docs/dyndbg: explain flags parse 1st

When writing queries to >control, flags are parsed 1st, since they are
the only required field, and they require specific compositions.  So
if the flags draw an error (on those specifics), then keyword errors
aren't reported.  This can be mildly confusing/annoying, so explain it
instead.

cc: linux-doc@vger.kernel.org
Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20260502-dyndbg-doc-v1-2-67cc4a93a77e@gmail.com>

diff --git a/Documentation/admin-guide/dynamic-debug-howto.rst b/Documentation/admin-guide/dynamic-debug-howto.rst
index 4b14d9fd0300978a1dc1451df9ecaf8806636a29..9c2f096ed1d81b67bc4e719633fbc72cb43f3486 100644 (file)
--- a/Documentation/admin-guide/dynamic-debug-howto.rst
+++ b/Documentation/admin-guide/dynamic-debug-howto.rst
@@ -109,10 +109,19 @@ The match-spec's select *prdbgs* from the catalog, upon which to apply
 the flags-spec, all constraints are ANDed together.  An absent keyword
 is the same as keyword "*".
 
-
-A match specification is a keyword, which selects the attribute of
-the callsite to be compared, and a value to compare against.  Possible
-keywords are:::
+Note that since the match-spec can be empty, the flags are checked 1st,
+then the pairs of keyword and value.  Flag errs will hide keyword errs::
+
+  bash-5.2# ddcmd mod bar +foo
+  dyndbg: read 13 bytes from userspace
+  dyndbg: query 0: "mod bar +foo" mod:*
+  dyndbg: unknown flag 'o'
+  dyndbg: flags parse failed
+  dyndbg: processed 1 queries, with 0 matches, 1 errs
+
+So a match-spec is a keyword, which selects the attribute of the
+callsite to be compared, and a value to compare against.  Possible
+keywords are::
 
   match-spec ::= 'func' string |
                 'file' string |