Feature Removal: disable "mtest" command by default

The "mtest" command is of little practical use (if any), and
experience has shown that a large number of board configurations
define useless or even dangerous start and end addresses.  If not even
the board maintainers are able to figure out which memory range can be
reliably tested, how can we expect such from the end users?  As this
problem comes up repeatedly, we rather do not enable this command by
default, so only people who know what they are doing will be
confronted with it.

As this changes the user interface, we allow for a grace period
before this change takes effect. For now, we make "mtest"
configurable through the CONFIG_CMD_MEMTEST variable, which is defined
in include/config_cmd_default.h;  we also add an entry to
doc/feature-removal-schedule.txt which announces the removal of this
default setting in two releases from now, i. e. with v2013.07.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Tom Rini <trini@ti.com>

2 files changed, 115 insertions, 0 deletions

diff --git a/doc/README.memory-test b/doc/README.memory-test
new file mode 100644
index 0000000..eb60e8d
--- /dev/null
+++ b/doc/README.memory-test
@@ -0,0 +1,98 @@
+The most frequent cause of problems when porting U-Boot to new
+hardware, or when using a sloppy port on some board, is memory errors.
+In most cases these are not caused by failing hardware, but by
+incorrect initialization of the memory controller.  So it appears to
+be a good idea to always test if the memory is working correctly,
+before looking for any other potential causes of any problems.
+
+U-Boot implements 3 different approaches to perform memory tests:
+
+1. The get_ram_size() function (see "common/memsize.c").
+
+   This function is supposed to be used in each and every U-Boot port
+   determine the presence and actual size of each of the potential
+   memory banks on this piece of hardware.  The code is supposed to be
+   very fast, so running it for each reboot does not hurt.  It is a
+   little known and generally underrated fact that this code will also
+   catch 99% of hardware related (i. e. reliably reproducible) memory
+   errors.  It is strongly recommended to always use this function, in
+   each and every port of U-Boot.
+
+2. The "mtest" command.
+
+   This is probably the best known memory test utility in U-Boot.
+   Unfortunately, it is also the most problematic, and the most
+   useless one.
+
+   There are a number of serious problems with this command:
+
+   - It is terribly slow.  Running "mtest" on the whole system RAM
+     takes a _long_ time before there is any significance in the fact
+     that no errors have been found so far.
+
+   - It is difficult to configure, and to use.  And any errors here
+     will reliably crash or hang your system.  "mtest" is dumb and has
+     no knowledge about memory ranges that may be in use for other
+     purposes, like exception code, U-Boot code and data, stack,
+     malloc arena, video buffer, log buffer, etc.  If you let it, it
+     will happily "test" all such areas, which of course will cause
+     some problems.
+
+   - It is not easy to configure and use, and a large number of
+     systems are seriously misconfigured.  The original idea was to
+     test basically the whole system RAM, with only exempting the
+     areas used by U-Boot itself - on most systems these are the areas
+     used for the exception vectors (usually at the very lower end of
+     system memory) and for U-Boot (code, data, etc. - see above;
+     these are usually at the very upper end of system memory).  But
+     experience has shown that a very large number of ports use
+     pretty much bogus settings of CONFIG_SYS_MEMTEST_START and
+     CONFIG_SYS_MEMTEST_END; this results in useless tests (because
+     the ranges is too small and/or badly located) or in critical
+     failures (system crashes).
+
+   Because of these issues, the "mtest" command is considered depre-
+   cated.  It should not be enabled in most normal ports of U-Boot,
+   especially not in production.  If you really need a memory test,
+   then see 1. and 3. above resp. below.
+
+3. The most thorough memory test facility is available as part of the
+   POST (Power-On Self Test) sub-system, see "post/drivers/memory.c".
+
+   If you really need to perform memory tests (for example, because
+   it is mandatory part of your requirement specification), then
+   enable this test which is generic and should work on all archi-
+   tectures.
+
+WARNING:
+
+It should pointed out that _all_ these memory tests have one
+fundamental, unfixable design flaw:  they are based on the assumption
+that memory errors can be found by writing to and reading from memory.
+Unfortunately, this is only true for the relatively harmless, usually
+static errors like shorts between data or address lines, unconnected
+pins, etc.  All the really nasty errors which will first turn your
+hair gray, only to make you tear it out later, are dynamical errors,
+which usually happen not with simple read or write cycles on the bus,
+but when performing back-to-back data transfers in burst mode.  Such
+accesses usually happen only for certain DMA operations, or for heavy
+cache use (instruction fetching, cache flushing).  So far I am not
+aware of any freely available code that implements a generic, and
+efficient, memory test like that.  The best known test case to stress
+a system like that is to boot Linux with root file system mounted over
+NFS, and then build some larger software package natively (say,
+compile a Linux kernel on the system) - this will cause enough context
+switches, network traffic (and thus DMA transfers from the network
+controller), varying RAM use, etc. to trigger any weak spots in this
+area.
+
+Note: An attempt was made once to implement such a test to catch
+memory problems on a specific board.  The code is pretty much board
+specific (for example, it includes setting specific GPIO signals to
+provide triggers for an attached logic analyzer), but you can get an
+idea how it works: see "examples/standalone/test_burst*".
+
+Note 2: Ironically enough, the "test_burst" did not catch any RAM
+errors, not a single one ever.  The problems this code was supposed
+to catch did not happen when accessing the RAM, but when reading from
+NOR flash.
diff --git a/doc/feature-removal-schedule.txt b/doc/feature-removal-schedule.txt
index e04ba2d..d9a0cc2 100644
--- a/doc/feature-removal-schedule.txt
+++ b/doc/feature-removal-schedule.txt
@@ -7,6 +7,23 @@ file.
 
 ---------------------------
 
+What:	Remove CONFIG_CMD_MEMTEST from default list
+When:	Release v2013.07
+
+Why:	The "mtest" command is of little practical use (if any), and
+	experience has shown that a large number of board configu-
+	rations define useless or even dangerous start and end
+	addresses.  If not even the board maintainers are able to
+	figure out which memory range can be reliably tested, how can
+	we expect such from the end users?  As this problem comes up
+	repeatedly, we rather do not enable this command by default,
+	so only people who know what they are doing will be confronted
+	with it.
+
+Who:	Wolfgang Denk <wd@denx.de>
+
+---------------------------
+
 What:	Users of the legacy miiphy_* code
 When:	undetermined