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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Wed, 10 Nov 2021 20:07:33 +0100
Message-ID: <CAMuHMdX_h9Tz8C-2SYyrS_G5BBbBNctqRA1mgKbhmYJxWzF-hg@mail.gmail.com>
Subject: Re: [PATCH/RFC] of: Shrink struct of_device_id
To:     Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Cc:     Rob Herring <robh+dt@kernel.org>,
        Frank Rowand <frowand.list@gmail.com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        "open list:BROADCOM NVRAM DRIVER" <linux-mips@vger.kernel.org>,
        linux-riscv <linux-riscv@lists.infradead.org>,
        Linux-Renesas <linux-renesas-soc@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Precedence: bulk

Hi Rasmus,

On Wed, Nov 10, 2021 at 5:51 PM Rasmus Villemoes
<rasmus.villemoes@prevas.dk> wrote:
> On 10/11/2021 17.23, Geert Uytterhoeven wrote:
> > Currently struct of_device_id is 196 (32-bit) or 200 (64-bit) bytes
> > large.  It contains fixed-size strings for a name, a type, and a
> > compatible value, but the first two are barely used.
> > OF device ID tables contain multiple entries, plus an empty sentinel
> > entry.
> >
> > Statistics for my current kernel source tree:
> >   - 4487 tables with 16836 entries (3367200 bytes)
> >   - 176 names (average 6.7 max 23 chars)
> >   - 66 types (average 5.1 max 21 chars)
> >   - 12192 compatible values (average 18.0 max 45 chars)
> > Taking into account the minimum needed size to store the strings, only
> > 6.9% of the allocated space is used...
> >
> > Reduce kernel size by reducing the sizes of the fixed strings by one
> > half.
>
> Tried something like this 2.5 years ago:
> https://lore.kernel.org/lkml/20190425203101.9403-1-linux@rasmusvillemoes.dk/

I wasn't aware of that.  I reworked some code which used multiple
of_find_compatible_node() calls before, and noticed the end result
had grown a lot due to the sheer size of of_device_id
("[PATCH] soc: renesas: Consolidate product register handling",
 https://lore.kernel.org/all/057721f46c7499de4133135488f0f3da7fb39265.1636570669.git.geert+renesas@glider.be).

> I think that there might be some not-in-tree code that relies on the
> existing layout. I considered adding a CONFIG_ knob, either for these
> sizes in particular, or more generally a def_bool y "CONFIG_LEGACY"
> which embedded folks that build the entire distro from source and don't
> have any legacy things can turn off, and then get more sensible defaults
> all around.

Most of that should have been gone since the #ifdef KERNEL was removed
from include/linux/mod_devicetable.h in commit 6543becf26fff612
("mod/file2alias: make modalias generation safe for cross compiling").
Of course you can never know for sure...

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds