[PATCH, RFC] remove slowport mode setting from ixp2000 map driver

[Lennert Buytenhek]

On Wed, Feb 23, 2005 at 11:26:44AM -0000, Mialkowski, Andrzej wrote:

> Lennert,

Hi,


> 	I'm OK with this change I just not caught that you are moving
> this setting to other place in ixp2000 global code.

Ok, great.  Can you commit it, or can someone else do that?


> Little surprise for
> me is that you are saying that you have(seen?) problem with access to
> devices when FRM=0, I never noticed that. This may of course happen in
> some hardware implementations but this means that there is no access to
> devices when executing from flash :(.

There is, but it's somewhat tricky -- you have to keep byteswapping
in mind, and the slowport might do more reads than you expect it to.

I have a byte-addressable device on my slowport region #0 which has
byte-sized registers.  The first four registers of the device are
normally '31 13 19 00'.  When the xscale runs in big-endian, I get
this:

		word read	halfword reads		byte reads
SP_FRM=0	00191331	0019 1331		00 19 13 31
SP_FRM=1	31313131	3113 1900		31 13 19 00

In SP_FRM=0, the output still looks 'fixable'.

But note that when SP_FRM=0 and you're doing a byte or halfword read,
what actually happens is that the slowport does four byte reads, combines
that to a word using little-endian ordering (00191331), presents that
word to the xscale, and the xscale then picks one of the {byte,halfword}s
from that word.

So, even if you do only a single byte read, it can still be doing four
byte reads on the slowport.  This can obviously confuse attached devices.


cheers,
Lennert