Re: [Openocd-development] MIPS target, big endian host

[Drasko D. <dra...@gm...>]

On Fri, Jul 8, 2011 at 6:30 PM, Mahr, Stefan <Ste...@sp...> wrote:
>>>> How do they convert then, when they do not know from which endianes to
>>>> convert from ?
>>>
>>> Conversion is done from byte array of jtag chain.
>> How ?
>
> buf_get_u32 does conversion from uint8* array
>
> example:
> mips_ejtag_get_impcode (mips_ejtag.c)
>
>  field.in_value is filled by jtag_add_dr_scan with 4 times uint8.
>  buf_get_u32 converts byte array to host endian uint32

How ? It does not convert anything. It just takes the data and puts it
into array, in the order that it comes.




>>> The endianness of MIPS EJTAG tap seems to have always the same endianness,
>>> no matter of MIPS CPU memory endianness.
>> What makes you think so ?
>
> Because openocd works in all combinations of BE and LE host and target. (If you
> revert commit 2482244b0788c007dd789c21a4416379c229ea5c.) So yes, it's just a guess.

Which might mean that all commands are sent to EJTAG in appropriate
target format in which CPU expects them (be it BE or LE, depends in
which mode it runs).


> Could this be the explanation:
>
> http://downloads.buffalo.nas-central.org/LS2_MIPSel/DevelopmentTools/JTAG/MD00047-2B-EJTAG-SPC-03.10.pdf
> Page 97:
> Byte 0 refers to bits 7:0, byte 1 refers to bits 15:8, byte 2 refers to bits 23:16, and byte 3 refers to bits
> 31:24, independent of endianess.
>
This is _always_ the case in _all_ architectures. But, as you see,
this Byte0 is put to lower addresses for LE and to higher for BE. This
is always the case.


> Please also see Chapter 6.3.3, 6.3.6, 6.4.1, etc. TAPs are alway LSB first, so endianness doesn't matter here.
This is always the case, for all JTAG TAPS.

No, there must be something else...

BR,
Drasko