dim_setup: Allow to use other setup as reference

On a slow network, most of the time for a setup is taken by the initial
clone of the kernel repository. It's common for kernel developers to
have a kernel clone lying around, so allow to borrow the git objects
from there.

Testing on a relatively fast connection, the initial checkout goes down
from ~7m45s to 2m21s.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>

dim

@@ -53,6 +53,10 @@ fi
 # prefix for repo directories
 DIM_PREFIX=${DIM_PREFIX:-$HOME/linux}
 
+# location of another dim setup used to speedup clones; default
+# to something that doesn't exist so no cache is used
+DIM_KERNEL_PREFIX=${DIM_KERNEL_REFERENCE:-/dev/fd/-1}
+
 # main maintainer repo under $DIM_PREFIX
 DIM_REPO=${DIM_REPO:-${DIM_DRM_INTEL:-src}}
 
@@ -2321,7 +2325,7 @@ function parse_opt_dim_setup # options
 {
 	local OPTS arg
 
-	OPTS=$(getopt --option y --long yes -n 'dim setup' -- "$@")
+	OPTS=$(getopt --option y --long yes,reference: -n 'dim setup' -- "$@")
 
 	eval set -- "$OPTS"
 
@@ -2331,6 +2335,10 @@ function parse_opt_dim_setup # options
 			ASK_USER_ASSUME_YES=1
 			shift
 			;;
+		--reference)
+			DIM_KERNEL_REFERENCE=$2
+			shift 2
+			;;
 		--)
 			shift; break ;;
 		esac
@@ -2361,7 +2369,8 @@ function dim_setup # options
 
 	if [[ ! -d "$DIM_REPO" ]]; then
 		if ask_user "The DIM_REPO maintainer kernel repository '$DIM_REPO' doesn't exist. Clone upstream?"; then
-			git clone "$linux_upstream_git" "$DIM_REPO"
+			git clone --reference-if-able "$DIM_KERNEL_REFERENCE" --dissociate \
+				"$linux_upstream_git" "$DIM_REPO"
 		fi
 	fi
 

dim.rst

@@ -120,6 +120,13 @@ Available options:
                         Assume "yes" to all prompts or default values so
                         it can be executed non-attended
 
+--reference <directory>
+                        Use another kernel checkout to borrow the git objects
+                        from while cloning so it reduces the network transfer
+                        amount.  The final repo is then dissociated from the
+                        passed reference.  See --reference and --dissociate in
+                        GIT-CLONE(1).
+
 update-branches
 ---------------
 Updates all maintainer branches. Useful to synchronize all branches when other