linux-kernel - Re: [PATCH 1/2] kconfig: remove unneeded symbol

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Date: Tue, 23 Jan 2024 16:11:04 +0100
From: Yoann Congal <yoann.congal@...le.fr>
To: Geert Uytterhoeven <geert@...ux-m68k.org>,
 Masahiro Yamada <masahiroy@...nel.org>
Cc: linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
 Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
 Vegard Nossum <vegard.nossum@...cle.com>
Subject: Re: [PATCH 1/2] kconfig: remove unneeded symbol_empty variable

Le 23/01/2024 à 13:54, Geert Uytterhoeven a écrit :
> Hi Yamada-san,

Hello,
 
> On Sat, Nov 25, 2023 at 5:36 PM Masahiro Yamada <masahiroy@...nel.org> wrote:
>> This is used only for initializing other variables.
>>
>> Use the empty string "".
>>
>> Please note newval.tri is unused for S_INT/HEX/STRING.
>>
>> Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
> 
> Thanks for your patch, which is now commit 4e244c10eab345a7
> ("kconfig: remove unneeded symbol_empty variable") in v6.8-rc1.
> 
> When running "make <foo>_defconfig" with <foo>_defconfig an SMP
> defconfig without explicit configuration of CONFIG_LOG_CPU_MAX_BUF_SHIFT,
> the aforementioned commit causes a change in the generated .config:
> 
> -CONFIG_LOG_CPU_MAX_BUF_SHIFT=12
> +CONFIG_LOG_CPU_MAX_BUF_SHIFT=0
> 
> It looks like CONFIG_BASE_SMALL=0 is treated as a string instead of
> the integer number zero?
> 
> init/Kconfig=config LOG_CPU_MAX_BUF_SHIFT
> init/Kconfig-   int "CPU kernel log buffer size contribution (13 => 8
> KB, 17 => 128KB)"
> init/Kconfig-   depends on SMP
> init/Kconfig-   range 0 21
> init/Kconfig:   default 12 if !BASE_SMALL
> init/Kconfig:   default 0 if BASE_SMALL
> 
> Note that reverting 4e244c10eab345a7 is not sufficient to fix the issue.
> Also reverting commit 6262afa10ef7cc8f ("kconfig: default to zero if
> int/hex symbol lacks default property") does fix it.

(Since I'd really like 6262afa10ef7cc8f ("kconfig: default to zero if int/hex symbol lacks default property") to stay, allow me to try to help)

The problem is quite easy to reproduce:
  $ make x86_64_defconfig
  $ grep 'LOG_CPU_MAX_BUF_SHIFT\|BASE_SMALL\|BASE_FULL' .config
  CONFIG_LOG_CPU_MAX_BUF_SHIFT=0
  CONFIG_BASE_FULL=y
  CONFIG_BASE_SMALL=0
Here, CONFIG_LOG_CPU_MAX_BUF_SHIFT should be 12 not 0.

For what it is worth, CONFIG_BASE_SMALL is defined as an int but is only used as a bool :
   $ git grep BASE_SMALL
  arch/x86/include/asm/mpspec.h:#if CONFIG_BASE_SMALL == 0
  drivers/tty/vt/vc_screen.c:#define CON_BUF_SIZE (CONFIG_BASE_SMALL ? 256 : PAGE_SIZE)
  include/linux/threads.h:#define PID_MAX_DEFAULT (CONFIG_BASE_SMALL ? 0x1000 : 0x8000)
  include/linux/threads.h:#define PID_MAX_LIMIT (CONFIG_BASE_SMALL ? PAGE_SIZE * 8 : \
  include/linux/udp.h:#define UDP_HTABLE_SIZE_MIN         (CONFIG_BASE_SMALL ? 128 : 256)
  include/linux/xarray.h:#define XA_CHUNK_SHIFT           (CONFIG_BASE_SMALL ? 4 : 6)
  init/Kconfig:   default 12 if !BASE_SMALL
  init/Kconfig:   default 0 if BASE_SMALL
  init/Kconfig:config BASE_SMALL
  kernel/futex/core.c:#if CONFIG_BASE_SMALL
  kernel/user.c:#define UIDHASH_BITS      (CONFIG_BASE_SMALL ? 3 : 7)

Maybe we should change CONFIG_BASE_SMALL to the bool type?

I'll poke around to see if I can understand why a int="0" is true for kconfig.

Regards,
-- 
Yoann Congal
Smile ECS - Tech Expert