edac: Improve the comments to better describe the memory concepts

The Computer memory terminology has changed with time since EDAC was
originally written: new concepts were introduced, and some things have
different meanings, depending on the memory architecture.

Improve the definition of all related terms.

Also, describe each memory type in a more detailed fashion.

No functional changes. Just comments were touched.

Acked-by: Borislav Petkov <borislav.petkov@amd.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

1 files changed, 111 insertions, 46 deletions

diff --git a/include/linux/edac.h b/include/linux/edac.h
index 1cd3947987e5..0714d67a6e1a 100644
--- a/include/linux/edac.h
+++ b/include/linux/edac.h
@@ -66,25 +66,64 @@ enum dev_type {
 #define DEV_FLAG_X32            BIT(DEV_X32)
 #define DEV_FLAG_X64            BIT(DEV_X64)
 
-/* memory types */
+/**
+ * enum mem_type - memory types. For a more detailed reference, please see
+ *                      http://en.wikipedia.org/wiki/DRAM
+ *
+ * @MEM_EMPTY           Empty csrow
+ * @MEM_RESERVED:       Reserved csrow type
+ * @MEM_UNKNOWN:        Unknown csrow type
+ * @MEM_FPM:            FPM - Fast Page Mode, used on systems up to 1995.
+ * @MEM_EDO:            EDO - Extended data out, used on systems up to 1998.
+ * @MEM_BEDO:           BEDO - Burst Extended data out, an EDO variant.
+ * @MEM_SDR:            SDR - Single data rate SDRAM
+ *                      http://en.wikipedia.org/wiki/Synchronous_dynamic_random-access_memory
+ *                      They use 3 pins for chip select: Pins 0 and 2 are
+ *                      for rank 0; pins 1 and 3 are for rank 1, if the memory
+ *                      is dual-rank.
+ * @MEM_RDR:            Registered SDR SDRAM
+ * @MEM_DDR:            Double data rate SDRAM
+ *                      http://en.wikipedia.org/wiki/DDR_SDRAM
+ * @MEM_RDDR:           Registered Double data rate SDRAM
+ *                      This is a variant of the DDR memories.
+ *                      A registered memory has a buffer inside it, hiding
+ *                      part of the memory details to the memory controller.
+ * @MEM_RMBS:           Rambus DRAM, used on a few Pentium III/IV controllers.
+ * @MEM_DDR2:           DDR2 RAM, as described at JEDEC JESD79-2F.
+ *                      Those memories are labed as "PC2-" instead of "PC" to
+ *                      differenciate from DDR.
+ * @MEM_FB_DDR2:        Fully-Buffered DDR2, as described at JEDEC Std No. 205
+ *                      and JESD206.
+ *                      Those memories are accessed per DIMM slot, and not by
+ *                      a chip select signal.
+ * @MEM_RDDR2:          Registered DDR2 RAM
+ *                      This is a variant of the DDR2 memories.
+ * @MEM_XDR:            Rambus XDR
+ *                      It is an evolution of the original RAMBUS memories,
+ *                      created to compete with DDR2. Weren't used on any
+ *                      x86 arch, but cell_edac PPC memory controller uses it.
+ * @MEM_DDR3:           DDR3 RAM
+ * @MEM_RDDR3:          Registered DDR3 RAM
+ *                      This is a variant of the DDR3 memories.
+ */
 enum mem_type {
-        MEM_EMPTY = 0,          /* Empty csrow */
-        MEM_RESERVED,           /* Reserved csrow type */
-        MEM_UNKNOWN,            /* Unknown csrow type */
-        MEM_FPM,                /* Fast page mode */
-        MEM_EDO,                /* Extended data out */
-        MEM_BEDO,               /* Burst Extended data out */
-        MEM_SDR,                /* Single data rate SDRAM */
-        MEM_RDR,                /* Registered single data rate SDRAM */
-        MEM_DDR,                /* Double data rate SDRAM */
-        MEM_RDDR,               /* Registered Double data rate SDRAM */
-        MEM_RMBS,               /* Rambus DRAM */
-        MEM_DDR2,               /* DDR2 RAM */
-        MEM_FB_DDR2,            /* fully buffered DDR2 */
-        MEM_RDDR2,              /* Registered DDR2 RAM */
-        MEM_XDR,                /* Rambus XDR */
-        MEM_DDR3,               /* DDR3 RAM */
-        MEM_RDDR3,              /* Registered DDR3 RAM */
+        MEM_EMPTY = 0,
+        MEM_RESERVED,
+        MEM_UNKNOWN,
+        MEM_FPM,
+        MEM_EDO,
+        MEM_BEDO,
+        MEM_SDR,
+        MEM_RDR,
+        MEM_DDR,
+        MEM_RDDR,
+        MEM_RMBS,
+        MEM_DDR2,
+        MEM_FB_DDR2,
+        MEM_RDDR2,
+        MEM_XDR,
+        MEM_DDR3,
+        MEM_RDDR3,
 };
 
 #define MEM_FLAG_EMPTY          BIT(MEM_EMPTY)
@@ -162,8 +201,9 @@ enum scrub_type {
 #define OP_OFFLINE              0x300
 
 /*
- * There are several things to be aware of that aren't at all obvious:
+ * Concepts used at the EDAC subsystem
  *
+ * There are several things to be aware of that aren't at all obvious:
  *
  * SOCKETS, SOCKET SETS, BANKS, ROWS, CHIP-SELECT ROWS, CHANNELS, etc..
  *
@@ -172,36 +212,61 @@ enum scrub_type {
  * creating a common ground for discussion, terms and their definitions
  * will be established.
  *
- * Memory devices:      The individual chip on a memory stick.  These devices
- *                      commonly output 4 and 8 bits each.  Grouping several
- *                      of these in parallel provides 64 bits which is common
- *                      for a memory stick.
+ * Memory devices:      The individual DRAM chips on a memory stick.  These
+ *                      devices commonly output 4 and 8 bits each (x4, x8).
+ *                      Grouping several of these in parallel provides the
+ *                      number of bits that the memory controller expects:
+ *                      typically 72 bits, in order to provide 64 bits +
+ *                      8 bits of ECC data.
  *
  * Memory Stick:        A printed circuit board that aggregates multiple
- *                      memory devices in parallel.  This is the atomic
- *                      memory component that is purchaseable by Joe consumer
- *                      and loaded into a memory socket.
+ *                      memory devices in parallel.  In general, this is the
+ *                      Field Replaceable Unit (FRU) which gets replaced, in
+ *                      the case of excessive errors. Most often it is also
+ *                      called DIMM (Dual Inline Memory Module).
  *
- * Socket:              A physical connector on the motherboard that accepts
- *                      a single memory stick.
+ * Memory Socket:       A physical connector on the motherboard that accepts
+ *                      a single memory stick. Also called as "slot" on several
+ *                      datasheets.
  *
- * Channel:             Set of memory devices on a memory stick that must be
- *                      grouped in parallel with one or more additional
- *                      channels from other memory sticks.  This parallel
- *                      grouping of the output from multiple channels are
- *                      necessary for the smallest granularity of memory access.
- *                      Some memory controllers are capable of single channel -
- *                      which means that memory sticks can be loaded
- *                      individually.  Other memory controllers are only
- *                      capable of dual channel - which means that memory
- *                      sticks must be loaded as pairs (see "socket set").
+ * Channel:             A memory controller channel, responsible to communicate
+ *                      with a group of DIMMs. Each channel has its own
+ *                      independent control (command) and data bus, and can
+ *                      be used independently or grouped with other channels.
  *
- * Chip-select row:     All of the memory devices that are selected together.
- *                      for a single, minimum grain of memory access.
- *                      This selects all of the parallel memory devices across
- *                      all of the parallel channels.  Common chip-select rows
- *                      for single channel are 64 bits, for dual channel 128
- *                      bits.
+ * Branch:              It is typically the highest hierarchy on a
+ *                      Fully-Buffered DIMM memory controller.
+ *                      Typically, it contains two channels.
+ *                      Two channels at the same branch can be used in single
+ *                      mode or in lockstep mode.
+ *                      When lockstep is enabled, the cacheline is doubled,
+ *                      but it generally brings some performance penalty.
+ *                      Also, it is generally not possible to point to just one
+ *                      memory stick when an error occurs, as the error
+ *                      correction code is calculated using two DIMMs instead
+ *                      of one. Due to that, it is capable of correcting more
+ *                      errors than on single mode.
+ *
+ * Single-channel:      The data accessed by the memory controller is contained
+ *                      into one dimm only. E. g. if the data is 64 bits-wide,
+ *                      the data flows to the CPU using one 64 bits parallel
+ *                      access.
+ *                      Typically used with SDR, DDR, DDR2 and DDR3 memories.
+ *                      FB-DIMM and RAMBUS use a different concept for channel,
+ *                      so this concept doesn't apply there.
+ *
+ * Double-channel:      The data size accessed by the memory controller is
+ *                      interlaced into two dimms, accessed at the same time.
+ *                      E. g. if the DIMM is 64 bits-wide (72 bits with ECC),
+ *                      the data flows to the CPU using a 128 bits parallel
+ *                      access.
+ *
+ * Chip-select row:     This is the name of the DRAM signal used to select the
+ *                      DRAM ranks to be accessed. Common chip-select rows for
+ *                      single channel are 64 bits, for dual channel 128 bits.
+ *                      It may not be visible by the memory controller, as some
+ *                      DIMM types have a memory buffer that can hide direct
+ *                      access to it from the Memory Controller.
  *
  * Single-Ranked stick: A Single-ranked stick has 1 chip-select row of memory.
  *                      Motherboards commonly drive two chip-select pins to
@@ -214,8 +279,8 @@ enum scrub_type {
  *
  * Double-sided stick:  DEPRECATED TERM, see Double-Ranked stick.
  *                      A double-sided stick has two chip-select rows which
- *                      access different sets of memory devices.  The two
- *                      rows cannot be accessed concurrently.  "Double-sided"
+ *                      access different sets of memory devices. The two
+ *                      rows cannot be accessed concurrently. "Double-sided"
  *                      is irrespective of the memory devices being mounted
  *                      on both sides of the memory stick.
  *