[PATCH] PowerPC: Program Priority Register support

[Adhemerval Zanella <azanella at linux dot vnet dot ibm dot com>]

Hi, some time ago I proposed a patch to save/restore the PowerPC Program
Priority Register along with platform specific functions to set its value.
However I later found out due a kernel limitation observed in the testcase
(the value was reset to default value in every interruption) I didn't try
to push.

However now with Linux kernel 3.9 the PPR save/restore is implemented correctly
within the kernel. So I'm simplifying the patch to just add the platform inline
function to adjust the PPR value. The instruction are just nops on architectures
that does not support PPR (pre-ISA 2.05) and on older kernel it will adjust the
PPR on current process, but its value will be set to reset on a interrupt.

Any comments, tips, advices?

---

2013-05-23  Adhemerval Zanella  <azanella@linux.vnet.ibm.com>

	* manual/platform.texi: Add PowerPC PPR function set documentation.
	* sysdeps/powerpc/sys/platform/ppc.h: Add PowerPC PPR set function
	implementation.

--

diff --git a/manual/platform.texi b/manual/platform.texi
index e387ff4..9803ba8 100644
--- a/manual/platform.texi
+++ b/manual/platform.texi
@@ -34,3 +34,24 @@ This frequency is not related to the processor clock or the bus clock.
 It is also possible that this frequency is not constant.  More information is
 available in @cite{Power ISA 2.06b - Book II - Section 5.2}.
 @end deftypefun
+
+@deftypefun {void} __ppc_set_ppr_med (void)
+Set the Program Priority Register to medium value (default).
+
+The @dfn{Program Priority Register} (PPR) is a 64-bit register that controls
+the program's priority. By adjusting the PPR value the programmer may
+improve system throughput by causing the system resources to be used
+more efficiently, especially in contention situations.
+The three unprivileged states available are covered by the functions
+__ppc_set_ppr_med (medium - default), __ppc_set_ppc_low (low) and
+__ppc_set_ppc_med_low (medium low).  More information
+available in @cite{Power ISA 2.06b - Book II - Section 3.1}.
+@end deftypefun
+
+@deftypefun {void} __ppc_set_ppr_low (void)
+Set the Program Priority Register to low value.
+@end deftypefun
+
+@deftypefun {void} __ppc_set_ppr_med_low (void)
+Set the Program Priority Register to medium low value.
+@end deftypefun
diff --git a/sysdeps/powerpc/sys/platform/ppc.h b/sysdeps/powerpc/sys/platform/ppc.h
index 740831e..7cc0f1c 100644
--- a/sysdeps/powerpc/sys/platform/ppc.h
+++ b/sysdeps/powerpc/sys/platform/ppc.h
@@ -50,4 +50,34 @@ __ppc_get_timebase (void)
 #endif
 }
 
+
+/* ISA 2.05 and beyond support the Program Priority Register (PPR) to adjust
+   thread priorities based on lock acquisition, wait and release. The ISA
+   defines the use of form 'or Rx,Rx,Rx' as the way to modify the PRI field.
+   The unprivileged priorities are:
+     Rx = 1 (low)
+     Rx = 2 (medium)
+     Rx = 6 (medium-low/normal)
+   The 'or' instruction form is a nop in previous hardware, so it is safe to
+   use unguarded. The default value is 'medium'.
+ */
+
+static inline void
+__ppc_set_ppr_med (void)
+{
+  __asm__ volatile ("or 2,2,2");
+}
+
+static inline void
+__ppc_set_ppr_med_low (void)
+{
+  __asm__ volatile ("or 6,6,6");
+}
+
+static inline void
+__ppc_set_ppr_low (void)
+{
+  __asm__ volatile ("or 1,1,1");
+}
+
 #endif  /* sys/platform/ppc.h */