Android cannot automatically create a USB printer node/dev/usb/lp0 "Go"

This article was reproduced from: http://blog.csdn.net/u013686019/article/details/50165059

[HTML]View PlainCopy

1. android:4.4.4

First, the problem analysis

When the USB printer is plugged into an Android device, there is no printer node (/DEV/USB/LP0) generated in the System/dev directory.

The first reaction is to check the printer device and access the Ubuntu-equipped PC, normal: /dev/usb/lp0

The second reaction is to view the printer driver information:

[HTML]View PlainCopy

2. [178931.515572] USB 1-1: New Full-speed USB device number, using XHCI_HCD
4. [178931.644906] USB 1-1: New USB device found, idvendor=0483, idproduct=5720
6. [178931.644909] USB 1-1: New USB Device strings: mfr=1, product=2, serialnumber=3
8. [178931.644911] USB 1-1: Product:sprt Printer
10. [178931.644912] USB 1-1: Manufacturer:spirit
12. [178931.644913] USB 1-1: serialnumber:11101800002
14. [178931.645918] usblp 1-1:1.0:usblp0:usb bidirectional printer dev 0 alt 0 Proto 2 vid 0x0483 pid 0x5720

also normal. and PC display the information is the key part of the same.

The problem is that the system cannot create the node . Then create it manually!

Second, the problem of a preliminary solution

Under embedded Linux systems, device nodes can be created through Mdev. Mdev is to obtain the device information by scanning the system/sys/class/directory, and then create the node under/dev/ . Pass:

[HTML]View PlainCopy

1. ls/sys/class/

You can see that there is information about the printer device in the system:

[HTML]View PlainCopy

1. /sys/class/usbmisc

Then you can perform the following:

[HTML]View PlainCopy

1. BusyBox mdev-s

See/dev/again, surprise discovery:/dev/usb/lp0, there!

Third, the problem is fundamentally solved

After the above detection, we know:

1, Drive Normal

2, the system has printer equipment information

So the question is: The system cannot automatically create a printer node for us/dev/usb/lp0

Under Linux, hot plug-related can be added as follows in the/etc/init.d/rcs script:

[HTML]View PlainCopy

1. Echo/system/bin/mdev >/proc/sys/kernel/hotplug
2. /system/bin/mdev-s

However, in Android, it is associated with the device node creation in the file:

[CPP]View PlainCopy

1. SYSTEM/CORE/INIT/DEVICES.C  
2. static void handle_generic_device_event ( struct uevent *uevent)   
3. {   
4.     if  (!strncmp (Uevent->subsystem,  
5.          if  (!strcmp (Uevent->subsystem,  "USB")  {  
6.              ....  
7.          }  
8.     }  
9. }  

By adding the following print information to the devices.c file:

We found that the USB printer information is as follows:

[HTML]View PlainCopy

1. uevent->subsystem = "Usbmisc"
2. uevent->device_name = "Usb/lp0"

The code simply return after the "Usbmisc" processing logic is not found.

At this point, the solution comes out:

Add the code that handles Uevent->subsystem = "Usbmisc":

Compile, burn write, restart, insert, ok!

Iv. Testing

Node/dev/usb/lp0 is there, can not use, but also to test ah, we are using the printer, not watching the node fun.

output characters to/dev/usb/lp0 via the echo command to print:

[HTML]View PlainCopy

1. echo "111" >/dev/usb/lp0

Android cannot automatically create a USB printer node/dev/usb/lp0 "Go"