How to learn embedded Linux?

Many beginners will ask, how to learn embedded Linux? Let's discuss this issue below.

Embedded Linux can be divided into two parts: the underlying system and application development .

For application development: C language, data structure, JAVA, what needs to be learned. There is nothing special to note about embedded application development and application development on PCs. Maybe you said that you need to do some optimization on the embedded, yes, to optimize, but the unoptimized program and the program development on the PC is no different. In addition, when you have the ability to optimize, you are not used to ask this question. Specific to some examples, such as the development interface, we use VC on the PC; in embedded Linux, we may use Android with QT, this time you should learn QT, Android programming. But the basics are still C or JAVA, based on which to familiarize themselves with their interfaces. If you have studied VC, you also need to take the time to understand the classes and controls.

If you want to learn the underlying system, this is my specialty, but I can say a little.

Before answering this question, I will answer first: Many people ask me, is it a learning drive or an application? I can only say that I am interested, and the drivers and applications are not completely separate.

1. The driver we are talking about is not limited to the operation of hardware, but also the concept of operating system, sleep wake-up scheduling of processes and so on. I want to write a good application, I want to solve the problem that the application encounters better. You should understand this knowledge. 2. The application threshold is low, especially the current ANDROID, pure JAVA. The development path of the application is personally considered to be a good business. For example, in the communications industry, IPTV industry, mobile phone industry, you understand the needs of the industry. Therefore, when leading people, most of them are applied. 3. To be a driver, in fact, I don't want to call it “doing a driver”, but I want to call it “doing the underlying system”. This is a good way to kill all industries. I have been working for a few years, doing cell phone, IPTV, conference TV, but these products are no different to me, because I only do the bottom. Their business has nothing to do with me. When there is a problem with the application and they can't solve it, I will give them an idea from a kernel perspective and provide them with tools. Do the underlying development direction, personally think it is a technical expert. 4. In fact, do the bottom layer or do the application, there is no boundary between them, there is the underlying experience, and then apply, you will feel very practical. With business experience, you can understand the bottom layer and you can form a team soon.

Go back to the question of how to learn. What is included in the embedded Linux underlying system? Don't worry, just give an example and you will know.

1. When the computer is turned on, who displays those interfaces? Is the BIOS, what does it do? Some self-tests, then read in windows from the hard drive and launch it. Similarly, this BIOS corresponds to the bootloader in embedded Linux. The bootloader goes to Flash to read into the Linux kernel and launch it.

2. What is the purpose of starting windows? Of course, what is online chat? Where are these Internet and chat tools? On the C drive, D drive. Therefore, windows must first identify the C drive, D drive. Under Linux we call the root file system.

3. Windows can recognize the C drive, D drive, then you can certainly read and write the hard drive. This involves something called a driver. Of course not only hard drives, but also network cards, USB and more. Embedded Linux can read and execute applications from Flash, and certainly have Flash drivers, and of course not just Flash.

Let me talk about it here. Embedded Linux contains 4 big blocks of bootloader, kernel, driver and root file system.

First, bootloader:

It is a slightly more complicated bare board program. However, it is not easy to understand and write this bare board program. The easy-to-use tools under Windows have weakened our programming skills.

Many people use ADS and KEIL when they play embedded. Can you answer these questions?

1. When the power is turned on, where does the CPU fetch instructions? A: Generally speaking from Flash.

2. But Flash is generally only read and can't be written directly. If I use global variables, where are these global variables? A: Global variables should be in memory

3. So who put the global variables in memory? A: Friends who have used ADS and KEIL for a long time, can you answer? This requires "relocation". In ADS or KEIL, the relocation code is written for the company that made these tools. Have you ever read it?

4. How big is the memory, how do I know which address to read "the contents of the original Flash" to the memory? A: This address is determined by the "link script". There are scatter files in ADS, and similar files are available in KEIL. But have you studied it?

5. You said that relocation is to copy the program from Flash to memory, then this program can read Flash? A: Yes, you need to be able to operate Flash. Of course not only these, but also setting the clock to make the system run faster and so on. Let me ask myself to answer it here. Bootloader is a bare board program. In fact, there are 3 main points:

1. Operation on the hardware 2. Understanding of the ARM system processor 3. The basic concept of the program: relocation, stack, code segment data segment BSS segment or something.

For hardware operation, you need to see the schematic and chip manual. This requires a certain amount of hardware knowledge, not asking you to design the hardware, but at least understand it; not to understand the analog circuit, but to understand the digital circuit. I have learned this ability in school. The two books (the book name is forgotten) are enough. But I doubt if you have the patience to read these 2 books. I don't know if there is a faster book now. If you want to be quick, just let go of this one. If you don't understand, ask GOOGLE and post it.

In addition, the chip manual is definitely to be read. If you don't go to Chinese, you can read English. It is very painful to start at first, and later you will find that it is easy to read any chip manual once the grammar and vocabulary are familiar.

For an understanding of the ARM system processor, see Du Chunlei's, there are assembly instructions, there are abnormal modes, MMU and so on. That's why these three pieces of content need you to understand. Architecture and Programming >

The basic concept of the program, of course, is to see the principle of compilation. Unfortunately, this type of book is absolutely heavenly. Advise you not to see if you are not a super genius. Just look at the <embedded Linux application development complete manual> and the first video I wrote, don't worry, don't spend money. The concept of the hardware-related experiments was done according to the video. I haven't found the second set of books or videos about these concepts, allowing me to brag about it. For the bootloader, I read it first, and then I wrote the program to do all the hardware experiments, such as GPIO, clock, SDRAM, UART, NAND. Make them all clear, it is easy to understand u-boot when you are together. To sum up, read the hardware schematics and look at the chip manual. This requires you to find the information yourself. For the rest, follow the <Full Manual for Embedded Linux Application Development> and the chapter directory of the first video to learn. Architecture and Programming >

Second, the kernel:

Those who want to be quick, first learn across the kernel and learn how to write drivers.

To become a master, the kernel must be deeply understood. Note, I am talking about understanding, I did not expect to write a kernel.

Have a understanding of the scheduling mechanism, memory management mechanism, file management mechanism, and so on.

Recommend two books:

1. Read through, please look at the thin one (the speed of the society is fast, huh), 2. Select, want to know which piece to read which kernel scenario analysis > kernel full comment >

Third, the driver:

The driver consists of two parts: the operation of the hardware itself, and the framework of the driver.

It is hardware, you still have to understand the schematic, read the chip manual, and practice more. Speaking of the driver framework, there are some books to introduce. LDD3, the one written by a foreigner, introduces a lot of concepts and is worth reading. However, its role is limited to introducing concepts. I basically used it to familiarize myself with the concept before getting started. I threw it away after getting started. Device driver>

A relatively comprehensive introduction to the driver should be Song Baohua. To be honest, I have only read the catalogue. Many people say it well. I recommend it here. Detailed information on device driver development >

To get a deeper understanding of a certain piece, it is definitely a super 5 star recommendation. Don't expect to read it, more than 1,800 pages, two volumes. When I am not sure about a piece, I will flip it over. In any part, this book can be said to be 2, 3 hundred pages, very detailed. And with a certain goal to take you to analyze the kernel source. It takes linux 2.4 as an example, but the principle is the same, and it is also applicable to other versions of linux. Is there any other introduction? Oh, of course, there is Weidongshan Linux video 2nd. The <embedded Linux application development complete manual> does not talk much about the driver, not enough depth. So I recorded this video. Not only teach you how to write how to change the driver, but also teach you why to write this change driver. Kernel Scenario Analysis >

Each driver is written on site:

1. Use the drawing board to draw a picture - the equivalent of the teacher in the school to draw a picture on the blackboard, very intuitive is definitely not to the PPT.
2. Use source insight to write the program on the spot, starting from the first line, every class is like this. I talked about more than 20 drivers and wrote more than 20 programs.
3. Compile and test after writing.
4. Very comprehensive, character device driver, block device, network card driver 3 categories complete, hardware introduction, drive framework analysis, testing 3 categories complete.

The content taught in the training institutions is far less than the richness of this video. I have taught in a number of training institutions. I have never seen a teacher who dares to explain on the spot on the spot to write code on the spot, except for me! I haven't seen any training institutions to finish these things - because there is not enough time, it will take at least a month to finish, but this part is basically only 2 weeks of teaching time. Try to write a driver for the hardware involved in the development board in your hand. If you have a problem, you should "think hard" first. In the process of thinking, you will connect a lot of unrelated knowledge and finally connect.

Fourth, the root file system:

Have you ever thought about these two questions:
1. For Linux products, some are used for monitoring, some for mobile phones, and some for tablets. Then, after the kernel is started, which application should be started after the root file system is mounted? A: The kernel does not know and does not matter which user program should be started. It only launches an init application, which corresponds to /sbin/init. Obviously, this application will read the configuration file and start the user program according to the configuration file (monitoring, manual interface, tablet interface, etc., this question prompts us that the content of the file system has some conventions, such as having /sbin /init, there must be a configuration file.

2. The hello, world program you wrote, have you ever thought about who implemented the printf used inside? A: This function is not implemented by you, it is implemented by the library function. When it runs, it has to find the library.

This question prompts us that there is a library in the file system. Simple self-answering to this place, in order to understand in depth, you can look at the init.c of busybox, you can know what the init process is doing. Of course, you can also look at the chapter on building a root file system in the Embedded Linux Application Development Complete Manual. Tell me about my learning experience.

1. When I was at school, I was studying physical electronics. In fact, I didn't teach how to design circuits in the course. I only taught some knowledge about electronic circuits. The design of the PCB was self-learning in the lab. Only two layers of the board were designed. Now I forgot about it. However, the ability to see the schematic and see the chip manual is retained.
2. I took a software degree and was very interested in software design, but I only learned C language and database. I have done a lot of competition questions with my interest. Not able to participate in the competition, but the C language is very solid.
3. In the lab, in the first company, design a simple PCI card and write the windows driver.
4. In the second company, using 51 MCUs to make car phones, began to embark on the road of pure software.
5. I started to feel the shortcomings of the MCU. I resigned for half a year to learn Linux, starting with how to operate the red hat. The steps are to look at it first, then write the bare board program to operate the hardware yourself, and then analyze u-boot. At the same time, I have some understanding of the LINUX framework. When writing bare boards, it is recommended that you strengthen the understanding of interrupts, the kernel is to use interrupts to complete various functions.
6. After analyzing u-boot, simple driver programming is started. At this time, the ability is still weak.
7. Start to work in ZTE, work for 2 years, write all kinds of drivers, solve various problems (drive problems, help locate application problems), and the ability to be tempered. Kernel Full Comments > Architecture and Programming >

in conclusion:

1. Hardware books: Microcomputer principle, digital circuits, teaching materials in colleges and universities.
2. Linux book: <Full manual for embedded Linux application development>, the one written by a foreigner.
3. Video: Weidongshan Linux Video Phase 1 (based on S3C2440 recording): ARM experiment, u-boot, file system, primary driver Weidongshan Linux video first phase (based on S3C6410 recording): bare board program Weidongshan Linux video Phase 2: Advanced Driver Kernel Scenario Analysis > Kernel Full Notes > Device Driver Development Details > Device Drivers > Architecture and Programming >