git grep/sed to standardize "/* SPDX-License-Identifier: "

[Joe Perches]

Almost all source files in the kernel use a standardized SPDX header
at line 1 with a comment /* initiator and terminator */:

/* SPDX-License-Identifier: <license> */

$ git grep -PHn '^/\* SPDX-License-Identifier:.*\*/\s*$' | \
  wc -l
17847

$ git grep -PHn '^/\* SPDX-License-Identifier:.*\*/\s*$' | \
  grep ":1:" | cut -f1 -d":" | grep -oP '\.\w+$' | \
  sort | uniq -c | sort -rn
  16769 .h
    972 .S
     87 .c
      6 .lds
      3 .l
      2 .y
      2 .py
      2 .dtsi
      1 .sh
      1 .dts
      1 .cpp
      1 .bc

But about 2% of the files do not use a use comment termination at
line 1 and use either:

/* SPDX-License-Identifier: <license>
 * additional comment or blank

or

/* SPDX-License-Identifier: <license>
<blank line>

$ git grep -PHn '^/\* SPDX-License-Identifier:(?!.*\*/\s*$)' | \
  wc -l
407

$ git grep -PHn '^/\* SPDX-License-Identifier:(?!.*\*/\s*$)' | \
  grep '\:1:' | cut -f1 -d':' | grep -oP '\.\w+$' | \
  sort | uniq -c | sort -rn
    357 .h
     34 .S
     16 .c

Here's a trivial script to convert and standardize the
first and second lines of these 407 files to make it easier
to categorize and sort.

$ git grep -PHn '^/\* SPDX-License-Identifier:(?!.*\*/\s*$)' | \
  grep ':1:' | cut -f1 -d":" | \
  xargs sed -i -e '1s@[[:space:]]*$@ */@' -r -e '2s@^( \*|)@/*@'