[Buildroot] [PATCH] linux: provide symlink dtc->linux-dtc is there is no dtc yet

[Arnout Vandecappelle <arnout@mind.be>]

 Hi Thomas,

 In the subject: is -> if

On 08-01-16 12:30, Thomas De Schampheleire wrote:
> From: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
> 
> Commit ab74e09eb4e28dab8bed8d783c5f0464d39a32e7 renamed the dtc host tool
> provided by linux to linux-dtc to avoid clashes with the dtc host tool
> provided by host-dtc.
> 
> However, external scripting may well rely on the existence of a device tree
> compiler as $(HOST_DIR)/usr/bin/dtc, regardless of its source. Changing
> these external scripts to use linux-dtc means that the scripts need to be
> aware of the buildroot release they are working with, which is not very
> nice.
> 
> Add a symlink dtc->linux-dtc when no $(HOST_DIR)/usr/bin/dtc is present.
> When host-dtc is not enabled, the end result will be dtc and
> linux-dtc representing the same thing.
> When host-dtc is enabled, either it is build before linux and no symlink
> is created at any time, or it is build after linux, and the 'install'
> command in host-dtc will overwrite the symlink with a proper dtc. In both
> cases, the end result will be dtc and linux-dtc representing a different
> thing.

 Or, when we eventually enable top-level parallel build, dtc and linux-dtc may
be installed in parallel...

 Why do you always have these controversial patches, Thomas :-)

 How about a simple:

	ln -s linux-dtc $(HOST_DIR)/usr/bin/dtc 2>/dev/null

 i.e. just try to create the symlink and ignore errors. Creating a symlink is
atomic.

 Argh, no, that leaves another race condition: the install of the dtc package
will do stat, (unlink), open, so between the stat or unlink and the open there
is a window in which the symlink will succeed. Then the version from host-dtc
will overwrite linux-dtc, which is not what we want...

 I just love race conditions :-)


 Perhaps a better solution is to revisit ab74e09e completely. The intention of
that commit was to have the linux version of dtc available for scripts. But is
there ever any reason for a script _not_ to use the linux version? So what we
would actually like is to keep a single dtc: the linux one if it is available,
that one will be used, and host-dtc is a NOP. If not, then host-dtc kicks into
action.

 So we'd add a dependency from host-dtc to linux if BR2_LINUX_KERNEL=y, and in
the build and install commands test if dtc is available in the kernel (testing
$(HOST_DIR)/usr/bin/dtc isn't sufficient, because it's possible that we rebuilt
host-dtc and then we do want to overwrite it). It's a bit kludgy but it would
work I think.

 PeterS, PeterK, what do you think?


 That said, we don't have parallel build at the moment, and my proposed
alternative is definitely going to be controversial, so we should probably
accept this one as is and revisit later. So:

Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>


 Regards,
 Arnout

> 
> Cc: Peter Korsgaard <peter@korsgaard.com>
> Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
> ---
>  linux/linux.mk | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/linux/linux.mk b/linux/linux.mk
> index 045294b..f5c629c 100644
> --- a/linux/linux.mk
> +++ b/linux/linux.mk
> @@ -339,6 +339,9 @@ define LINUX_INSTALL_HOST_TOOLS
>  	# Installing dtc (device tree compiler) as host tool, if selected
>  	if grep -q "CONFIG_DTC=y" $(@D)/.config; then 	\
>  		$(INSTALL) -D -m 0755 $(@D)/scripts/dtc/dtc $(HOST_DIR)/usr/bin/linux-dtc ;	\
> +		if [ ! -e $(HOST_DIR)/usr/bin/dtc ]; then	\
> +			ln -sf linux-dtc $(HOST_DIR)/usr/bin/dtc ;	\
> +		fi	\
>  	fi
>  endef
>  
> 


-- 
Arnout Vandecappelle                          arnout at mind be
Senior Embedded Software Architect            +32-16-286500
Essensium/Mind                                http://www.mind.be
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