It is interesting to understand MMAP in this way!

Probably Emperor Yongzheng never thought of it , I am in the Western calendar 2022 In the eyes of young boys and girls , There will be two different images .

1

Based on my observation of my classmates and friends around me , Most boys like to watch 《 Yongzheng Dynasty 》, Yongzheng in their eyes , It's about a promotion “ The fire is consumed ”、“ Spread small into Mu ” And so on , Tax relief and other policies Reformer , It's a thrilling experience of nine sons' seizing the throne 、 Dark belly Politician , He was also conscientious after he ascended the throne , Stay up late 996 Of workaholic .

And most girls like to watch 《 Zhen Huan preach 》, Yongzheng in their eyes , yes “ Big fat orange ”, yes “ unfaithful man ”, It was played by concubines in the harem , Yes N A green hat , Finally, he was angry with Niu cobalt Lu Zhen Huan Cheating on men who play with women's affections .

I haven't seen the legend of Zhen Huan completely , But I was lucky to watch several episodes with my family during dinner .

As the saying goes There are three thousand beauties in the harem , Grind an iron pestle into an embroidery needle ... （ No

There are too many concubines , The emperor is only one , It is inevitable that they will be jealous of each other . The high ranking imperial concubine's bullying , Some little concubines can't fight back directly , Naturally, I will use some other strange skills , Prick a villain It is one of the most frequent methods .

According to my summary , The core idea of this technology is ： The user can't get to the master , You can only take a piece of cloth and other objects around you to simulate a little man , Draw the meridians and veins of Zhengzhu on it , Write your name , Cast some kind of magic , Then use a needle to pierce a certain acupoint of the villain at hand , The corresponding part of the remote Lord will suffer the same torture .

Although it is a bit magical , Enviable but not available . But in linux Kernel development , But you can use mmap A similar effect is achieved by the mechanism of .

2

mmap The core idea of ： The user cannot directly operate the physical address of the register in the user state , So through mmap Method for memory mapping , Map the physical address to the virtual address of the user state , Then the user reads and writes the virtual address at hand , You can read the physical address / write in .

What they have in common is , because The target cannot be operated directly , So in some way , Set up a kind of mapping Relationship , So as to achieve arm and finger , Where to hit , The effect of hitting and hurting .

As long as the mapping to the target can be established , What can we do with this mapping , Naturally, there is a lot of imagination . therefore mmap There are many uses , Some people use it to realize interprocess communication , Someone uses it to carry data , For our embedded engineers , We can use it to light a lamp . Hey , Surprised? ！

Listen to me .

3

As an embedded Engineer , Fancy lighting is a necessary skill . Whether it's writing bare metal code GPIO mouth , Or remote control through the cloud of the Internet of things LED, From a hardware point of view , The core principle is to find the connection LED Of GPIO mouth , Let it output an electrical signal . From the software point of view , The ultimate goal is to find this GPIO Mouth correspondence The address of the register , According to the actual circuit requirements , Give Way CPU Write it a 1 perhaps 0.

During bare metal development , We can directly find the physical address for operation . And in the Linux The system is slightly different . Because there is kernel space in the operating system , The programs we write run in user mode , You need to drive the hardware through the kernel , Cannot directly manipulate physical address .

Of course you can choose for this LED Write a driver , Thus in the user space through read,write To operate its state . However, some students have to write a drive when they listen to it , I just want to sing a song about how difficult it is to express my flinch . So if you haven't learned about driver, you can also choose to use a more direct way ：mmap.

It's like you can choose to drug the imperial concubine to control her , However, this method of medication needs to be proficient in Pharmacology 、 Control the timing , The high cost , Is difficult . As long as the goal can be achieved , Sometimes it's more economical to stick with a villain .

When I was in my last company, I used ARM Cortex-A9 Chip has done a project , The development process is probably to agree a protocol with the hardware colleagues first , Then I go through GPIO The input and output of the port simulate this Protocol , It is used to read and write registers , Driver hardware ADC sampling , Then the collected data is passed DMA transmission , Finally, it goes to the application layer for analysis and processing . Which drives GPIO The mouth part is used mmap.

Great project , For more than half a year , It's unrealistic to want to be completely clear , But we can drive GPIO This is the starting point , Experience the mysterious process of software driving hardware . Smart you can draw inferences from one instance .

4

As a software engineer , When you get the board , The hardware engineer will usually give you a document , Like this ：

This document will indicate , If you want to operate this GPIO Words of mouth , You need to use GPIO Add the base address of the peripheral and the offset address to find the corresponding register address , And then the bit operation is applied to the specified bit Write command .

But because of FPGA There will also be some , So in our company FPGA Of Block Diagram I built it all . finish building FPGA After the hardware engineering, do a comprehensive , from Address Map You can see what I want to use GPIO The address is . Pictured ：

No matter what , Now you have the so-called hardware register address , Then we can stab it with a villain .

I got the above picture 0x43C00000 For example , This is the address of the register , Can I put... Directly in the application 0x43C00000 Assign to a pointer , Then read and write it ？

This is true when playing bare metal . But it says ,Linux The system has Virtual memory The existence of , You can't do that . Because theoretically, I can drive in the system 100 A process , this 100 There are all processes 0x43C00000 This address , What about this 100 Which address is the real register address ？ It may not be . Because in the process 0x43C00000 It's virtual. , Where is its real physical address , No one knows . To map a virtual address to a physical address , You have to use it. mmap Do memory mapping .

5

mmap The function interface of is defined as follows ：

void mmap(void addr,size_t length,
          int prot,int flags,
          int fd,off_t offset);

There are many parameters . among addr Generally designated as NULL,prot Is used to set the permissions of the mapping area , For example, whether it is readable or writable ;flags Is used to specify whether the mapping is shared or private ; and fd,offset,length These three parameters indicate that fd Corresponding file , from offset From the position , The length will be length The content of the process is mapped to the address space of the process .

We need to pay attention to mmap The operation unit of is page , That is, the last mapped offset Parameter must be An integer multiple of the size of a memory page , and Linux The system memory page size is generally 4096 byte .

An example of my call in the program ：

#define AXI_GPIO_BASEADDR 0x43C00000 

int memfd = open("/dev/mem", O_RDWR
                 | O_SYNC); 

if (-1 == memfd) {   
    printf("Can't open /dev/mem\n");   
    return -1; 
} 

unsigned int* led_gpio =    
    (unsigned int*)(mmap(
            NULL, MMAP_SIZE, 
            PROT_READ | PROT_WRITE,
            MAP_SHARED, memfd, 
            AXI_GPIO_BASEADDR));

call mmap after , We get a pointer , Do any operation on the pointed address through this pointer , The corresponding register physical address will have the same effect . So we assign it circularly 0101, Controlled by the corresponding register GPIO The port outputs an electrical signal , So the light on the board flashes successfully , Similar to the energy lamp when Altman is out of strength .

6

Say two more words , Except to operate GPIO/ Outside the character device ,mmap Another common scenario is to operate a block device . It is different from the traditional use read,write The difference between , The key is Save a copy .

For example, to read the data of a file on the disk , use read write Words , Because it involves system calls , Processes cannot access the kernel directly , So in read Before the system call returns , The kernel needs to put the data From the kernel Copy to process designated buffer in .

But if use mmap Words , Then this data will be copied to memory as Page caching （ namely page cache). use mmap Will this period of Memory mapping To user space , Then the process can read and write directly through the pointer page cache, No more unnecessary system calls and memory copies .

But although one copy is missing , but mmap Will trigger Missing pages interruption （page fault）, Compared to memory copy , Page missing interrupt It's more expensive . So in terms of performance mmap In most cases, it is not better than read/write It is better to .

Speaking of page caching , When I was developing projects in my last company , Has also been Dirty page delay This thing has gone through . And the limit of the length , Page cache involves page break , Dirty page , Delay writing back ,sync Forced writeback, etc , Let's talk more about it next time .

7

Okay , So we learned to use mmap Light up a light . Imagine the next scene ：

You tell the most beautiful female colleague in the R & D Department of the company ,

“hi, The leader gave our group a new task , You write a drive to control it LED Well ？”

“ ah ？ Driving is so hard , I won't ”

“ Oh, it's okay , That you use mmap Well ”

“ Hey, how can you swear ！”