__ppc_set_ppr_med.3: Document PPC functions providing access to PPR

GNU C Library 2.18 adds functions (__ppc_set_ppr_low(3),
__ppc_set_ppr_med(3), __ppc_set_ppr_med_low(3)) that provide
access to the Program Priority Register (PPR).

Signed-off-by: Gabriel F. T. Gomes <gftg@linux.vnet.ibm.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

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+.\" Copyright (c) 2015, IBM Corporation.
+.\"
+.\" %%%LICENSE_START(VERBATIM)
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+.\" manual provided the copyright notice and this permission notice are
+.\" preserved on all copies.
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+.\" Formatted or processed versions of this manual, if unaccompanied by
+.\" the source, must acknowledge the copyright and authors of this work.
+.\" %%%LICENSE_END
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+.\" Process this file with
+.\" groff -man -Tascii foo.1
+.\"
+.TH __PPC_SET_PPR_MED 3 2015-08-05 "GNU C Library" "Linux\
+Programmer's Manual"
+.SH NAME
+__ppc_set_ppr_med, __ppc_set_ppr_low, __ppc_set_ppr_med_low \-
+Set the Program Priority Register
+.SH SYNOPSIS
+.B #include <sys/platform/ppc.h>
+.sp
+.B void __ppc_set_ppr_med(void);
+.br
+.B void __ppc_set_ppr_low(void);
+.br
+.B void __ppc_set_ppr_med_low(void);
+.SH DESCRIPTION
+The functions
+.BR __ppc_set_ppr_med (),
+.BR __ppc_set_ppr_low ()
+and
+.BR __ppc_set_ppr_med_low ()
+provide access to the
+.I Program Priority Register
+(PPR).
+.P
+The 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
+.BR __ppc_set_ppr_med (),
+.BR __ppc_set_ppr_low ()
+and
+.BR __ppc_set_ppr_med_low ().
+More information available in Power ISA 2.06b - Book II - Section 3.1.
+.P
+.BR __ppc_set_ppr_med ()
+sets the Program Priority Register to medium value (default).
+.P
+.BR __ppc_set_ppr_low ()
+sets the Program Priority Register to low value.
+.P
+.BR __ppc_set_ppr_med_low ()
+sets the Program Priority Register to medium low value.
+.SH VERSIONS
+GNU C Library added support for
+.BR __ppc_set_ppr_med (),
+.BR __ppc_set_ppr_low ()
+and
+.BR __ppc_set_ppr_med_low ()
+in version 2.18.
+.SH ATTRIBUTES
+For an explanation of the terms used in this section, see
+.BR attributes (7).
+.ad l
+.TS
+allbox;
+lb lb lb
+l l l.
+Interface      Attribute       Value
+T{
+.BR __ppc_set_ppr_med (),
+.BR __ppc_set_ppr_low (),
+.BR __ppc_set_ppr_med_low ()
+T}     Thread safety   MT-Safe
+.TE
+.ad
+.SH CONFORMING TO
+These functions are nonstandard GNU extensions.
+.SH SEE ALSO
+.BR __ppc_yield (3)