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Subject: Re: [PATCH v3] bitfield.h: add FIELD_MAX() and field_max()
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Maxim Kuvyrkov <maxim.kuvyrkov@linaro.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        "David S. Miller" <davem@davemloft.net>,
        Johannes Berg <johannes@sipsolutions.net>,
        Jakub Kicinski <kuba@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Network Development <netdev@vger.kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>
References: <20200311024240.26834-1-elder@linaro.org>
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 <3659efd7-4e72-6bff-5657-c1270e8553f4@linaro.org>
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 <3c878065-8d25-8177-b7c4-9813b60c9ff6@linaro.org>
 <CAKwvOdnZ-QNeYQ_G-aEuo8cC_m68E5mAC4cskwAQpJJQPc1BSg@mail.gmail.com>
From:   Alex Elder <elder@linaro.org>
Message-ID: <efd2c8b1-4efd-572e-10c5-c45f705274d0@linaro.org>
Date:   Wed, 1 Apr 2020 15:21:19 -0500
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On 4/1/20 2:54 PM, Nick Desaulniers wrote:
> On Wed, Apr 1, 2020 at 12:44 PM Alex Elder <elder@linaro.org> wrote:
>>
>> On 4/1/20 2:13 PM, Nick Desaulniers wrote:
>>> On Wed, Apr 1, 2020 at 11:24 AM Alex Elder <elder@linaro.org> wrote:
>>>>
>>>> On 4/1/20 12:35 PM, Nick Desaulniers wrote:
>>>>>> Define FIELD_MAX(), which supplies the maximum value that can be
>>>>>> represented by a field value.  Define field_max() as well, to go
>>>>>> along with the lower-case forms of the field mask functions.
>>>>>>
>>>>>> Signed-off-by: Alex Elder <elder@linaro.org>
>>>>>> Acked-by: Jakub Kicinski <kuba@kernel.org>
>>>>>> ---
>>>>>> v3: Rebased on latest netdev-next/master.
>>>>>>
>>>>>> David, please take this into net-next as soon as possible.  When the
>>>>>> IPA code was merged the other day this prerequisite patch was not
>>>>>> included, and as a result the IPA driver fails to build.  Thank you.
>>>>>>
>>>>>>   See: https://lkml.org/lkml/2020/3/10/1839
>>>>>>
>>>>>>                                      -Alex
>>>>>
>>>>> In particular, this seems to now have regressed into mainline for the 5.7
>>>>> merge window as reported by Linaro's ToolChain Working Group's CI.
>>>>> Link: https://github.com/ClangBuiltLinux/linux/issues/963
>>>>
>>>> Is the problem you're referring to the result of a build done
>>>> in the midst of a bisect?
>>>>
>>>> The fix for this build error is currently present in the
>>>> torvalds/linux.git master branch:
>>>>     6fcd42242ebc soc: qcom: ipa: kill IPA_RX_BUFFER_ORDER
>>>
>>> Is that right? That patch is in mainline, but looks unrelated to what
>>> I'm referring to.
>>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=6fcd42242ebcc98ebf1a9a03f5e8cb646277fd78
>>> From my github link above, the issue I'm referring to is a
>>> -Wimplicit-function-declaration warning related to field_max.
>>> 6fcd42242ebc doesn't look related.
>>
>> I'm very sorry, I pointed you at the wrong commit.  This one is
>> also present in torvalds/linux.git master:
>>
>>   e31a50162feb bitfield.h: add FIELD_MAX() and field_max()
>>
>> It defines field_max() as a macro in <linux/bitfield.h>, and
>> "gsi.c" includes that header file.
>>
>> This was another commit that got added late, after the initial
>> IPA code was accepted.
> 
> Yep, that looks better.

Sorry about that.  The two actually are related in a way, because
without the first one I pointed you at, a *different* problem
involving field_max() gets triggered.  But that's irrelevant to
this discussion...

>>>> I may be mistaken, but I believe this is the same problem I discussed
>>>> with Maxim Kuvyrkov this morning.  A different build problem led to
>>>> an automated bisect, which conluded this was the cause because it
>>>> landed somewhere between the initial pull of the IPA code and the fix
>>>> I reference above.
>>>
>>> Yes, Maxim runs Linaro's ToolChain Working Group (IIUC, but you work
>>> there, so you probably know better than I do), that's the CI I was
>>> referring to.
>>>
>>> I'm more concerned when I see reports of regressions *in mainline*.
>>> The whole point of -next is that warnings reported there get fixed
>>> BEFORE the merge window opens, so that we don't regress mainline.  Or
>>> we drop the patches in -next.
>>
>> Can you tell me where I can find the commit id of the kernel
>> that is being built when this error is reported?  I would
>> like to examine things and build it myself so I can fix it.
>> But so far haven't found what I need to check out.
> 
> From the report: https://groups.google.com/g/clang-built-linux/c/pX-kr_t5l_A

That link doesn't work for me.

> Configuration details:
> rr[llvm_url]="https://github.com/llvm/llvm-project.git"
> rr[linux_url]="https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git"
> rr[linux_branch]="7111951b8d4973bda27ff663f2cf18b663d15b48"

That commit is just the one in which Linux v5.6 is tagged.
It doesn't include any of this code (but it's the last
tagged release that current linus/master is built on).

It doesn't answer my question about what commit id was
used for this build, unfortunately.

> the linux_branch looks like a SHA of what the latest ToT of mainline
> was when the CI ran.
> 
> I was suspecting that maybe there was a small window between the
> regression, and the fix, and when the bot happened to sync.  But it
> seems that: e31a50162feb352147d3fc87b9e036703c8f2636 landed before
> 7111951b8d4973bda27ff663f2cf18b663d15b48 IIUC.

Yes, this:
  e31a50162feb bitfield.h: add FIELD_MAX() and field_max()
landed about 200 commits after the code that needed it.

So there's a chance the kernel that got built was somewhere
between those two, and I believe the problem you point out
would happen in that case.  This is why I started by asking
whether it was something built during a bisect.

It's still not clear to me what happened here.  I can explain
how this *could* happen, but I don't believe problem exists
in the latest upstream kernel commit.

Is there something else you think I should do?

					-Alex

> So I think the bot had your change when it ran, so still seeing a
> failure is curious.  Unless I've misunderstood something.