Re: [POWERPC] convert string i/o operations to C

On Tue, 19 Sep 2006 13:29:53 -0500
linas@austin.ibm.com (Linas Vepstas) wrote:

What does this twi do? According to my powerpc docs, this would be a
no-op.  Does this have some magic synchronizing powers on certain
implementations? If so, there should be at least a comment card added
about why the twi is there. (This special ability of twi might be
well-known to some, but still, this is not immediately obvious,
and not immedately documented in e.g. the PEM.)

include/asm-p[ower]pc/io.h sheds some light on the matter:

 * With the sequence below (twi; isync; nop), we have found that
 * the machine check occurs on one of the three instructions on
 * all PPC implementations tested so far.  The twi and isync are
 * needed on the 601 (in fact twi; sync works too), the isync and
 * nop are needed on 604[e|r], and any of twi, sync or isync will
 * work on 603[e], 750, 74xx.
 * The twi creates an explicit data dependency on the returned
 * value which seems to be needed to make the 601 wait for the
 * load to finish.

Kim