[U-Boot-Users] [Q] memtest doubt

[Jerry Van Baren <gerald.vanbaren@ge.com>]

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