Guennadi Liakhovetski wrote:
Hi all,
looking through the common/cmd_mem.c::do_mem_mtest() function, I couldn't understand the following place:
addr_mask = ((ulong)end - (ulong)start)/sizeof(vu_long);... for (offset = 1; (offset & addr_mask) != 0; offset <<= 1) { start[offset] = pattern; }
why (offset & addr_mask) != 0 and not just offset < addr_mask? Suppose
end = 0xbf; start = 0;
addr_mask = 0x2f;
The loop will iterate over offset = 1, 2, 4, 8, and on 0x10 it will abort and 0x10 and 0x20 will stay untested. Whereas if we just had "offset < addr_mask" it would just function correctly, wouldn't it? Yes, I do realise, that it is at least unusual to set the end address to anything other than start address + ((1 << x) - 1), but still.
Thanks Guennadi
Hi Guennadi,
The address test is stepping through the address lines 0x01, 0x02, 0x04, 0x08, 0x10, 0x20 Your end of 0xBF with a mask of 0x2F indicates that the address lines 0x10, 0x40, and 0x80 are not present (even though address line 0x80 looks like it is part of the test since 0xBF includes 0x80 - but it isn't tested).
This is nonsensical with respect to what the address line test is testing (address lines!) and how it is testing them - by stepping through the address lines and looking for inadvertent overlapped memory accesses. Address lines are inherently powers of two, skipping certain ones of them doesn't make much sense, and ending not on a power of two (minus one) doesn't make any sense at all.
I fail to see what your change would benefit as an address line test. If there is a benefit, you will have to rewrite the address line test because having a contiguous mask and a ((2^n) - 1) end is all fundamental to how the address line test works, is fundamental to what address lines are, and is based on the symptoms that are observable when an address line fails. Simply using an arbitrary end address and a funky mask will cause the current test to fail and likely will cause some real failures to be undetected.
Best regards, gvb