Prerequisites

In the article I posted last time, [Preparation] I played with cheap Linux board G-cluster (no screen output) ", from a personal computer with UART It starts from accessing the shell of G-cluster! !!

This time, we will proceed with the goal of "making it possible to display free character strings on the screen"! (This time I will not introduce Python that I planned last time. I may do it next time)

Even if you say that you want to display a character string, it's boring to just display the characters, so I'd like to make it possible to display a simple shell.

Rough procedure

Analyze various apps to find out about screen display
Make it possible to get information from a USB keyboard
Write a suitable program to be a shell

Analyze various apps to find out about screen display

When you hear "put a shell on the screen", you might think that "Ctrl + Alt + F1" will switch to tty. But that's the story on X. Since X is not running on G-cluster (I want to do it in the future), I have to make it from the screen by myself.

On the file system, there was a pipe called "remotectl_pipe" that seems to be used for screen display, so I tried stringing with this name.

Then I found something like the following

echo "firstboot $WELCOME" > /cavium/remotectl_pipe
echo 'firstboot 5' > /cavium/remotectl_pipe
echo gc_warning Firmware found > /cavium/remotectl_pipe
echo wifi_err ap > /cavium/remotectl_pipe
echo network_connect > /cavium/remotectl_pipe
echo network_disconnect > /cavium/remotectl_pipe

In this way, it seems that basically the screen that is always prepared is requested and displayed with a pipe.

Here, while hitting this command in various ways, I noticed something.

In ʻecho gc_warning Firmware found> / cavium / remotectl_pipehere, it seems that theFirmware found` part can be changed freely.

That's why you can now display any character on the screen. You can also use \ n and Japanese, so it seems to be difficult to display it.

Make it possible to get information from a USB keyboard

At the Linux kernel level, there is a function that automatically mounts when you insert a USB keyboard. When you actually stab it, / dev / event * is added, and if you touch the keyboard while it is open with cat, characters will come out.

But it's not well-formed, so I don't know what it is. I tried to display the key code on the TV. The code is Accessing Keys from Linux Input Device Almost the same site, only the display part is changed.

#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <linux/input.h>
#include <string.h>
#include <stdio.h>

static const char *const evval[3] = {
    "RELEASED",
    "PRESSED ",
    "REPEATED"
};

int main(void)
{
    const char *dev = "/dev/input/event1";
    struct input_event ev;
    ssize_t n;
    int fd;

    fd = open(dev, O_RDONLY);
    if (fd == -1) {
        fprintf(stderr, "Cannot open %s: %s.\n", dev, strerror(errno));
        return EXIT_FAILURE;
    }

    while (1) {
        n = read(fd, &ev, sizeof ev);
        if (n == (ssize_t)-1) {
            if (errno == EINTR)
                continue;
            else
                break;
        } else
        if (n != sizeof ev) {
            errno = EIO;
            break;
        }
        if (ev.type == EV_KEY && ev.value >= 0 && ev.value <= 2)
        printf("%s 0x%04x (%d)\n", evval[ev.value], (int)ev.code, (int)ev.code);
    }

    fflush(stdout);
    fprintf(stderr, "%s.\n", strerror(errno));
    return EXIT_FAILURE;
}

You can now display the key code on the screen.

Write a suitable program to be a shell

I wrote it without thinking about it, so I haven't implemented capital letters or scrolling, but I can move the shell like this.

#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <linux/input.h>
#include <string.h>
#include <stdio.h>

#define N 100000

static const char *const evval[3] = {
    "RELEASED",
    "PRESSED ",
    "REPEATED"
};

int main(void)
{
    const char *dev = "/dev/event1";
    // const char *dev = "/dev/input/event3";

    // ref: https://gist.github.com/rickyzhang82/8581a762c9f9fc6ddb8390872552c250
    const int keytable[127] = {' ', ' ', '1', '2', '3', '4', '5', '6', '7', '8', 
                               '9', '0', '-', '=', ' ', ' ', 'q', 'w', 'e', 'r', 
                               't', 'y', 'u', 'i', 'o', 'p', '[', ']', ' ', ' ', 
                               'a', 's', 'd', 'f', 'g', 'h', 'j', 'k', 'l', ';', 
                               '\'', '`', ' ', '\\', 'z', 'x', 'c', 'v', 'b', 'n', 
                               'm', ',', '.', '/', ' ', '*', ' ', ' ', ' ', ' ', 
                               ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', 
                               ' ', ' ', ' ', ' ', '-', ' ', ' ', ' ', '+', ' ', 
                               ' ', ' ', ' ', ' ', ' ', '.', ' ', ' ', ' ', ' ', 
                               ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', 
                               ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', 
                               ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', 
                               ' ', ' ', ' ', ' ', ' ', ' ', ' '
   };
  
    struct input_event ev;
    ssize_t n;
    int fd;
    char result[N] = {'\0'};
    
    char inputing[N] = {'\0'};
    char inputkey;
    int inputi = 0;
  
    FILE *file;
	  char line[N];

    fd = open(dev, O_RDONLY);
    if (fd == -1) {
        fprintf(stderr, "Cannot open %s: %s.\n", dev, strerror(errno));
        return EXIT_FAILURE;
    }

    while (1) {
        n = read(fd, &ev, sizeof ev);
        if (n == (ssize_t)-1) {
            if (errno == EINTR)
                continue;
            else
                break;
        } else
        if (n != sizeof ev) {
            errno = EIO;
            break;
        }
        if (ev.type == EV_KEY && ev.value == 1) {
          inputkey = keytable[(int)ev.code];
          if((int)ev.code == 28) {
            sprintf(result, "%s &> cmdtmp", inputing);
            system(result);
            
            strcat(inputing, "\\n");
            file = fopen("cmdtmp", "r");
	          while(fgets(line, N, file)!=NULL){
              line[strcspn(line, "\r\n")] = 0;
		          strcat(inputing, line);
              strcat(inputing, "\\n");
	          }          
	          fclose(file);
                                   
            sprintf(result, "echo \"gc_warning > %s\" > /cavium/remotectl_pipe", inputing);
            printf("%s\n", result);
            system(result);
            
            inputing[0] = '\0';
            inputi = 0;
          } else {
            inputing[inputi] = inputkey;
            inputing[inputi+1] = '\0';
            inputi++;
            sprintf(result, "echo \"gc_warning > %s\" > /cavium/remotectl_pipe", inputing);
            printf("%s\n", result);
            system(result);
          }
          //printf("%s 0x%04x (%d)\n", evval[ev.value], (int)ev.code, (int)ev.code);
          //sprintf(result, "echo \"gc_warning %s 0x%04x (%d)\" > /cavium/remotectl_pipe", evval[ev.value], (int)ev.code, (int)ev.code);
          //sprintf(result, "echo \"gc_warning %s 0x%04x (%d)\" > remotectl_pipe", evval[ev.value], (int)ev.code, (int)ev.code);
          //system(result);
          //printf("%s\n", result);
        }
        
    }

    fflush(stdout);
    fprintf(stderr, "%s.\n", strerror(errno));
    return EXIT_FAILURE;
}

Now you can move the shell on your TV like the gif at the beginning.

from now on

This time, I'm honestly not very satisfied because I just wrote that I can run the shell and use the TV screen for the time being.

The next goal is to use the USB firmware update function of G-cluster to burn the image and run any program without hardware modification. It will take more time, so please be patient. ..

After that, since I have purchased a large amount of g-cluster, I plan to play various things such as arranging them as Wi-Fi modules at regular intervals and using them for position detection.

I will post it when progress is made! Thank you very much.

[Shell startup] I tried to display the shell on the TV with a cheap Linux board G-cluster

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