[PATCH]: powerpc: clarify use of twi/isync in io macros

[Linas Vepstas]

On Tue, Sep 19, 2006 at 09:57:11PM +0200, Segher Boessenkool wrote:
> ..."before the isync has completed, and it won't complete until
> the branch is resolved;"...

New improved patch attched.

> You shouldn't post my test and put your signed-off on it, esp.
> if I forgot to type half a sentence :-)  Here, have one though:

My moral fiber did twinge before I hit the send button,
but I confused it with a muscle spasm.

> Signed-off-by: Segher Boessenkool <segher at kernel.crashing.org>

--------

Clarify why twi appears in the i/o macros.

Signed-off-by: Linas Vepstas <linas at austin.ibm.com>
Signed-off-by: Segher Boessenkool <segher at kernel.crashing.org>

----
 include/asm-powerpc/io.h |   12 ++++++++++++
 1 file changed, 12 insertions(+)

Index: linux-2.6.18-rc7-git1/include/asm-powerpc/io.h
===================================================================
--- linux-2.6.18-rc7-git1.orig/include/asm-powerpc/io.h	2006-09-14 13:38:48.000000000 -0500
+++ linux-2.6.18-rc7-git1/include/asm-powerpc/io.h	2006-09-19 15:04:30.000000000 -0500
@@ -278,6 +278,18 @@ static inline void iosync(void)
  * These routines do not perform EEH-related I/O address translation,
  * and should not be used directly by device drivers.  Use inb/readb
  * instead.
+ *
+ * Read operations have additional twi & isync to make sure the read
+ * is actually performed (i.e. the data has come back) before we start
+ * executing any following instructions.
+ *
+ * A data-dependent branch followed by an isync ensures that no
+ * instructions after the isync in program order will be
+ * (speculatively) executed before the isync has completed, and
+ * the isync won't complete until the branch is resolved. The
+ * the load that the twi depends on has to complete before
+ * anything else is executed; in particular, it's a barrier to
+ * keep MMIO reads ordered before main-storage accesses.
  */
 static inline int in_8(const volatile unsigned char __iomem *addr)
 {