From: linas@austin•ibm.com (Linas Vepstas)
To: Segher Boessenkool <segher@kernel•crashing.org>
Cc: Stephen Rothwell <sfr@canb•auug.org.au>,
paulus@samba•org, ppc-dev <linuxppc-dev@ozlabs•org>
Subject: Re: [PATCH]: powerpc: clarify use of twi/isync in io macros
Date: Tue, 19 Sep 2006 15:10:53 -0500 [thread overview]
Message-ID: <20060919201053.GP29167@austin.ibm.com> (raw)
In-Reply-To: <72674737-8325-48FA-B04F-C4ADCA2E2BF9@kernel.crashing.org>
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@kernel•crashing.org>
--------
Clarify why twi appears in the i/o macros.
Signed-off-by: Linas Vepstas <linas@austin•ibm.com>
Signed-off-by: Segher Boessenkool <segher@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)
{
next prev parent reply other threads:[~2006-09-19 20:10 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-09-19 19:30 [PATCH]: powerpc: clarify use of twi/isync in io macros Linas Vepstas
2006-09-19 19:57 ` Segher Boessenkool
2006-09-19 20:10 ` Linas Vepstas [this message]
2006-09-19 20:30 ` Segher Boessenkool
2006-09-21 12:41 ` Paul Mackerras
2006-09-21 15:06 ` Segher Boessenkool
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