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.