linux-kernel - RE: [PATCH] ARM: simplify the build rule of mach-types.h

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Message-ID: <60fb7c6374e74b6d9e7597b19772209a@AcuMS.aculab.com>
Date:   Mon, 31 May 2021 08:59:52 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Masahiro Yamada' <masahiroy@...nel.org>,
        "patches@....linux.org.uk" <patches@....linux.org.uk>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Linus Walleij" <linus.walleij@...aro.org>,
        Russell King <linux@...linux.org.uk>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: RE: [PATCH] ARM: simplify the build rule of mach-types.h

From: Masahiro Yamada
> Sent: 28 May 2021 04:49
> 
> The directory of mach-types.h is created a couple of lines above:
> 
>   _dummy := $(shell [ -d '$(kapi)' ] || mkdir -p '$(kapi)') \
> 
> The 'mkdir -p' command is redundant.
(In the rule itself)
...
>  quiet_cmd_gen_mach = GEN     $@
> -      cmd_gen_mach = mkdir -p $(dir $@) && \
> -		     $(AWK) -f $(filter-out $(PHONY),$^) > $@
> +      cmd_gen_mach = $(AWK) -f $(real-prereqs) > $@

There is a much easier way to get directories created.
gmake lets you define dependencies that only need to exist
(ie there file timestamps are't checked)
These are ideal for creating directories.

So if can define:

%/.:
	mkdir -p $@

You can just use

$(OBJ)/$(FILE): | ${OBJ)/.

to get any directories created.

Annoyingly gmake doesn't seem to support the 'dynamic dependencies'
of SYSV make (it's only useful feature).
So you can't use:
xxxxx: | $$@D/.

	David

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