linux-kernel - Re: [PATCH v2] relay: Add global mode support for buffer-only channels

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Message-ID: <20160712092445.GN6157@nuc-i3427.alporthouse.com>
Date:	Tue, 12 Jul 2016 10:24:45 +0100
From:	Chris Wilson <chris@...is-wilson.co.uk>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	akash.goel@...el.com, linux-kernel@...r.kernel.org,
	viro@...iv.linux.org.uk,
	Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>,
	Tom Zanussi <tzanussi@...il.com>
Subject: Re: [PATCH v2] relay: Add global mode support for buffer-only
 channels

On Mon, Jul 11, 2016 at 01:17:09PM -0700, Andrew Morton wrote:
> On Mon, 11 Jul 2016 12:47:36 +0530 akash.goel@...el.com wrote:
> 
> > From: Akash Goel <akash.goel@...el.com>
> > 
> > The following patch added support to use channels with no associated files.
> > 	relay: add buffer-only channels; useful for early logging
> 
> hm, 8 years ago.  Normally we refer to previous commits using the form
> 20d8b67c06fa5e74f44e ("relay: add buffer-only channels; useful for
> early logging").  But this one is so old that we should inform readers
> about its vintage, so this form:
> 
> commit 20d8b67c06fa5e74f44e80b0a0fd68c8327f7c6a
> Author: Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>
> Date:   Fri Jul 25 19:45:12 2008 -0700
> 
>     relay: add buffer-only channels; useful for early logging
> 
> would be better.
> 
> > This is useful when the exact location of relay file is not known or the
> > the parent directory of relay file is not available, while creating the
> > channel and the logging has to start right from the boot.
> > 
> > But there was no provision to use global mode with buffer-only channels,
> > which is added by this patch, without modifying the interface where initially
> > there will be a dummy invocation of create_buf_file callback through which
> > kernel client can convey the need of a global buffer.
> > 
> > For the use case where drivers/kernel clients want a simple interface for the
> > userspace, which enables them to capture data/logs from relay file in order &
> > without any post processing, support of Global buffer mode is warranted.
> >
> > ...
> >
> > @@ -706,6 +727,7 @@ int relay_late_setup_files(struct rchan *chan,
> >  
> >  	return err;
> >  }
> > +EXPORT_SYMBOL_GPL(relay_late_setup_files);
> 
> The export is unneeded and undocumented.

Something like:

diff --git a/kernel/relay.c b/kernel/relay.c
index 04d7cf3ef8cf..fd86f01de4b2 100644
--- a/kernel/relay.c
+++ b/kernel/relay.c
@@ -562,6 +562,10 @@ static int relay_hotcpu_callback(struct notifier_block *nb,
  *     attributes specified.  The created channel buffer files
  *     will be named base_filename0...base_filenameN-1.  File
  *     permissions will be %S_IRUSR.
+ *
+ *     If opening a buffer (@parent = NULL) that you later wish to register
+ *     in a filesystem, call relay_late_setup_files() once the @parent dentry
+ *     is available.
  */
 struct rchan *relay_open(const char *base_filename,
                         struct dentry *parent,
@@ -640,8 +644,12 @@ static void __relay_set_buf_dentry(void *info)
  *
  *     Returns 0 if successful, non-zero otherwise.
  *
- *     Use to setup files for a previously buffer-only channel.
- *     Useful to do early tracing in kernel, before VFS is up, for example.
+ *     Use to setup files for a previously buffer-only channel created
+ *     by relay_open() with a NULL parent dentry.
+ *
+ *     For example, this is useful for perfomring early tracing in kernel,
+ *     before VFS is up and then exposing the early results once the dentry
+ *     is available.
  */
 int relay_late_setup_files(struct rchan *chan,
                           const char *base_filename,


with a comment in the changelog that modules using relay_open() in early
init also want to later register their buffer-only relays once debugfs is
available, e.g. i915.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre