Add README

Add a file that describes how to configure the modem to use its IPA
connection to the AP in loopback mode.

Signed-off-by: Alex Elder <elder@linaro.org>

1 files changed, 110 insertions, 0 deletions

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+Thu Jun 30 05:43:09 PM CDT 2022
+
+To put the modem in loopback mode, you must configure it using "NV"
+(non-volatile) data on the target.
+
+With "newer" systems (not sure how that's defined), loopback is
+configured by placing a "text file" configuration into a spot in the
+modem's "EFS" file system.  That's the focus of this document.
+
+I have yet to discover how to install this configuration without the
+use of a Qualcomm Windows tool.  More on that below; first a summary.
+
+The location in the file system is:
+	/nv/item_files/modem/data/3gpp/ps/loopback_config.txt
+
+Initially, this much exists:
+	/nv/item_files/modem/
+And that directory contains other information, but no "data"
+subdirectory.  So the remaining directory path must be created:
+	data/3gpp/ps/
+And the configuration file must be placed into that.
+	loopback_config.txt
+
+The configuration file I have looks like this (ignore leading tabs):
+	LOOPBACK_ENABLED:1;
+	L2_LOOPBACK_ENABLED:0;
+	SYS MODE:9;
+	REP_FACTOR:1;
+	NR5G_ONLY:1;
+(Note: each line is terminated with '\r\n, except the last which is
+not terminated.  This might not matteri.)
+
+So you use a Qualcomm tool to create the directory to contain the
+loopback config, then copy the config file into that directory.
+After that, you must sync and reboot the target system.
+
+-----
+
+The Windows system must have direct access to the target machine.
+I do this using Ethernet, using a crossover cable.  (It's possible a
+"normal" Ethernet cable would work, but that depends on hardware.)
+
+-----------            ----------
+|         |            |        |
+| Windows +============+ Target |
+|         |     ^      |        |
+-----------     |      ----------
+                |
+     Ethernet crossover cable
+
+The Windows Ethernet adapter will automatically get a link-local IP
+address assigned, if it doesn't have a fixed IP set up and doesn't
+find a DHCP server.  If you run "ipconfig" in a command window
+you'll see "Ethernet adapter Ethernet".  The top two bytes are
+169.254, and the lower two bytes will based on your hardware.  Mine
+shows up as 169.254.156.89.
+
+Because there is nothing else on the network, you can safely pick
+whatever link-local IP address you want for the target.  So I use
+169.254.1.1.  Run this on the target:
+    ifconfig eth0 169.254.1.1
+
+At this point you should be able to verify this crossover connection
+works:
+    ping 169.254.1.1		# On Windows
+    ping 169.254.156.89		# On the target
+
+-----
+
+The Qualcomm tool I use to write the config file is "EFS Explorer",
+which is part of the "QPST" package.  I assume you'll be able to get
+this installed on a Windows system.
+
+You need to be running "diag-router" on the target, and tell it
+where to find the server.  Run this on the target:
+
+    diag-router -s 169.254.156.89 &     <- Windows link-local IP
+
+Then open "EFS Explorer on Windows.  It should show a "Phone
+Selection" dialog, and if all goes well, you should see an entry
+below the "Phone ESN (etc.)" headers.  Select that.
+
+At that point a new window opens, saying "Please Wait..." and
+showing that it's retrieving the embedded file system contents.
+This takes a little while.
+
+Once that's done, the EFS Explorer window shows two panels, with the
+left showing the file path hierarchy, and the right showing what's
+in the currently-selected path element.
+
+Traverse the left panel, selecting "nv", then "item-files", then
+"modem".  With that selected, select "File->New->Directory..." and
+add to the path shown, so the box shows:
+	/nv/item_files/modem/data/3gpp/ps
+Click OK.  Those directiories are created, and "ps" should be
+selected as the current path element.
+
+Now copy in the file (it might be possible to enter it manually, but
+I've always just had a copy in the Windows file system and browsed
+to it).  Select "File->New->File..." and click on the "Browse..."
+button in the top text box.  (Make sure the second text box shows
+the full path to the "ps" directory.)  Select "loopback_config.txt"
+and click the "Open" button.  The file should get created.
+
+Finally, you can quit "EFS Explorer" and then sync and reboot the
+target system.
+
+Once this NV configuration is in place, you should be able to use
+QMI requests and "ip" commands to bring up the modem interface in
+loopback mode.