Re: [patch] linux 2.4.17: The second mb() rework (final)

On Fri, 15 Feb 2002, Hartvig Ekner wrote:

But in-order for the CPU is not enough - it also needs to make sure all
written data is visible for DMA accesses to memory from the outside, which
is why the writebuffer needs to be flushed by a SYNC as well. From the
definition of "SYNC":

	"A store is completed when the stored value is visible to every other
	processor in the system". 

Which presumably also includes DMA I/O devices...

 The description found in the R4k manual seems to contradict.  A "sync" on
a single CPU only assures transactions on it's external bus will happen in
order wrt the "sync".  In a multiple-CPU environment all CPUs interested
in a barrier need to execute a "sync".  That's seems natural -- how would 
you define a synchronization point for a CPU that does not execute a
"sync".

 Since I/O devices have no means to signal a CPU a "sync" operation, how
can you expect a "sync" on the CPU to complete all preceding writes to the
device?  It's only a following read that may imply it.

-- 
+  Maciej W. Rozycki, Technical University of Gdansk, Poland   +
+--------------------------------------------------------------+
+        e-mail: macro@ds2.pg.gda.pl, PGP key available        +