[PATCHv4 3/5] net/lwip: add doc/develop/net_lwip.rst

[Maxim Uvarov]

Just add inital doc.

Signed-off-by: Maxim Uvarov <maxim.uvarov at linaro.org>
---
 doc/develop/index.rst    |  1 +
 doc/develop/net_lwip.rst | 59 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 60 insertions(+)
 create mode 100644 doc/develop/net_lwip.rst

diff --git a/doc/develop/index.rst b/doc/develop/index.rst
index 97c526e997..a092c33df0 100644
--- a/doc/develop/index.rst
+++ b/doc/develop/index.rst
@@ -43,6 +43,7 @@ Implementation
    smbios
    spl
    uefi/index
+   net_lwip
    vbe
    version
 
diff --git a/doc/develop/net_lwip.rst b/doc/develop/net_lwip.rst
new file mode 100644
index 0000000000..567234fff2
--- /dev/null
+++ b/doc/develop/net_lwip.rst
@@ -0,0 +1,59 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+LWIP IP stack intergation for U-Boot
+====================================
+
+Intro
+-----
+
+LWIP is a library for implementation network protocols, which is commonly used
+on embedded devices.
+
+https://savannah.nongnu.org/projects/lwip/
+
+LwIP  license:
+LwIP is licensed under a BSD-style license: http://lwip.wikia.com/wiki/License.
+
+Main features include:
+
+* Protocols: IP, IPv6, ICMP, ND, MLD, UDP, TCP, IGMP, ARP, PPPoS, PPPoE
+
+* DHCP client, DNS client (incl. mDNS hostname resolver), AutoIP/APIPA (Zeroconf), SNMP agent (v1, v2c, v3, private MIB support & MIB compiler)
+
+* APIs: specialized APIs for enhanced performance, optional Berkeley-alike socket API
+
+* Extended features: IP forwarding over multiple network interfaces, TCP congestion control, RTT estimation and fast recovery/fast retransmit
+
+* Addon applications: HTTP(S) server, SNTP client, SMTP(S) client, ping, NetBIOS nameserver, mDNS responder, MQTT client, TFTP server
+
+U-Boot implementation details
+-----------------------------
+
+1. In general we can build lwIP as .a library and link it against u-boot or compile it in
+the U-Boot tree in the same way as other U-Boot files. There are few reasons why I selected
+the second variant: LwIP is very customizable with defines for features, memory size, types of
+allocation, some internal types and platform specific code. And it was more easy to enable/disable
+debug which is also done with defines, and is needed periodically.
+
+2. lwIP has 2 APIs - raw mode and sequential (as lwIP names it, or socket API as we name it in Linux).
+For now only raw API is supported.
+
+Raw IP means that the call back function for RX path is registered and will be called when packet
+data passes the IP stack and is ready for the application.
+
+Example is unmodified working ping example from lwip sources which registeres the callback:
+
+.. code-block:: c
+
+        ping_pcb = raw_new(IP_PROTO_ICMP);
+        raw_recv(ping_pcb, ping_recv, NULL); <- ping_recv is app callback.
+        raw_bind(ping_pcb, IP_ADDR_ANY)
+
+Socket API also gives nice advantages due it will be easy to port linux socket applications to u-boot.
+I.e. LwIP sockets compatible with the linux ones. But that will require RX thread running in the background.
+So that means we need some kind of scheduler, locking and threading support or find some other solution.
+
+3.  Input and output
+
+RX packet path is injected to U-Boot eth_rx() polling loop and TX patch is in eth_send() accordingly.
+So we do not touch any drivers code and just eat packets when they are ready.
-- 
2.30.2