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Date: Mon, 6 May 2024 12:22:07 +0900
From: Sergey Senozhatsky <senozhatsky@chromium.org>
To: Andrew Morton <akpm@linux-foundation.org>, linux-kbuild@vger.kernel.org
Cc: kernel test robot <lkp@intel.com>, Minchan Kim <minchan@kernel.org>,
	llvm@lists.linux.dev, oe-kbuild-all@lists.linux.dev,
	Linux Memory Management List <linux-mm@kvack.org>,
	linux-kernel@vger.kernel.org, linux-block@vger.kernel.org,
	Sergey Senozhatsky <senozhatsky@chromium.org>
Subject: Re: [PATCH 08/14] zram: check that backends array has at least one
 backend
Message-ID: <20240506032207.GD8623@google.com>
References: <20240503091823.3616962-9-senozhatsky@chromium.org>
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 <20240504071416.GH14947@google.com>
 <20240504161004.f5a0aab5e5aa1033d4696c20@linux-foundation.org>
 <20240505043957.GA8623@google.com>
 <20240505051305.GB8623@google.com>
 <20240505064832.GC8623@google.com>
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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?