[RFC PATCH] kbuild: only prompt for compressors that are actually usable

[Vegard Nossum <vegard.nossum@oracle.com>]

If a given compression algorithm for the kernel image is not usable on
the host system, there is no point prompting for it.

We can use the kconfig preprocessing feature to check if the command is
available or not. I've chosen to test this using "which", which exits
with success if the given command exists in PATH (or it is an absolute
path), which mimics exactly how it would be found in the kernel's
Makefiles.

This uses the make variables that are set in Makefile and/or the
command line, so you can do e.g.

  make KGZIP=pigz menuconfig

and it will test for the correct program.

I am intentionally adding these dependencies to e.g. KERNEL_LZ4, as
opposed to HAVE_KERNEL_LZ4, since the latter are "select"-ed
unconditionally by the architectures that use them, so they are not
suitable for depending on anything else.

I've put RFC in the subject as maybe there are downsides to this that
I'm not aware of.

Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
---
 init/Kconfig | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/init/Kconfig b/init/Kconfig
index 11f8a845f259d..f03f2b7962027 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -250,6 +250,7 @@ choice
 config KERNEL_GZIP
 	bool "Gzip"
 	depends on HAVE_KERNEL_GZIP
+	depends on $(success,which $(KGZIP))
 	help
 	  The old and tried gzip compression. It provides a good balance
 	  between compression ratio and decompression speed.
@@ -257,6 +258,7 @@ config KERNEL_GZIP
 config KERNEL_BZIP2
 	bool "Bzip2"
 	depends on HAVE_KERNEL_BZIP2
+	depends on $(success,which $(KBZIP2))
 	help
 	  Its compression ratio and speed is intermediate.
 	  Decompression speed is slowest among the choices.  The kernel
@@ -267,6 +269,7 @@ config KERNEL_BZIP2
 config KERNEL_LZMA
 	bool "LZMA"
 	depends on HAVE_KERNEL_LZMA
+	depends on $(success,which $(LZMA))
 	help
 	  This compression algorithm's ratio is best.  Decompression speed
 	  is between gzip and bzip2.  Compression is slowest.
@@ -275,6 +278,7 @@ config KERNEL_LZMA
 config KERNEL_XZ
 	bool "XZ"
 	depends on HAVE_KERNEL_XZ
+	depends on $(success,which $(XZ))
 	help
 	  XZ uses the LZMA2 algorithm and instruction set specific
 	  BCJ filters which can improve compression ratio of executable
@@ -290,6 +294,7 @@ config KERNEL_XZ
 config KERNEL_LZO
 	bool "LZO"
 	depends on HAVE_KERNEL_LZO
+	depends on $(success,which $(KLZOP))
 	help
 	  Its compression ratio is the poorest among the choices. The kernel
 	  size is about 10% bigger than gzip; however its speed
@@ -298,6 +303,7 @@ config KERNEL_LZO
 config KERNEL_LZ4
 	bool "LZ4"
 	depends on HAVE_KERNEL_LZ4
+	depends on $(success,which $(LZ4))
 	help
 	  LZ4 is an LZ77-type compressor with a fixed, byte-oriented encoding.
 	  A preliminary version of LZ4 de/compression tool is available at
@@ -310,6 +316,7 @@ config KERNEL_LZ4
 config KERNEL_ZSTD
 	bool "ZSTD"
 	depends on HAVE_KERNEL_ZSTD
+	depends on $(success,which $(ZSTD))
 	help
 	  ZSTD is a compression algorithm targeting intermediate compression
 	  with fast decompression speed. It will compress better than GZIP and
-- 
2.23.0.718.g5ad94255a8