lwIP Module Adaptation

Lightweight IP (lwIP) is an open-source TCP/IP stack designed for embedded systems. LiteOS-M has adapted to lwIP and provides enhanced lwIP features. The lwIP code consists of the following:

lwIP source code in the third_party/lwip directory: Only a few intrusive modifications have been made for enhanced features.
Code for lwIP adaptation and enhancement in the kernel/liteos_m/components/net/lwip-2.1 directory: The default lwIP configuration file is provided.

If you want to use the lwIP module, perform the following steps to complete adaptation:

Create a directory, for example, lwip_adapter, in the device directory to store its adaptation files.
Create the include directory in the lwip_adapter directory to store the adaptation header files.

Create the lwip directory in the include directory and then create the header file lwipopts.h in the lwip directory. The code is as follows. If the default configuration cannot meet service requirements, modify the configuration, for example, disable the DHCP function.

#ifndef _LWIP_ADAPTER_LWIPOPTS_H_
#define _LWIP_ADAPTER_LWIPOPTS_H_

#include_next "lwip/lwipopts.h"

#undef LWIP_DHCP
#define LWIP_DHCP                       0 // Disable the DHCP function.

#endif /* _LWIP_ADAPTER_LWIPOPTS_H_ */

Copy BUILD.gn in the kernel/liteos_m/components/net/lwip-2.1/porting directory to the lwip_adapter directory and modify the file as follows:

import("//kernel/liteos_m/liteos.gni")
import("$LITEOSTHIRDPARTY/lwip/lwip.gni")
import("$LITEOSTOPDIR/components/net/lwip-2.1/lwip_porting.gni")

module_switch = defined(LOSCFG_NET_LWIP_SACK)
module_name = "lwip"
kernel_module(module_name) {
  sources = LWIP_PORTING_FILES + LWIPNOAPPSFILES - [ "$LWIPDIR/api/sockets.c" ]
  include_dirs = [ "//utils/native/lite/include" ]
}

# Add include, the directory of the new adaptation header file.
config("public") {
  include_dirs = [ "include" ] + LWIP_PORTING_INCLUDE_DIRS + LWIP_INCLUDE_DIRS
}

In the device configuration file (for example, config.json), set the lwIP build path to the path of BUILD.gn in step 4.

{
  "subsystem": "kernel",
  "components": [
    { "component": "liteos_m", "features":["ohos_kernel_liteos_m_lwip_path = \"//xxx/lwip_adapter\"" ] }
  ]
},

In the kernel build configuration file of the device, for example, kernel_config/debug.config, enable lwIP build.
```
LOSCFG_NET_LWIP=y
```