powerpc/fadump: Improve fadump documentation

The figures depicting FADump's (Firmware-Assisted Dump) memory layout
are missing some finer details like different memory regions and what
they represent. Improve the documentation by updating those details.

Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/156821322070.5656.8194734198500730487.stgit@hbathini.in.ibm.com

Documentation/powerpc/firmware-assisted-dump.rst

@@ -76,8 +76,9 @@ as follows:
    there is crash data available from a previous boot. During
    the early boot OS will reserve rest of the memory above
    boot memory size effectively booting with restricted memory
-   size. This will make sure that the second kernel will not
-   touch any of the dump memory area.
+   size. This will make sure that this kernel (also, referred
+   to as second kernel or capture kernel) will not touch any
+   of the dump memory area.
 
 -  User-space tools will read /proc/vmcore to obtain the contents
    of memory, which holds the previous crashed kernel dump in ELF
@@ -128,42 +129,46 @@ space memory except the user pages that were present in CMA region::
 
   o Memory Reservation during first kernel
 
-  Low memory                                         Top of memory
-  0      boot memory size                                       |
-  |           |                |<--Reserved dump area -->|      |
-  V           V                |   Permanent Reservation |      V
-  +-----------+----------/ /---+---+----+-----------+----+------+
-  |           |                |CPU|HPTE|  DUMP     |ELF |      |
-  +-----------+----------/ /---+---+----+-----------+----+------+
-        |                                           ^
-        |                                           |
-        \                                           /
-         -------------------------------------------
-          Boot memory content gets transferred to
-          reserved area by firmware at the time of
-          crash
+  Low memory                                                Top of memory
+  0      boot memory size      |<--Reserved dump area --->|      |
+  |           |                | (Permanent Reservation)  |      |
+  V           V                |                          |      V
+  +-----------+----------/ /---+---+----+--------+---+----+------+
+  |           |                |CPU|HPTE|  DUMP  |HDR|ELF |      |
+  +-----------+----------/ /---+---+----+--------+---+----+------+
+        |                                   ^      ^
+        |                                   |      |
+        \                                   /      |
+         -----------------------------------     FADump Header
+          Boot memory content gets transferred   (meta area)
+          to reserved area by firmware at the
+          time of crash
+
                    Fig. 1
 
+
   o Memory Reservation during second kernel after crash
 
-  Low memory                                        Top of memory
-  0      boot memory size                                       |
-  |           |<------------- Reserved dump area ----------- -->|
-  V           V                                                 V
-  +-----------+----------/ /---+---+----+-----------+----+------+
-  |           |                |CPU|HPTE|  DUMP     |ELF |      |
-  +-----------+----------/ /---+---+----+-----------+----+------+
+  Low memory                                                Top of memory
+  0      boot memory size                                        |
+  |           |<----------- Crash preserved area --------------->|
+  V           V                |<-- Reserved dump area -->|      V
+  +-----------+----------/ /---+---+----+--------+---+----+------+
+  |           |                |CPU|HPTE|  DUMP  |HDR|ELF |      |
+  +-----------+----------/ /---+---+----+--------+---+----+------+
         |                                              |
         V                                              V
    Used by second                                /proc/vmcore
    kernel to boot
                    Fig. 2
 
-Currently the dump will be copied from /proc/vmcore to a
-a new file upon user intervention. The dump data available through
-/proc/vmcore will be in ELF format. Hence the existing kdump
-infrastructure (kdump scripts) to save the dump works fine with
-minor modifications.
+Currently the dump will be copied from /proc/vmcore to a new file upon
+user intervention. The dump data available through /proc/vmcore will be
+in ELF format. Hence the existing kdump infrastructure (kdump scripts)
+to save the dump works fine with minor modifications. KDump scripts on
+major Distro releases have already been modified to work seemlessly (no
+user intervention in saving the dump) when FADump is used, instead of
+KDump, as dump mechanism.
 
 The tools to examine the dump will be same as the ones
 used for kdump.