linux-kernel - Re: [PATCH v5 2/3] ACPI: allow longer device IDs

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Date:   Mon, 28 Feb 2022 00:27:09 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Alexander Graf <graf@...zon.com>
Cc:     Len Brown <lenb@...nel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v5 2/3] ACPI: allow longer device IDs

Hey again,

On Sun, Feb 27, 2022 at 1:43 PM Jason A. Donenfeld <Jason@...c4.com> wrote:
>
> Hi Alex,
>
> On Sun, Feb 27, 2022 at 12:42:03PM +0100, Alexander Graf wrote:
> > > To allow device drivers to match identifiers that exceed the 9 byte
> > > limit, this simply ups the length to 16, just like it was before the
> > > aforementioned commit. Empirical testing indicates that this
> >
> >
> > This is only true for 64bit systems where padding automatically bloated
> > to 9 byte array to 16. I still believe the patch is fine as it is, but
> > there will be minor .rodata overhead on 32bit targets which you may want
> > to quantify in the patch description.
>
> Good point. So I just tried this out with a 32-bit i686 kernel and the
> results are the same again for the size of vmlinux. I then ran `objdump
> --headers` and looked at the size of the .rodata section, where it's
> also the same. I'm not quite sure what to make of this, as it's not what
> I was expecting, but I think I tested it right. So maybe we're lucky
> here?

I tried a little harder to get _some_ difference on 32-bit, and
managed to get one by doing i386_defconfig and then switching off
modules to make all M into Y, and then compared sizes:

vmlinux: 25590780 -> 25598972, so a 0.032% increase.
bzImage: 8698944 -> 8699424, so a 0.0055% increase.

So it does increase, ever so slightly, but a) on 32-bit, and b) a
super, super tiny amount.

In other words, I still think this patch is very much a-okay. But very
eager to hear from Rafael on the approach.

Jason