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

[Jerry Van Baren]

Guennadi Liakhovetski wrote:
> Hi Jerry
> 
> On Thu, 7 Feb 2008, Jerry Van Baren wrote:
> 
>> 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).
> 
> I think, you are making a mistake here. Look above how addr_mask is 
> calculated:
> 
>>> 		addr_mask = ((ulong)end - (ulong)start)/sizeof(vu_long);
> 
> That means, it is just calculated based upon your memory range being 
> tested. So if for some reason you want to test a strange address range 
> like (0x10000) to (0x10000 + 0xbf) then the 0x40 address line will not be 
> tested, although it is needed to cover all addresses in this range:-) Or 
> am I still missing anything?
> 
> Thanks
> Guennadi

Ahh, yes.  In your example the 0x80 line isn't tested either, even 
though it could (and should) be.

The address line test has to be careful to not exceed the end of memory 
because that is wrong and will cause errors.  I think that the problem 
you are pointing out is that, when we reserve memory space at the end of 
memory, our "end" address isn't really the memory end address (and 
rightly so), but the *algorithmic* calculation of the "addr_mask" is not 
really correct if "end" isn't really the end of memory.

The result is an address line that *could be* (and should be) tested is 
missed.  FWIIW, in similar code I've written in the past, I've hardcoded 
(#defined) the mask to a sensible value based on my knowledge of the 
specific hardware.  The problem with hardcoding the mask is that it 
doesn't work with dynamic memory sizing.

Your original question/concept still used the name "addr_mask" which 
caused confusion on my part.  It really should be a length, not a mask. 
  Illustrating with a hand-generated patch, what I am now hearing is the 
following change:

- 		addr_mask = ((ulong)end - (ulong)start)/sizeof(vu_long);
+ 		len = ((ulong)end - (ulong)start)/sizeof(vu_long);
  ...
- 		for (offset = 1; (offset & addr_mask) != 0; offset <<= 1) {
+ 		for (offset = 1; offset < len; offset <<= 1) {
  			start[offset] = pattern;
  		}

That makes sense to me.

Best regards,
gvb