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63 lines
3.3 KiB
Markdown
63 lines
3.3 KiB
Markdown
Writing a "bare metal" operating system for Raspberry Pi 4 (Part 15)
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====================================================================
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Adding a TCP/IP stack
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---------------------
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Having achieved "proof of life" from our Ethernet module in _part14-spi-ethernet_, you're doubtless wondering how to go from there to serving web pages, posting tweets on Twitter or perhaps even just simply responding to a ping!
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This is where you'll need a fully-fledged TCP/IP stack that goes way beyond handcrafted ARPs, implementing many more protocols to achieve efficient bi-directional communication.
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In this part we make use of some code from Guido Socher of [tuxgraphics.org](http://tuxgraphics.org/), designed to be a lightweight TCP/IP stack for embedded devices. I chose this because it was super simple to get working (or "port"), but you might want to look at [LwIP](https://en.wikipedia.org/wiki/LwIP) if you need something more advanced.
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The code
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--------
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Most of the new code is in the _tcpip/_ subdirectory. I actually came across it in [this Github repository](https://github.com/ussserrr/maglev-ti-rtos) and, again, made only a very few cosmetic changes (`diff` is your friend!).
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It did require me to expose the `strlen()` function we implemented in _lib/fb.c_, so that's added to _include/fb.h_. Similarly, we expose the `memcpy()` function we implemented in _kernel/kernel.c_, so that's added to _kernel/kernel.h_.
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I also needed a single function that tells the ENC to send a packet. Nothing new here, just different packaging:
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```c
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void enc28j60PacketSend(unsigned short buflen, void *buffer) {
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if (ENC_RestoreTXBuffer(&handle, buflen) == 0) {
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ENC_WriteBuffer((unsigned char *) buffer, buflen);
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handle.transmitLength = buflen;
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ENC_Transmit(&handle);
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}
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}
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```
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This was also added to _kernel/kernel.h_. Finally, _kernel/kernel.c_ now calls a function called `net_test()` instead of our original `arp_test()`.
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The changes to _arp.c_
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----------------------
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We initialise the network card in exactly the same way but, when we're done, we call this function in Guido's code:
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```c
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init_udp_or_www_server(myMAC, deviceIP);
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```
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This tells the TCP/IP library who we are, so we're all on the same page!
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Finally, and aside from a little cleanup, the major change is the new `net_test()` function:
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```c
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void net_test(void)
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{
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while (1) {
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while (!ENC_GetReceivedFrame(&handle));
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uint16_t len = handle.RxFrameInfos.length;
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uint8_t *buffer = (uint8_t *)handle.RxFrameInfos.buffer;
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packetloop_arp_icmp_tcp(buffer, len);
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}
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}
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```
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This is an infinite loop which waits for an incoming packet and then simply passes it to Guido's `packetloop_arp_icmp_tcp()` function. This function implements some useful things, like responding to pings. I modified the routine to print a message to the screen when it sends a "pong" (look from line 1381 of _tcpip/ip_arp_udp_tcp.c_), so we can see when it's in action!
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_Imagine my excitement when I built, ran and could ping my RPi4 at 192.168.0.66 and get a response to both my laptop and my iPhone!_
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I recommend reading [this page](http://tuxgraphics.org/electronics/200905/embedded-tcp-ip-stack.shtml) to give you some ideas about what else you might achieve with Guido's library...
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