[U-Boot] [PATCH] include/ns16550.h: Unify structure declaration for registers

[Shinya Kuribayashi <skuribay@pobox.com>]

Hi Jerry-san,

Jerry Van Baren wrote:
>>> I might be unclear. I used to use REG_SIZE = -16, as 16550 registers
>>> are located at 0, +0x10, +0x20, ..., .
> 
> 16 byte stride.  That is seriously odd.

Well, 8 or 16 byte stride is not so odd, IMHO.

>>> I don't know much about precise hardware logics, but the byte addresses
>>> under 16-bytes-border are ignored.  I'm using a big-endian mips machine.
>>
>> This does not make much sense to me, sorry.
> 
> The "16" of the "16-bytes-border" statement confuses me too.

Sorry for my poor vocabularies :-(

> It sounds like Shinya has some pretty odd (read "broken") hardware that 
> is decoding the registers with a 16 byte stride, although his example 
> above shows a 4 byte stride (less broken).

Let me reword:

* my UART registers are located with 16 byte stride.

* The address decoder in my UART block rounds +1/+2/+3 offsets down
  to zero offset.  Therefore we can't do byte read/write to ns16550
  registers properly; i.e. the return value of readb(x + 3) will be
  equal to readb(x).

> I would further deduce his hardware does not support byte write 
> operations (I've never seen hardware that didn't support byte reads). 
> I've had hardware that did not support byte writes, so s/w needed to 
> write a word instead (given Shinya's description, the extra bytes are 
> "don't care").  (I've also dealt with flash connections that only 
> supported 64 bit writes - PITA!).

Thanks for deducing :-)  Yes, I wanted to say 'don't care'.

> My guess is his processor limitations prevent byte writes so he has to 
> do 32bit (4byte) writes, but his hardware decoding results in a 16 byte 
> stride.  The result is setting REG_SIZE to 4 gives him the r/w access he 
> needs (32 bits), but fails the stride.  Setting it to 16 gives him the 
> stride he needs, but a 16 byte register is nonsensical and breaks the 
> software.  My guess is Shinya needs another customization dial (I'm 
> making this up) "REG_STRIDE" = 16 as well as "REG_SIZE" = 4.

Let me clarify:

* My processor MIPS 4KEc doesn't have limitations on byte accesses.

* My address decoder in UART block, can't handle +1/+2/+3 offsets
  properly.  This is the reason I need 32-bit word accesses.  And
  16 byte stride is not related here.

Thanks again for you translation,

  Shinya