[dpdk-dev] Understanding of Acked-By

[dpdk-dev] Understanding of Acked-By

[Van Haaren, Harry]

( Was [dpdk-dev] [PATCH v4] ethdev: fix MAC address replay, CC-ed are participants of that thread http://dpdk.org/ml/archives/dev/2017-January/056278.html )


Hi All,


There was an idea (from Thomas) to better document the Acked-by and Reviewed-By in the above thread, which I think is worth doing to make the process clearer. I'll kick off a thread*, and offer to submit a patch for the documentation when a consensus is reached.


The question that needs to be addressed is "What is the most powerful signoff to add as somebody who checked a patch?"


The documentation mentions Acked, Reviewed, and Tested by[1], as signoffs that can be commented on patches. The Review Process[2] section mentions Reviewed and Tested by, but nowhere specifically states what any of these indicate.


Offered below is my current understanding of the Acked-by; Reviewed-by; and Tested-by tags, in order of least-powerful first:


3) Tested-by: (least powerful)
  - Indicates having passed testing of functionality, and works as expected for Tester
  - Does NOT include full code review (instead use Reviewed by)
  - Does NOT indicate that the Tester understands architecture (instead use Acked by)


2) Reviewed-by:
  - Indicates having passed code-review, checkpatch and compilation testing by Reviewer
  - Does NOT include full testing of functionality (instead use Tested-by)
  - Does NOT indicate that the Reviewer understands architecture (instead use Acked by)


1) Acked-by: (most powerful)
   - Indicates Reviewed-by, but also:
   - Acker understands impact to architecture (if any) and agrees with changes
   - Acker has performed runtime sanity check
   - Requests "please merge" to maintainer
   - Level of trust in Acked-by based on previous contributions to DPDK/networking community


The above is a suggested interpretation, alternative interpretations welcomed.
-Harry



[1] http://dpdk.org/doc/guides/contributing/patches.html#commit-messages-body
[2] http://dpdk.org/doc/guides/contributing/patches.html#the-review-process

* Apologies for the slightly bike-shed topic

Re: [dpdk-dev] Understanding of Acked-By

[Thomas Monjalon]

2017-01-25 13:53, Van Haaren, Harry:
> There was an idea (from Thomas) to better document the Acked-by and Reviewed-By in the above thread, which I think is worth doing to make the process clearer. I'll kick off a thread*, and offer to submit a patch for the documentation when a consensus is reached.
> 
> 
> The question that needs to be addressed is "What is the most powerful signoff to add as somebody who checked a patch?"

I do not see the benefit of knowing the most powerful.
Anyway, the most powerful tags are done by trusted people.
And people are trusted after delivering good reviews or patches ;)

The question should be "How to use the tags?"

> The documentation mentions Acked, Reviewed, and Tested by[1], as signoffs that can be commented on patches. The Review Process[2] section mentions Reviewed and Tested by, but nowhere specifically states what any of these indicate.
> 
> Offered below is my current understanding of the Acked-by; Reviewed-by; and Tested-by tags, in order of least-powerful first:
> 
> 
> 3) Tested-by: (least powerful)
>   - Indicates having passed testing of functionality, and works as expected for Tester
>   - Does NOT include full code review (instead use Reviewed by)
>   - Does NOT indicate that the Tester understands architecture (instead use Acked by)
> 
> 
> 2) Reviewed-by:
>   - Indicates having passed code-review, checkpatch and compilation testing by Reviewer

Compilation testing is done by the CI.
The reviewer must just check the results.

>   - Does NOT include full testing of functionality (instead use Tested-by)
>   - Does NOT indicate that the Reviewer understands architecture (instead use Acked by)

I disagree here.
The reviewer must understand the impacts of the patch.
That's why a Reviewed-by tag is really strong.

> 1) Acked-by: (most powerful)
>    - Indicates Reviewed-by, but also:

A maintainer may want to approve the intent without doing a full review,
especially if he trusts the author or the reviewers.
That's why I think Acked-by should not include Reviewed-by.
If a maintainer does a full review, he should use Reviewed-by instead of Acked-by.

>    - Acker understands impact to architecture (if any) and agrees with changes
>    - Acker has performed runtime sanity check

Not sure about this one.
Personnaly I give some Acks without testing sometimes.
We may add a Tested-by to indicate we made some tests.

>    - Requests "please merge" to maintainer

Yes, "please merge" to tree maintainer (committer).

>    - Level of trust in Acked-by based on previous contributions to DPDK/networking community

The level of trust applies to any tag or comment.

> The above is a suggested interpretation, alternative interpretations welcomed.

Thanks Harry

Re: [dpdk-dev] Understanding of Acked-By

[Shreyansh Jain]

On Wednesday 25 January 2017 08:28 PM, Thomas Monjalon wrote:
> 2017-01-25 13:53, Van Haaren, Harry:
>> There was an idea (from Thomas) to better document the Acked-by and Reviewed-By in the above thread, which I think is worth doing to make the process clearer. I'll kick off a thread*, and offer to submit a patch for the documentation when a consensus is reached.
>>
>>
>> The question that needs to be addressed is "What is the most powerful signoff to add as somebody who checked a patch?"
>
> I do not see the benefit of knowing the most powerful.
> Anyway, the most powerful tags are done by trusted people.
> And people are trusted after delivering good reviews or patches ;)
>
> The question should be "How to use the tags?"
>
>> The documentation mentions Acked, Reviewed, and Tested by[1], as signoffs that can be commented on patches. The Review Process[2] section mentions Reviewed and Tested by, but nowhere specifically states what any of these indicate.
>>
>> Offered below is my current understanding of the Acked-by; Reviewed-by; and Tested-by tags, in order of least-powerful first:
>>
>>
>> 3) Tested-by: (least powerful)
>>   - Indicates having passed testing of functionality, and works as expected for Tester
>>   - Does NOT include full code review (instead use Reviewed by)
>>   - Does NOT indicate that the Tester understands architecture (instead use Acked by)
>>
>>
>> 2) Reviewed-by:
>>   - Indicates having passed code-review, checkpatch and compilation testing by Reviewer
>
> Compilation testing is done by the CI.
> The reviewer must just check the results.
>
>>   - Does NOT include full testing of functionality (instead use Tested-by)
>>   - Does NOT indicate that the Reviewer understands architecture (instead use Acked by)
>
> I disagree here.
> The reviewer must understand the impacts of the patch.
> That's why a Reviewed-by tag is really strong.

 From what I understand, 'Reviewed-by' and 'Acked-by' are the other way
around.
- Acked-by is intent that 'I agree with change'.
- Reviewed-by is 'I vouch for the changes' either through review or
   testing or both.

>
>> 1) Acked-by: (most powerful)
>>    - Indicates Reviewed-by, but also:
>
> A maintainer may want to approve the intent without doing a full review,
> especially if he trusts the author or the reviewers.
> That's why I think Acked-by should not include Reviewed-by.
> If a maintainer does a full review, he should use Reviewed-by instead of Acked-by.
>
>>    - Acker understands impact to architecture (if any) and agrees with changes
>>    - Acker has performed runtime sanity check
>
> Not sure about this one.
> Personnaly I give some Acks without testing sometimes.
> We may add a Tested-by to indicate we made some tests.
>
>>    - Requests "please merge" to maintainer
>
> Yes, "please merge" to tree maintainer (committer).
>
>>    - Level of trust in Acked-by based on previous contributions to DPDK/networking community
>
> The level of trust applies to any tag or comment.
>
>> The above is a suggested interpretation, alternative interpretations welcomed.
>
> Thanks Harry
>

Re: [dpdk-dev] Understanding of Acked-By

[Bruce Richardson]

On Fri, Jan 27, 2017 at 12:48:06PM +0530, Shreyansh Jain wrote:
> On Wednesday 25 January 2017 08:28 PM, Thomas Monjalon wrote:
> > 2017-01-25 13:53, Van Haaren, Harry:
> > > There was an idea (from Thomas) to better document the Acked-by and Reviewed-By in the above thread, which I think is worth doing to make the process clearer. I'll kick off a thread*, and offer to submit a patch for the documentation when a consensus is reached.
> > > 
> > > 
> > > The question that needs to be addressed is "What is the most powerful signoff to add as somebody who checked a patch?"
> > 
> > I do not see the benefit of knowing the most powerful.
> > Anyway, the most powerful tags are done by trusted people.
> > And people are trusted after delivering good reviews or patches ;)
> > 
> > The question should be "How to use the tags?"
> > 
> > > The documentation mentions Acked, Reviewed, and Tested by[1], as signoffs that can be commented on patches. The Review Process[2] section mentions Reviewed and Tested by, but nowhere specifically states what any of these indicate.
> > > 
> > > Offered below is my current understanding of the Acked-by; Reviewed-by; and Tested-by tags, in order of least-powerful first:
> > > 
> > > 
> > > 3) Tested-by: (least powerful)
> > >   - Indicates having passed testing of functionality, and works as expected for Tester
> > >   - Does NOT include full code review (instead use Reviewed by)
> > >   - Does NOT indicate that the Tester understands architecture (instead use Acked by)
> > > 
> > > 
> > > 2) Reviewed-by:
> > >   - Indicates having passed code-review, checkpatch and compilation testing by Reviewer
> > 
> > Compilation testing is done by the CI.
> > The reviewer must just check the results.
> > 
> > >   - Does NOT include full testing of functionality (instead use Tested-by)
> > >   - Does NOT indicate that the Reviewer understands architecture (instead use Acked by)
> > 
> > I disagree here.
> > The reviewer must understand the impacts of the patch.
> > That's why a Reviewed-by tag is really strong.
> 
> From what I understand, 'Reviewed-by' and 'Acked-by' are the other way
> around.
> - Acked-by is intent that 'I agree with change'.
> - Reviewed-by is 'I vouch for the changes' either through review or
>   testing or both.
> 

Other way round in what way - compared to proposed by Harry or by
Thomas? Which do you view as the stronger indication that the patch is
ok?

Regards,
/Bruce

Re: [dpdk-dev] Understanding of Acked-By

[Shreyansh Jain]

On Friday 27 January 2017 03:43 PM, Bruce Richardson wrote:
> On Fri, Jan 27, 2017 at 12:48:06PM +0530, Shreyansh Jain wrote:
>> On Wednesday 25 January 2017 08:28 PM, Thomas Monjalon wrote:
>>> 2017-01-25 13:53, Van Haaren, Harry:
>>>> There was an idea (from Thomas) to better document the Acked-by and Reviewed-By in the above thread, which I think is worth doing to make the process clearer. I'll kick off a thread*, and offer to submit a patch for the documentation when a consensus is reached.
>>>>
>>>>
>>>> The question that needs to be addressed is "What is the most powerful signoff to add as somebody who checked a patch?"
>>>
>>> I do not see the benefit of knowing the most powerful.
>>> Anyway, the most powerful tags are done by trusted people.
>>> And people are trusted after delivering good reviews or patches ;)
>>>
>>> The question should be "How to use the tags?"
>>>
>>>> The documentation mentions Acked, Reviewed, and Tested by[1], as signoffs that can be commented on patches. The Review Process[2] section mentions Reviewed and Tested by, but nowhere specifically states what any of these indicate.
>>>>
>>>> Offered below is my current understanding of the Acked-by; Reviewed-by; and Tested-by tags, in order of least-powerful first:
>>>>
>>>>
>>>> 3) Tested-by: (least powerful)
>>>>   - Indicates having passed testing of functionality, and works as expected for Tester
>>>>   - Does NOT include full code review (instead use Reviewed by)
>>>>   - Does NOT indicate that the Tester understands architecture (instead use Acked by)
>>>>
>>>>
>>>> 2) Reviewed-by:
>>>>   - Indicates having passed code-review, checkpatch and compilation testing by Reviewer
>>>
>>> Compilation testing is done by the CI.
>>> The reviewer must just check the results.
>>>
>>>>   - Does NOT include full testing of functionality (instead use Tested-by)
>>>>   - Does NOT indicate that the Reviewer understands architecture (instead use Acked by)
>>>
>>> I disagree here.
>>> The reviewer must understand the impacts of the patch.
>>> That's why a Reviewed-by tag is really strong.
>>
>> From what I understand, 'Reviewed-by' and 'Acked-by' are the other way
>> around.
>> - Acked-by is intent that 'I agree with change'.
>> - Reviewed-by is 'I vouch for the changes' either through review or
>>   testing or both.
>>
>
> Other way round in what way - compared to proposed by Harry or by
> Thomas? Which do you view as the stronger indication that the patch is
> ok?

Sorry, I should have posted this against Harry's mail rather than
Thomas'.
'Other way round' as compared to Harry's text.
Reviewed-by is a strong indication, in my understanding.

>
> Regards,
> /Bruce
>

-
Shreyansh

Re: [dpdk-dev] Understanding of Acked-By

[Mcnamara, John]

> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Shreyansh Jain
> Sent: Friday, January 27, 2017 10:25 AM
> To: Richardson, Bruce <bruce.richardson@intel.com>
> Cc: Thomas Monjalon <thomas.monjalon@6wind.com>; Van Haaren, Harry
> <harry.van.haaren@intel.com>; dev@dpdk.org; Yigit, Ferruh
> <ferruh.yigit@intel.com>; Igor Ryzhov <iryzhov@nfware.com>; Steve Shin
> <jonshin@cisco.com>
> Subject: Re: [dpdk-dev] Understanding of Acked-By
> 
> On Friday 27 January 2017 03:43 PM, Bruce Richardson wrote:
> > On Fri, Jan 27, 2017 at 12:48:06PM +0530, Shreyansh Jain wrote:
> >> On Wednesday 25 January 2017 08:28 PM, Thomas Monjalon wrote:
> >>> 2017-01-25 13:53, Van Haaren, Harry:
> >>>> There was an idea (from Thomas) to better document the Acked-by and
> Reviewed-By in the above thread, which I think is worth doing to make the
> process clearer. I'll kick off a thread*, and offer to submit a patch for
> the documentation when a consensus is reached.
> >>>>
> >>>>
> >>>> The question that needs to be addressed is "What is the most powerful
> signoff to add as somebody who checked a patch?"
> >>>
> >>> I do not see the benefit of knowing the most powerful.
> >>> Anyway, the most powerful tags are done by trusted people.
> >>> And people are trusted after delivering good reviews or patches ;)
> >>>
> >>> The question should be "How to use the tags?"
> >>>
> >>>> The documentation mentions Acked, Reviewed, and Tested by[1], as
> signoffs that can be commented on patches. The Review Process[2] section
> mentions Reviewed and Tested by, but nowhere specifically states what any
> of these indicate.
> >>>>
> >>>> Offered below is my current understanding of the Acked-by; Reviewed-
> by; and Tested-by tags, in order of least-powerful first:
> >>>>
> >>>>
> >>>> 3) Tested-by: (least powerful)
> >>>>   - Indicates having passed testing of functionality, and works as
> expected for Tester
> >>>>   - Does NOT include full code review (instead use Reviewed by)
> >>>>   - Does NOT indicate that the Tester understands architecture
> >>>> (instead use Acked by)
> >>>>
> >>>>
> >>>> 2) Reviewed-by:
> >>>>   - Indicates having passed code-review, checkpatch and compilation
> >>>> testing by Reviewer
> >>>
> >>> Compilation testing is done by the CI.
> >>> The reviewer must just check the results.
> >>>
> >>>>   - Does NOT include full testing of functionality (instead use
> Tested-by)
> >>>>   - Does NOT indicate that the Reviewer understands architecture
> >>>> (instead use Acked by)
> >>>
> >>> I disagree here.
> >>> The reviewer must understand the impacts of the patch.
> >>> That's why a Reviewed-by tag is really strong.
> >>
> >> From what I understand, 'Reviewed-by' and 'Acked-by' are the other
> >> way around.
> >> - Acked-by is intent that 'I agree with change'.
> >> - Reviewed-by is 'I vouch for the changes' either through review or
> >>   testing or both.
> >>
> >
> > Other way round in what way - compared to proposed by Harry or by
> > Thomas? Which do you view as the stronger indication that the patch is
> > ok?
> 
> Sorry, I should have posted this against Harry's mail rather than Thomas'.
> 'Other way round' as compared to Harry's text.
> Reviewed-by is a strong indication, in my understanding.


Hi,

Maybe we should just follow the Kernel guidelines on this:

https://www.kernel.org/doc/html/latest/process/submitting-patches.html#when-to-use-acked-by-and-cc


And to, save-you-a-click(tm) here is the relevant sections of the doc:


12) When to use Acked-by: and Cc:
---------------------------------

The Signed-off-by: tag indicates that the signer was involved in the
development of the patch, or that he/she was in the patch's delivery path.

If a person was not directly involved in the preparation or handling of a
patch but wishes to signify and record their approval of it then they can
ask to have an Acked-by: line added to the patch's changelog.

Acked-by: is often used by the maintainer of the affected code when that
maintainer neither contributed to nor forwarded the patch.

Acked-by: is not as formal as Signed-off-by:.  It is a record that the acker
has at least reviewed the patch and has indicated acceptance.  Hence patch
mergers will sometimes manually convert an acker's "yep, looks good to me"
into an Acked-by: (but note that it is usually better to ask for an
explicit ack).

Acked-by: does not necessarily indicate acknowledgement of the entire patch.
For example, if a patch affects multiple subsystems and has an Acked-by: from
one subsystem maintainer then this usually indicates acknowledgement of just
the part which affects that maintainer's code.  Judgement should be used here.
When in doubt people should refer to the original discussion in the mailing
list archives.

If a person has had the opportunity to comment on a patch, but has not
provided such comments, you may optionally add a ``Cc:`` tag to the patch.
This is the only tag which might be added without an explicit action by the
person it names - but it should indicate that this person was copied on the
patch.  This tag documents that potentially interested parties
have been included in the discussion.


13) Using Reported-by:, Tested-by:, Reviewed-by:, Suggested-by: and Fixes:
--------------------------------------------------------------------------

The Reported-by tag gives credit to people who find bugs and report them and it
hopefully inspires them to help us again in the future.  Please note that if
the bug was reported in private, then ask for permission first before using the
Reported-by tag.

A Tested-by: tag indicates that the patch has been successfully tested (in
some environment) by the person named.  This tag informs maintainers that
some testing has been performed, provides a means to locate testers for
future patches, and ensures credit for the testers.

Reviewed-by:, instead, indicates that the patch has been reviewed and found
acceptable according to the Reviewer's Statement:

Reviewer's statement of oversight
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

By offering my Reviewed-by: tag, I state that:

	 (a) I have carried out a technical review of this patch to
	     evaluate its appropriateness and readiness for inclusion into
	     the mainline kernel.

	 (b) Any problems, concerns, or questions relating to the patch
	     have been communicated back to the submitter.  I am satisfied
	     with the submitter's response to my comments.

	 (c) While there may be things that could be improved with this
	     submission, I believe that it is, at this time, (1) a
	     worthwhile modification to the kernel, and (2) free of known
	     issues which would argue against its inclusion.

	 (d) While I have reviewed the patch and believe it to be sound, I
	     do not (unless explicitly stated elsewhere) make any
	     warranties or guarantees that it will achieve its stated
	     purpose or function properly in any given situation.

A Reviewed-by tag is a statement of opinion that the patch is an
appropriate modification of the kernel without any remaining serious
technical issues.  Any interested reviewer (who has done the work) can
offer a Reviewed-by tag for a patch.  This tag serves to give credit to
reviewers and to inform maintainers of the degree of review which has been
done on the patch.  Reviewed-by: tags, when supplied by reviewers known to
understand the subject area and to perform thorough reviews, will normally
increase the likelihood of your patch getting into the kernel.

A Suggested-by: tag indicates that the patch idea is suggested by the person
named and ensures credit to the person for the idea. Please note that this
tag should not be added without the reporter's permission, especially if the
idea was not posted in a public forum. That said, if we diligently credit our
idea reporters, they will, hopefully, be inspired to help us again in the
future.

A Fixes: tag indicates that the patch fixes an issue in a previous commit. It
is used to make it easy to determine where a bug originated, which can help
review a bug fix. This tag also assists the stable kernel team in determining
which stable kernel versions should receive your fix. This is the preferred
method for indicating a bug fixed by the patch. See :ref:`describe_changes`
for more details.

Re: [dpdk-dev] Understanding of Acked-By

[Ferruh Yigit]

On 1/27/2017 10:32 AM, Mcnamara, John wrote:
> 
> 
>> -----Original Message-----
>> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Shreyansh Jain
>> Sent: Friday, January 27, 2017 10:25 AM
>> To: Richardson, Bruce <bruce.richardson@intel.com>
>> Cc: Thomas Monjalon <thomas.monjalon@6wind.com>; Van Haaren, Harry
>> <harry.van.haaren@intel.com>; dev@dpdk.org; Yigit, Ferruh
>> <ferruh.yigit@intel.com>; Igor Ryzhov <iryzhov@nfware.com>; Steve Shin
>> <jonshin@cisco.com>
>> Subject: Re: [dpdk-dev] Understanding of Acked-By
>>
>> On Friday 27 January 2017 03:43 PM, Bruce Richardson wrote:
>>> On Fri, Jan 27, 2017 at 12:48:06PM +0530, Shreyansh Jain wrote:
>>>> On Wednesday 25 January 2017 08:28 PM, Thomas Monjalon wrote:
>>>>> 2017-01-25 13:53, Van Haaren, Harry:
>>>>>> There was an idea (from Thomas) to better document the Acked-by and
>> Reviewed-By in the above thread, which I think is worth doing to make the
>> process clearer. I'll kick off a thread*, and offer to submit a patch for
>> the documentation when a consensus is reached.
>>>>>>
>>>>>>
>>>>>> The question that needs to be addressed is "What is the most powerful
>> signoff to add as somebody who checked a patch?"
>>>>>
>>>>> I do not see the benefit of knowing the most powerful.
>>>>> Anyway, the most powerful tags are done by trusted people.
>>>>> And people are trusted after delivering good reviews or patches ;)
>>>>>
>>>>> The question should be "How to use the tags?"
>>>>>
>>>>>> The documentation mentions Acked, Reviewed, and Tested by[1], as
>> signoffs that can be commented on patches. The Review Process[2] section
>> mentions Reviewed and Tested by, but nowhere specifically states what any
>> of these indicate.
>>>>>>
>>>>>> Offered below is my current understanding of the Acked-by; Reviewed-
>> by; and Tested-by tags, in order of least-powerful first:
>>>>>>
>>>>>>
>>>>>> 3) Tested-by: (least powerful)
>>>>>>   - Indicates having passed testing of functionality, and works as
>> expected for Tester
>>>>>>   - Does NOT include full code review (instead use Reviewed by)
>>>>>>   - Does NOT indicate that the Tester understands architecture
>>>>>> (instead use Acked by)
>>>>>>
>>>>>>
>>>>>> 2) Reviewed-by:
>>>>>>   - Indicates having passed code-review, checkpatch and compilation
>>>>>> testing by Reviewer
>>>>>
>>>>> Compilation testing is done by the CI.
>>>>> The reviewer must just check the results.
>>>>>
>>>>>>   - Does NOT include full testing of functionality (instead use
>> Tested-by)
>>>>>>   - Does NOT indicate that the Reviewer understands architecture
>>>>>> (instead use Acked by)
>>>>>
>>>>> I disagree here.
>>>>> The reviewer must understand the impacts of the patch.
>>>>> That's why a Reviewed-by tag is really strong.
>>>>
>>>> From what I understand, 'Reviewed-by' and 'Acked-by' are the other
>>>> way around.
>>>> - Acked-by is intent that 'I agree with change'.
>>>> - Reviewed-by is 'I vouch for the changes' either through review or
>>>>   testing or both.
>>>>
>>>
>>> Other way round in what way - compared to proposed by Harry or by
>>> Thomas? Which do you view as the stronger indication that the patch is
>>> ok?
>>
>> Sorry, I should have posted this against Harry's mail rather than Thomas'.
>> 'Other way round' as compared to Harry's text.
>> Reviewed-by is a strong indication, in my understanding.
> 
> 
> Hi,
> 
> Maybe we should just follow the Kernel guidelines on this:
> 
> https://www.kernel.org/doc/html/latest/process/submitting-patches.html#when-to-use-acked-by-and-cc
> 
> 
> And to, save-you-a-click(tm) here is the relevant sections of the doc:
> 
> 
> 12) When to use Acked-by: and Cc:
> ---------------------------------
> 
> The Signed-off-by: tag indicates that the signer was involved in the
> development of the patch, or that he/she was in the patch's delivery path.
> 
> If a person was not directly involved in the preparation or handling of a
> patch but wishes to signify and record their approval of it then they can
> ask to have an Acked-by: line added to the patch's changelog.
> 
> Acked-by: is often used by the maintainer of the affected code when that
> maintainer neither contributed to nor forwarded the patch.
> 
> Acked-by: is not as formal as Signed-off-by:.  It is a record that the acker
> has at least reviewed the patch and has indicated acceptance.  

I want to highlight this part.
Ack means patch _at least_ reviewed, I was expecting a patch not acked
without a review.
That is why I believe Ack is a superset of Reviewed-by. Because it means
patch is reviewed and agreed by a knowledgeable person of that area.

> Hence patch
> mergers will sometimes manually convert an acker's "yep, looks good to me"
> into an Acked-by: (but note that it is usually better to ask for an
> explicit ack).
> 
> Acked-by: does not necessarily indicate acknowledgement of the entire patch.
> For example, if a patch affects multiple subsystems and has an Acked-by: from
> one subsystem maintainer then this usually indicates acknowledgement of just
> the part which affects that maintainer's code.  Judgement should be used here.
> When in doubt people should refer to the original discussion in the mailing
> list archives.
> 
> If a person has had the opportunity to comment on a patch, but has not
> provided such comments, you may optionally add a ``Cc:`` tag to the patch.
> This is the only tag which might be added without an explicit action by the
> person it names - but it should indicate that this person was copied on the
> patch.  This tag documents that potentially interested parties
> have been included in the discussion.
> 
> 
> 13) Using Reported-by:, Tested-by:, Reviewed-by:, Suggested-by: and Fixes:
> --------------------------------------------------------------------------
> 
> The Reported-by tag gives credit to people who find bugs and report them and it
> hopefully inspires them to help us again in the future.  Please note that if
> the bug was reported in private, then ask for permission first before using the
> Reported-by tag.
> 
> A Tested-by: tag indicates that the patch has been successfully tested (in
> some environment) by the person named.  This tag informs maintainers that
> some testing has been performed, provides a means to locate testers for
> future patches, and ensures credit for the testers.
> 
> Reviewed-by:, instead, indicates that the patch has been reviewed and found
> acceptable according to the Reviewer's Statement:
> 
> Reviewer's statement of oversight
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> 
> By offering my Reviewed-by: tag, I state that:
> 
> 	 (a) I have carried out a technical review of this patch to
> 	     evaluate its appropriateness and readiness for inclusion into
> 	     the mainline kernel.
> 
> 	 (b) Any problems, concerns, or questions relating to the patch
> 	     have been communicated back to the submitter.  I am satisfied
> 	     with the submitter's response to my comments.
> 
> 	 (c) While there may be things that could be improved with this
> 	     submission, I believe that it is, at this time, (1) a
> 	     worthwhile modification to the kernel, and (2) free of known
> 	     issues which would argue against its inclusion.
> 
> 	 (d) While I have reviewed the patch and believe it to be sound, I
> 	     do not (unless explicitly stated elsewhere) make any
> 	     warranties or guarantees that it will achieve its stated
> 	     purpose or function properly in any given situation.
> 
> A Reviewed-by tag is a statement of opinion that the patch is an
> appropriate modification of the kernel without any remaining serious
> technical issues.  Any interested reviewer (who has done the work) can
> offer a Reviewed-by tag for a patch.  

And this part, anyone can do the review, and add reviewed-by to state
his/her opinion. It can be strong or not depending who did the review.

> This tag serves to give credit to
> reviewers and to inform maintainers of the degree of review which has been
> done on the patch.  Reviewed-by: tags, when supplied by reviewers known to
> understand the subject area and to perform thorough reviews, will normally
> increase the likelihood of your patch getting into the kernel.

And Reviewed-by helps maintainers to Ack the patch.

> 
> A Suggested-by: tag indicates that the patch idea is suggested by the person
> named and ensures credit to the person for the idea. Please note that this
> tag should not be added without the reporter's permission, especially if the
> idea was not posted in a public forum. That said, if we diligently credit our
> idea reporters, they will, hopefully, be inspired to help us again in the
> future.
> 
> A Fixes: tag indicates that the patch fixes an issue in a previous commit. It
> is used to make it easy to determine where a bug originated, which can help
> review a bug fix. This tag also assists the stable kernel team in determining
> which stable kernel versions should receive your fix. This is the preferred
> method for indicating a bug fixed by the patch. See :ref:`describe_changes`
> for more details.
> 
> 
> 
> 
> 
>