88772C_772B_772A_760_772_178_LINUX_DRIVER_v4.22.0_Source.tar.bz2

$ cd AX88772C_772B_772A_760_772_178_LINUX_DRIVER_v4.22.0_Source

$ make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- -C ../ M=`pwd`

#当前目录下就会生成 asix.ko 这个内核模块，生成的内核模块拷贝到指定的目录即可正常工作

解决已知问题

使用上面的命令安装完成内核后，目前(2018.09.26)遇到的问题为，当插入ASIX AX88772系列的USB有线网卡之后，会导致内核崩溃，启动失败。

设备信息如下：

Bus 001 Device 034: ID 0b95:7720 ASIX Electronics Corp. AX88772

初步怀疑是USB设备的驱动依赖关系不正确导致内核崩溃。

目前的临时解决方法为要求设备启动时候优先加载USB设备相关的驱动，而不是等到网卡插入的时候再去加载。

也就是在/boot/config.txt文件尾部新增加一行dtoverlay=dwc2。这段代码本来是为树莓派通过USB访问网络的虚拟网卡准备的（是的，你没看错，树莓派本身可以不借助网卡直接通过USB接口跟电脑共享方式上网，不过需要设置一堆东西，最简单的还是外接真正的USB网卡）。我们加载这个模块，但是并不使用这个功能，造成的结果就是重新调整了模块加载顺序，规避了后续的问题。

$ sed -i '$a\\ndtoverlay=dwc2' /media/`whoami`/boot/config.txt

上述修改后，依旧存在动态插拔网卡，设备会重启的问题，不过已经不影响正常使用。

参考链接

---

• Ubuntu 16.04 (x64)树莓派B+ (Raspberry Pi B+)源代码编译
• #Raspberry Pi: Preempt-RT Patching Tutorial for Kernel 4.14.y
• 树莓派编译升级内核
• cross compile raspberry pi kernel module

树莓派在实时操作系统上的几个问题，原文如下：
继续阅读How to Perform Real-Time Processing on the Raspberry Pi