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

From: Goel, Akash
Date: Tue Jul 12 2016 - 08:51:08 EST

On 7/12/2016 2:54 PM, Chris Wilson wrote:
On Mon, Jul 11, 2016 at 01:17:09PM -0700, Andrew Morton wrote:
On Mon, 11 Jul 2016 12:47:36 +0530 akash.goel@xxxxxxxxx wrote:

From: Akash Goel <akash.goel@xxxxxxxxx>

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@xxxxxxxxxxx>
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;

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.

Thanks much, will update the documentation as well as the changelog as per the above.

But an export of symbol relay_late_setup_files() is still needed, just
like relay_open() is exported, in order to make it accessible to modules like i915 ?

Best regards