linux-kernel - [tip:perf/core] perf/x86/intel/pt: Document pt_buffer_reset

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Message-ID: <tip-5b1dbd17c0dee679b154ce47f534677b7e0f7ad6@git.kernel.org>
Date:	Wed, 27 May 2015 03:08:18 -0700
From:	tip-bot for Alexander Shishkin <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	paulus@...ba.org, alexander.shishkin@...ux.intel.com,
	torvalds@...ux-foundation.org, mingo@...nel.org,
	linux-kernel@...r.kernel.org, peterz@...radead.org, hpa@...or.com,
	tglx@...utronix.de
Subject: [tip:perf/core] perf/x86/intel/pt:
  Document pt_buffer_reset_offsets()

Commit-ID:  5b1dbd17c0dee679b154ce47f534677b7e0f7ad6
Gitweb:     http://git.kernel.org/tip/5b1dbd17c0dee679b154ce47f534677b7e0f7ad6
Author:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>
AuthorDate: Tue, 21 Apr 2015 16:16:16 +0300
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 27 May 2015 09:17:47 +0200

perf/x86/intel/pt: Document pt_buffer_reset_offsets()

Currently, the description of pt_buffer_reset_offsets() lacks information
about its calling constraints and ordering with regards to other buffer
management functions.

Add a clarification about when this function has to be called.

Signed-off-by: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Paul Mackerras <paulus@...ba.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: acme@...radead.org
Cc: adrian.hunter@...el.com
Cc: hpa@...or.com
Link: http://lkml.kernel.org/r/1429622177-22843-5-git-send-email-alexander.shishkin@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/kernel/cpu/perf_event_intel_pt.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/perf_event_intel_pt.c b/arch/x86/kernel/cpu/perf_event_intel_pt.c
index b2746ea..40ba5e4 100644
--- a/arch/x86/kernel/cpu/perf_event_intel_pt.c
+++ b/arch/x86/kernel/cpu/perf_event_intel_pt.c
@@ -705,7 +705,14 @@ static void pt_buffer_setup_topa_index(struct pt_buffer *buf)
  * @head:	Write pointer (aux_head) from AUX buffer.
  *
  * Find the ToPA table and entry corresponding to given @head and set buffer's
- * "current" pointers accordingly.
+ * "current" pointers accordingly. This is done after we have obtained the
+ * current aux_head position from a successful call to perf_aux_output_begin()
+ * to make sure the hardware is writing to the right place.
+ *
+ * This function modifies buf::{cur,cur_idx,output_off} that will be programmed
+ * into PT msrs when the tracing is enabled and buf::head and buf::data_size,
+ * which are used to determine INT and STOP markers' locations by a subsequent
+ * call to pt_buffer_reset_markers().
  */
 static void pt_buffer_reset_offsets(struct pt_buffer *buf, unsigned long head)
 {
--
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