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Date:   Wed, 01 Sep 2021 17:34:07 +0000
To:     Krish Jain <krishjain02939@gmail.com>,
        Greg KH <gregkh@linuxfoundation.org>
From:   Bryan Brattlof <hello@bryanbrattlof.com>
Cc:     "Fabio M. De Francesco" <fmdefrancesco@gmail.com>,
        linux-staging@lists.linux.dev, linux-kernel@vger.kernel.org
Reply-To: Bryan Brattlof <hello@bryanbrattlof.com>
Subject: Re: [PATCH] Declare the file_operations struct as const
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On this day, September  1, 2021, thus sayeth Greg KH:
> On Wed, Sep 01, 2021 at 05:34:36PM +0200, Krish Jain wrote:
> >
> > Can you tell me why this is the case?
>
> Again, it depends on your kernel configuration file as to what will, or
> will not, be built.
>
> If you have some things set as modules, they can be built as a module,
> but the ashmem code can not be built as a module, so you would never
> build it if you did the above line.
>
> Here, look at this sequence, starting with a tree that does nothing if I
> do a simple 'make' in it, as the whole kernel is already built, and
> ashmem is enabled in the kernel configuration
>
> $ grep ASHMEM .config
> CONFIG_ASHMEM=3Dy
> $ make
> $
>
> So, let's change the time stamp on the ashmem.c file and see what gets
> built if you use the M=3D option:
>
> $ touch drivers/staging/android/ashmem.c
> $ make M=3Ddrivers/staging/android
>   MODPOST drivers/staging/android/Module.symvers
> $
>
> Nothing gets built as ashmem is NOT a module, and M=3D only builds any
> modules in the directory you specified.
>
> But, if you tell make to just build the whole subdirectory, no matter
> what the setting is, it will be built:
>
> $ make drivers/staging/android/
>   CALL    scripts/checksyscalls.sh
>   CALL    scripts/atomic/check-atomics.sh
>   DESCEND objtool
>   CC      drivers/staging/android/ashmem.o
>   AR      drivers/staging/android/built-in.a
> $
>
> So that's the difference, "M=3D" builds modules in that directory, but if
> you tell it to build the subdir, everything in there that needs to be
> built, will be built.
>
> Be careful about your kernel configuration, that is the key for what
> will, and will not, be built.
>

Ouch...

I want to *really* apologize to you Krish for introducing so much
confusion while you, and apparently I, am still learning. And for your
persistence with seeking the correct answer here Krish.

I did not notice that this could only be build as a built-in object.
Thank you Greg for pointing out my mistake, and I apologize for dragging
this out longer than it had to and the frustration this caused.

It seems I will be reading the documentation again, along with Greg's
book recommendation, "Linux Kernel in a Nutshell" over this merge
window.

Thank you again Krish and Greg
~Bryan