Subject: Re: [RFC] uartlite driver MicroBlaze compatability

From: John Williams <[email-masked]>
Date: Wed, 02 May 2007 16:18:47 +1000

Grant Likely wrote:
> On 5/1/07, John Williams <[email-masked]> wrote:
> 
>> Grant Likely wrote:
>> > However, the uartlite is *not* an 8250.  The 8250 turns up all over
>> > the place and it's registers are defined as 8 bit wide.  The
>> > offset-by-3 stuff is part of the plat_serial8250_port structure which
>> > is also used to specify .regshift (increment between registers).
>> > Whereas the UARTLITE is defined as a 32 bit device and it doesn't show
>> > up in anywhere near as many designs.  Registers are always 4 bytes
>> > wide and are always located at multiples of 4 bytes off the base
> 
> Hmm, I think I was smoking something last night.  Address used for 8
> bit access should not be affected by CPU endianess.  After David's
> comments, I reread the uartlite documentation.  The current design is
> definately for 32bit OPB bus connections, but it looks like there is a
> posibility for xilinx to add a 16 or 8 bit attachment.  Since the
> uartlite design explicitly supports 8, 16 and 32 bit access, sticking
> with 8 bit io may be the safest.  

To be honest I don't think that will ever happen - just because the OPB 
bus data width is parameterisable, doesn't mean that it actually *works* 
or has been tested on anything other than 32-bits wide.  I've certainly 
never heard of anyone doing so, on either MicroBlaze or PPC.

but, I won't fight over it :)

Either way, it will still require a code change if/when someone does a 
16/8 bit wide OPB bus.  Whether they change the IO access operation, or 
a hardcoded constant, it's still not perfect.

Of course the real solution here is to create an OPB bus driver, with  a 
'width' field that you can pull out of XPAR, and so on... Use that 
instead of platform bus, and all this rubbish can be dealt with cleanly.

> However, I still think the
> application of the 3 byte offset should be done in the driver, and not
> in the platform bus registration.

If it has to be done, I agree the driver is the place to put it.

> I've reworked the patch with the following changes
> - remove 3 byte offset from platform bus registration.
> - added ulite_in/ulite_out macros to make changing bus attachment
> details simpler if xilinx changes the uartlite design.
> - stick with 8 bit IO.

It works fine, however perhaps a comment explaining the +3 offset might 
be appreciated by those who follow.

  > Tested on PPC.  John, can you please test on microblaze?

Acked-by: John Williams <[email-masked]>

John