linux-kernel - Re: [PATCH] kunit: tool: Force the use of the 'tty' console for UML

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Date:   Wed, 6 Jan 2021 12:29:12 +0800
From:   David Gow <davidgow@...gle.com>
To:     Shuah Khan <skhan@...uxfoundation.org>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Petr Mladek <pmladek@...e.com>,
        Brendan Higgins <brendanhiggins@...gle.com>,
        Senozhatsky <sergey.senozhatsky@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        KUnit Development <kunit-dev@...glegroups.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kunit: tool: Force the use of the 'tty' console for UML

On Wed, Jan 6, 2021 at 3:52 AM Shuah Khan <skhan@...uxfoundation.org> wrote:
>
> On 1/5/21 11:57 AM, Andy Shevchenko wrote:
> > On Tue, Jan 05, 2021 at 09:34:33AM -0700, Shuah Khan wrote:
> >> On 1/5/21 9:21 AM, Petr Mladek wrote:
> >>> On Mon 2021-01-04 09:23:57, Shuah Khan wrote:
> >>>> On 12/22/20 4:11 AM, Andy Shevchenko wrote:
> >>>>> On Mon, Dec 21, 2020 at 11:39:00PM -0800, David Gow wrote:
> >>>>>> kunit_tool relies on the UML console outputting printk() output to the
> >>>>>> tty in order to get results. Since the default console driver could
> >>>>>> change, pass 'console=tty' to the kernel.
> >>>>>>
> >>>>>> This is triggered by a change[1] to use ttynull as a fallback console
> >>>>>> driver which -- by chance or by design -- seems to have changed the
> >>>>>> default console output on UML, breaking kunit_tool. While this may be
> >>>>>> fixed, we should be less fragile to such changes in the default.
> >>>>>>
> >>>>>> [1]:
> >>>>>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=757055ae8dedf5333af17b3b5b4b70ba9bc9da4e
> >>>>>
> >>>>> Reported-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> >>>>> Tested-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> >>>>>
> >>>>
> >>>> Thank you all. Now in linux-kselftest kunit-fixes branch.
> >>>>
> >>>> Will send this up for rc3.
> >>>>
> >>>> Sorry for the delay - have been away from the keyboard for a
> >>>> bit.
> >>>
> >>> JFYI, I am not sure that this is the right solution. I am
> >>> looking into it, see
> >>> https://lore.kernel.org/linux-kselftest/X%2FSRA1P8t+ONZFKb@alley/
> >>> for more details.
> >>>
> >>
> >> Thanks Petr. I will hold off on sending the patch up to Linus and
> >> let you find a the right solution.
> >
> > Please. leave it in Linux Next at least. Otherwise kunit will be broken for a
> > long time which is not good.
> >
> >
>
> Yes. That is the plan. It will be in there until real fix comes in.
>

Thanks, Shuah.

Personally, I think that this patch makes some sense to keep even if
the underlying issue with ttynull is resolved. Given that kunit.py
requires the console output, explicitly stating we want console=tty
set is probably worth doing rather than relying on it being the
default. That being said, I definitely agree that this patch doesn't
fix the underlying issue with UML/ttynull: it just makes the kunit.py
script less sensitive to such changes (which, while unlikely, could
potentially occur legitimately down the track).

Cheers,
-- David