doc: samsung: Fix PXE doc for E850-96 board

As stated in PXELINUX doc [1], the PXE configuration file has to be in
the format of "01-MAC-address" for Ethernet connections:

    The hardware type (using its ARP "htype" code) and address, all in
    lowercase hexadecimal with dash separators. For example, for an
    Ethernet (i.e. ARP hardware type "1") with address
    "88:99:AA:BB:CC:DD", it would search for the filename
    "01-88-99-aa-bb-cc-dd".

Indeed, PXE implementation in U-Boot looks for files like that, as can
be seen from this call chain:

    format_mac_pxe()
    pxe_mac_path()
    pxe_get()
    extlinux_pxe_read_bootflow()

Mention the fact that PXE expects the configuration file to be prepended
with "01" in the PXE section of E850-96 documentation. While at it, fix
some other minor issues in PXE section.

[1] https://wiki.syslinux.org/wiki/index.php?title=PXELINUX

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>

diff --git a/doc/board/samsung/e850-96.rst b/doc/board/samsung/e850-96.rst
index 2b66e6568fb3b98ce47a16ae812a2f83bbaa5711..3174a7daff962608c55eb29e55809a5fcf47a449 100644 (file)
--- a/doc/board/samsung/e850-96.rst
+++ b/doc/board/samsung/e850-96.rst
@@ -637,9 +637,14 @@ machine:
     `-- pxelinux.cfg
         `-- 01-02-36-f5-1c-81-13
 
-  where ``'01-02-36-f5-1c-81-13'`` file contains extlinux configuration for
-  TFTP to load and boot. The name of this file is the Ethernet MAC address,
-  which can be looked up in the ``$ethaddr`` environment variable in U-Boot.
+  where ``'01-02-36-f5-1c-81-13'`` file contains extlinux configuration for TFTP
+  to load and boot. The name of this file has to be in the format of
+  ``'01-MAC-address'``. The Ethernet MAC address in this case is
+  ``02-36-f5-1c-81-13``, and it can be looked up in the ``$ethaddr`` environment
+  variable in U-Boot::
+
+    => env print ethaddr
+    ethaddr=02:36:f5:1c:81:13
 
   An example of such configuration file is::
 
@@ -667,11 +672,11 @@ machine:
     arch/arm64/configs/defconfig
     ----------------------------
 
-    # Needed for boot from USB storage (mounting rootfs from USB drive)
+    # Keep USB PHY driver built-in, as the Ethernet controller sits on USB bus
     CONFIG_PHY_EXYNOS5_USBDRD=y
     CONFIG_TYPEC=y
 
-    # Needed for NFS boot
+    # Keep Ethernet driver built-in
     CONFIG_USB_NET_SMSC95XX=y
     CONFIG_USB_USBNET=y