linux-kernel - Re: [PATCH 08/14] zram: check that backends array has at least one backend

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Date: Mon, 6 May 2024 14:20:37 +0900
From: Masahiro Yamada <masahiroy@...nel.org>
To: Sergey Senozhatsky <senozhatsky@...omium.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kbuild@...r.kernel.org, 
	kernel test robot <lkp@...el.com>, Minchan Kim <minchan@...nel.org>, llvm@...ts.linux.dev, 
	oe-kbuild-all@...ts.linux.dev, 
	Linux Memory Management List <linux-mm@...ck.org>, linux-kernel@...r.kernel.org, 
	linux-block@...r.kernel.org, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH 08/14] zram: check that backends array has at least one backend

On Mon, May 6, 2024 at 12:22 PM Sergey Senozhatsky
<senozhatsky@...omium.org> wrote:
>
> On (24/05/05 15:48), Sergey Senozhatsky wrote:
> > On (24/05/05 14:13), Sergey Senozhatsky wrote:
> > > On (24/05/05 13:39), Sergey Senozhatsky wrote:
> > > [..]
> > > > > I guess just pick one if none were selected.
> > >
> > > How do I pick one if none were selected? Does Kconfig support
> > > something like that?
> >
> > This triggers Kconfig error:
> >
> > config ZRAM_EMPTY_BACKENDS_FIXUP
> >        bool
> >        depends on ZRAM && !ZRAM_BACKEND_LZO && !ZRAM_BACKEND_LZ4 && \
> >                !ZRAM_BACKEND_LZ4HC && !ZRAM_BACKEND_ZSTD && \
> >                !ZRAM_BACKEND_DEFLATE
> >        select ZRAM_BACKEND_LZO
> >
> >
> > drivers/block/zram/Kconfig:17:error: recursive dependency detected!
> > drivers/block/zram/Kconfig:17:  symbol ZRAM_BACKEND_LZO is selected by ZRAM_EMPTY_BACKENDS_FIXUP
> > drivers/block/zram/Kconfig:52:  symbol ZRAM_EMPTY_BACKENDS_FIXUP depends on ZRAM_BACKEND_LZO
> >
> >
> > I'm a little surprised by this - EMPTY_BACKENDS_FIXUP does not depend
> > on ZRAM_BACKEND_LZO, it depends on NOT ZRAM_BACKEND_LZO.
> >
> > Let me Cc linux-kbuild. Kbuild folks, how do I workaround this?
>
> Is this how one does it?
>
> config ZRAM_BACKEND_LZO
>        bool "lzo and lzo-rle compression support"
>        depends on ZRAM
>        default y if !ZRAM_BACKEND_LZ4 && !ZRAM_BACKEND_LZ4HC && \
>                !ZRAM_BACKEND_ZSTD && !ZRAM_BACKEND_DEFLATE
>        default n
>        select LZO_COMPRESS
>        select LZO_DECOMPRESS
>
>
> User still can select N and then we'll have empty backends, but
> at least default is Y if none of the algorithms were selected.
> Is it good enough?


I interpret this sentence into:

"randconfig will eventually disable all ZRAM_BACKEND_*,
causing the build error again.
Is it good enough, Arnd?"



Some possible solutions:



config ZRAM_BACKEND_FORCE_LZO
        def_bool !ZRAM_BACKEND_LZ4 && !ZRAM_BACKEND_LZ4HC && \
                 !ZRAM_BACKEND_ZSTD && !ZRAM_BACKEND_DEFLATE
        depends on ZRAM
        select ZRAM_BACKEND_LZO

config ZRAM_BACKEND_LZO
        bool "lzo and lzo-rle compression support"
        depends on ZRAM
        select LZO_COMPRESS
        select LZO_DECOMPRESS


OR


config ZRAM_BACKEND_FORCE_LZO
        def_bool !ZRAM_BACKEND_LZ4 && !ZRAM_BACKEND_LZ4HC && \
                 !ZRAM_BACKEND_ZSTD && !ZRAM_BACKEND_DEFLATE

config ZRAM_BACKEND_LZO
        bool "lzo and lzo-rle compression support" if !ZRAM_BACKEND_FORCE_LZO
        depends on ZRAM
        default ZRAM_BACKEND_FORCE_LZO
        select LZO_COMPRESS
        select LZO_DECOMPRESS




BTW, "default n" you are adding are redundant.

-- 
Best Regards
Masahiro Yamada