linux-kernel - Re: [PATCH] kbuild: Enable armthumb BCJ filter for Thumb-2 kernel

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Date:   Fri, 19 Nov 2021 21:46:47 +0200
From:   Lasse Collin <lasse.collin@...aani.org>
To:     Jubin Zhong <zhongjubin@...wei.com>
Cc:     <akpm@...ux-foundation.org>, <wangfangpeng1@...wei.com>,
        <liaohua4@...wei.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kbuild: Enable armthumb BCJ filter for Thumb-2 kernel

On 2021-11-19 Jubin Zhong wrote:
> xz_wrap.sh use $SRCARCH to detect the BCJ filter. However, assigning
> arm BCJ filter to Thumb-2 kernel is not optimal. In my case, about 5%
> decrease of image size is observed with armthumb BCJ filter:
> 
> Test results:
>   hardware:      QEMU emulator version 3.1.0
>   config:        vexpress_defconfig with THUMB2_KERNEL & KERNEL_XZ on
>   arm BCJ:       4029808
>   armthumb BCJ:  3827280
> 
> Choose armthumb BCJ filter for Thumb-2 kernel to make smaller images.

I didn't test the patch but it looks reasonable to me. Below are a small
optimization idea and two very minor style suggestions.

> --- a/lib/decompress_unxz.c
> +++ b/lib/decompress_unxz.c
> @@ -131,6 +131,9 @@
>  #ifdef CONFIG_ARM
>  #	define XZ_DEC_ARM
>  #endif
> +#ifdef CONFIG_THUMB2_KERNEL
> +#	define XZ_DEC_ARMTHUMB
> +#endif
>  #ifdef CONFIG_IA64
>  #	define XZ_DEC_IA64
>  #endif

If a Thumb-2 kernel will always use the ARM-Thumb BCJ filter, one can
save a few bytes from the pre-boot code by omitting the ARM BCJ filter:

--- a/lib/decompress_unxz.c
+++ b/lib/decompress_unxz.c
@@ -129,7 +129,11 @@
 #	define XZ_DEC_POWERPC
 #endif
 #ifdef CONFIG_ARM
-#	define XZ_DEC_ARM
+#	ifdef CONFIG_THUMB2_KERNEL
+#		define XZ_DEC_ARMTHUMB
+#	else
+#		define XZ_DEC_ARM
+#	endif
 #endif
 #ifdef CONFIG_IA64
 #	define XZ_DEC_IA64

> --- a/scripts/xz_wrap.sh
> +++ b/scripts/xz_wrap.sh
> @@ -8,6 +8,7 @@
>  # This file has been put into the public domain.
>  # You can do whatever you want with this file.
>  #
> +. include/config/auto.conf

I suggest adding an empty line before this new line so that it is
clearly separated from the header comment.

> +if [ -n "${CONFIG_THUMB2_KERNEL}" ];then

I suggest adding a space after the semi-colon: ]; then

With or without the above modifications:

Acked-by: Lasse Collin <lasse.collin@...aani.org>

-- 
Lasse Collin