Message-Id: <20220401234309.21252-8-beaub@linux.microsoft.com>
Date:   Fri,  1 Apr 2022 16:43:09 -0700
From:   Beau Belgrave <beaub@...ux.microsoft.com>
To:     rostedt@...dmis.org, mhiramat@...nel.org,
        mathieu.desnoyers@...icios.com
Cc:     linux-trace-devel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, beaub@...ux.microsoft.com
Subject: [PATCH 7/7] tracing/user_events: Update ABI documentation to align to bits vs bytes

Update the documentation to reflect the new ABI requirements and how to
use the byte index with the mask properly to check event status.

Signed-off-by: Beau Belgrave <beaub@...ux.microsoft.com>
---
 Documentation/trace/user_events.rst | 46 +++++++++++++----------------
 1 file changed, 21 insertions(+), 25 deletions(-)

diff --git a/Documentation/trace/user_events.rst b/Documentation/trace/user_events.rst
index c180936f49fc..6aabdb126f2e 100644
--- a/Documentation/trace/user_events.rst
+++ b/Documentation/trace/user_events.rst
@@ -20,14 +20,15 @@ dynamic_events is the same as the ioctl with the u: prefix applied.
 
 Typically programs will register a set of events that they wish to expose to
 tools that can read trace_events (such as ftrace and perf). The registration
-process gives back two ints to the program for each event. The first int is the
-status index. This index describes which byte in the
+process gives back three ints to the program for each event. The first int is
+the status index. This index describes which byte in the
 /sys/kernel/debug/tracing/user_events_status file represents this event. The
-second int is the write index. This index describes the data when a write() or
-writev() is called on the /sys/kernel/debug/tracing/user_events_data file.
+second int is the mask to use on the byte to check the event status. The third
+int is the write index which describes the data when a write() or writev() is
+called on the /sys/kernel/debug/tracing/user_events_data file.
 
-The structures referenced in this document are contained with the
-/include/uap/linux/user_events.h file in the source tree.
+The structures referenced in this document are contained within the
+/include/uapi/linux/user_events.h file in the source tree.
 
 **NOTE:** *Both user_events_status and user_events_data are under the tracefs
 filesystem and may be mounted at different paths than above.*
@@ -44,6 +45,7 @@ This command takes a struct user_reg as an argument::
         u32 size;
         u64 name_args;
         u32 status_index;
+        u32 status_mask;
         u32 write_index;
   };
 
@@ -112,14 +114,19 @@ writev() calls when something is actively attached to the event.
 
 User programs call mmap() on /sys/kernel/debug/tracing/user_events_status to
 check the status for each event that is registered. The byte to check in the
-file is given back after the register ioctl() via user_reg.status_index.
+file is given back after the register ioctl() via user_reg.status_index. The
+byte needs to be AND'd with the user_reg.status_mask value to check if the
+event is enabled currently.
+
 Currently the size of user_events_status is a single page, however, custom
 kernel configurations can change this size to allow more user based events. In
 all cases the size of the file is a multiple of a page size.
 
-For example, if the register ioctl() gives back a status_index of 3 you would
-check byte 3 of the returned mmap data to see if anything is attached to that
-event.
+For example, if the register ioctl() gives back a status_index of 3 and a mask
+of 1 you would check byte 3 of the returned mmap data and then AND the result
+with 1 (e.g. status_page[3] & 1) to see if anything is attached to that event.
+A helper function is available called user_event_enabled() which does this for
+you.
 
 Administrators can easily check the status of all registered events by reading
 the user_events_status file directly via a terminal. The output is as follows::
@@ -137,7 +144,7 @@ For example, on a system that has a single event the output looks like this::
 
   Active: 1
   Busy: 0
-  Max: 4096
+  Max: 32768
 
 If a user enables the user event via ftrace, the output would change to this::
 
@@ -145,21 +152,10 @@ If a user enables the user event via ftrace, the output would change to this::
 
   Active: 1
   Busy: 1
-  Max: 4096
-
-**NOTE:** *A status index of 0 will never be returned. This allows user
-programs to have an index that can be used on error cases.*
-
-Status Bits
-^^^^^^^^^^^
-The byte being checked will be non-zero if anything is attached. Programs can
-check specific bits in the byte to see what mechanism has been attached.
-
-The following values are defined to aid in checking what has been attached:
-
-**EVENT_STATUS_FTRACE** - Bit set if ftrace has been attached (Bit 0).
+  Max: 32768
 
-**EVENT_STATUS_PERF** - Bit set if perf has been attached (Bit 1).
+**NOTE:** *A status index of 0 with a mask of 1 will never be returned. This
+allows user programs to have an index and mask that can be used on error cases.*
 
 Writing Data
 ------------
-- 
2.25.1

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