1 The most frequent cause of problems when porting U-Boot to new 2 hardware, or when using a sloppy port on some board, is memory errors. 3 In most cases these are not caused by failing hardware, but by 4 incorrect initialization of the memory controller. So it appears to 5 be a good idea to always test if the memory is working correctly, 6 before looking for any other potential causes of any problems. 7 8 U-Boot implements 3 different approaches to perform memory tests: 9 10 1. The get_ram_size() function (see "common/memsize.c"). 11 12 This function is supposed to be used in each and every U-Boot port 13 determine the presence and actual size of each of the potential 14 memory banks on this piece of hardware. The code is supposed to be 15 very fast, so running it for each reboot does not hurt. It is a 16 little known and generally underrated fact that this code will also 17 catch 99% of hardware related (i. e. reliably reproducible) memory 18 errors. It is strongly recommended to always use this function, in 19 each and every port of U-Boot. 20 21 2. The "mtest" command. 22 23 This is probably the best known memory test utility in U-Boot. 24 Unfortunately, it is also the most problematic, and the most 25 useless one. 26 27 There are a number of serious problems with this command: 28 29 - It is terribly slow. Running "mtest" on the whole system RAM 30 takes a _long_ time before there is any significance in the fact 31 that no errors have been found so far. 32 33 - It is difficult to configure, and to use. And any errors here 34 will reliably crash or hang your system. "mtest" is dumb and has 35 no knowledge about memory ranges that may be in use for other 36 purposes, like exception code, U-Boot code and data, stack, 37 malloc arena, video buffer, log buffer, etc. If you let it, it 38 will happily "test" all such areas, which of course will cause 39 some problems. 40 41 - It is not easy to configure and use, and a large number of 42 systems are seriously misconfigured. The original idea was to 43 test basically the whole system RAM, with only exempting the 44 areas used by U-Boot itself - on most systems these are the areas 45 used for the exception vectors (usually at the very lower end of 46 system memory) and for U-Boot (code, data, etc. - see above; 47 these are usually at the very upper end of system memory). But 48 experience has shown that a very large number of ports use 49 pretty much bogus settings of CONFIG_SYS_MEMTEST_START and 50 CONFIG_SYS_MEMTEST_END; this results in useless tests (because 51 the ranges is too small and/or badly located) or in critical 52 failures (system crashes). 53 54 Because of these issues, the "mtest" command is considered depre- 55 cated. It should not be enabled in most normal ports of U-Boot, 56 especially not in production. If you really need a memory test, 57 then see 1. and 3. above resp. below. 58 59 3. The most thorough memory test facility is available as part of the 60 POST (Power-On Self Test) sub-system, see "post/drivers/memory.c". 61 62 If you really need to perform memory tests (for example, because 63 it is mandatory part of your requirement specification), then 64 enable this test which is generic and should work on all archi- 65 tectures. 66 67 WARNING: 68 69 It should pointed out that _all_ these memory tests have one 70 fundamental, unfixable design flaw: they are based on the assumption 71 that memory errors can be found by writing to and reading from memory. 72 Unfortunately, this is only true for the relatively harmless, usually 73 static errors like shorts between data or address lines, unconnected 74 pins, etc. All the really nasty errors which will first turn your 75 hair gray, only to make you tear it out later, are dynamical errors, 76 which usually happen not with simple read or write cycles on the bus, 77 but when performing back-to-back data transfers in burst mode. Such 78 accesses usually happen only for certain DMA operations, or for heavy 79 cache use (instruction fetching, cache flushing). So far I am not 80 aware of any freely available code that implements a generic, and 81 efficient, memory test like that. The best known test case to stress 82 a system like that is to boot Linux with root file system mounted over 83 NFS, and then build some larger software package natively (say, 84 compile a Linux kernel on the system) - this will cause enough context 85 switches, network traffic (and thus DMA transfers from the network 86 controller), varying RAM use, etc. to trigger any weak spots in this 87 area. 88 89 Note: An attempt was made once to implement such a test to catch 90 memory problems on a specific board. The code is pretty much board 91 specific (for example, it includes setting specific GPIO signals to 92 provide triggers for an attached logic analyzer), but you can get an 93 idea how it works: see "examples/standalone/test_burst*". 94 95 Note 2: Ironically enough, the "test_burst" did not catch any RAM 96 errors, not a single one ever. The problems this code was supposed 97 to catch did not happen when accessing the RAM, but when reading from 98 NOR flash. 99
