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

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Date:	Tue, 12 Jul 2016 18:20:06 +0530
From:	"Goel, Akash" <akash.goel@...el.com>
To:	Chris Wilson <chris@...is-wilson.co.uk>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, viro@...iv.linux.org.uk,
	Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>,
	Tom Zanussi <tzanussi@...il.com>, akash.goel@...el.com
Subject: Re: [PATCH v2] relay: Add global mode support for buffer-only
 channels



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@...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.

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
Akash


> -Chris
>