4 files changed, 82 insertions, 26 deletions

diff --git a/docs/cli.md b/docs/cli.md
index 8ee8ab0e8..7c3effcc0 100644
--- a/docs/cli.md
+++ b/docs/cli.md
@@ -71,14 +71,36 @@ There are some limitations to the local CLI compared to the global CLI:
 
 ## `qmk cformat`
 
-This command formats C code using clang-format. Run it with no arguments to format all core code, or pass filenames on the command line to run it on specific files.
+This command formats C code using clang-format. 
 
-**Usage**:
+Run it with no arguments to format all core code that has been changed. Default checks `origin/master` with `git diff`, branch can be changed using `-b <branch_name>`
+
+Run it with `-a` to format all core code, or pass filenames on the command line to run it on specific files.
+
+**Usage for specified files**:
 
 ```
 qmk cformat [file1] [file2] [...] [fileN]
 ```
 
+**Usage for all core files**:
+
+```
+qmk cformat -a
+```
+
+**Usage for only changed files against origin/master**:
+
+```
+qmk cformat
+```
+
+**Usage for only changed files against branch_name**:
+
+```
+qmk cformat -b branch_name
+```
+
 ## `qmk compile`
 
 This command allows you to compile firmware from any directory. You can compile JSON exports from <https://config.qmk.fm>, compile keymaps in the repo, or compile the keyboard in the current working directory.
diff --git a/lib/python/qmk/cli/cformat.py b/lib/python/qmk/cli/cformat.py
index de55218ae..7e3a91dcf 100644
--- a/lib/python/qmk/cli/cformat.py
+++ b/lib/python/qmk/cli/cformat.py
@@ -1,16 +1,14 @@
 """Format C code according to QMK's style.
 """
-import os
 import subprocess
 from shutil import which
 
 from milc import cli
+import qmk.path
 
 
-@cli.argument('files', nargs='*', arg_only=True, help='Filename(s) to format.')
-@cli.subcommand("Format C code according to QMK's style.")
-def cformat(cli):
-    """Format C code according to QMK's style.
+def cformat_run(files, all_files):
+    """Spawn clang-format subprocess with proper arguments
     """
     # Determine which version of clang-format to use
     clang_format = ['clang-format', '-i']
@@ -19,27 +17,48 @@ def cformat(cli):
         if which(binary):
             clang_format[0] = binary
             break
-
-    # Find the list of files to format
-    if cli.args.files:
-        cli.args.files = [os.path.join(os.environ['ORIG_CWD'], file) for file in cli.args.files]
-    else:
-        ignores = ['tmk_core/protocol/usb_hid', 'quantum/template']
-        for dir in ['drivers', 'quantum', 'tests', 'tmk_core']:
-            for dirpath, dirnames, filenames in os.walk(dir):
-                if any(i in dirpath for i in ignores):
-                    dirnames.clear()
-                    continue
-
-                for name in filenames:
-                    if name.endswith(('.c', '.h', '.cpp')):
-                        cli.args.files.append(os.path.join(dirpath, name))
-
-    # Run clang-format on the files we've found
     try:
-        subprocess.run(clang_format + cli.args.files, check=True)
+        if not files:
+            cli.log.warn('No changes detected. Use "qmk cformat -a" to format all files')
+            return False
+        if files and all_files:
+            cli.log.warning('Filenames passed with -a, only formatting: %s', ','.join(cli.args.files))
+        # 3.6+: Can remove the str casting, python will cast implicitly
+        subprocess.run(clang_format + [str(file) for file in files], check=True)
         cli.log.info('Successfully formatted the C code.')
 
     except subprocess.CalledProcessError:
         cli.log.error('Error formatting C code!')
         return False
+
+
+@cli.argument('-a', '--all-files', arg_only=True, action='store_true', help='Format all core files.')
+@cli.argument('-b', '--base-branch', default='origin/master', help='Branch to compare to diffs to.')
+@cli.argument('files', nargs='*', arg_only=True, help='Filename(s) to format.')
+@cli.subcommand("Format C code according to QMK's style.")
+def cformat(cli):
+    """Format C code according to QMK's style.
+    """
+    # Empty array for files
+    files = []
+    # Core directories for formatting
+    core_dirs = ['drivers', 'quantum', 'tests', 'tmk_core']
+    ignores = ['tmk_core/protocol/usb_hid', 'quantum/template']
+    # Find the list of files to format
+    if cli.args.files:
+        files.extend(qmk.path.normpath(file) for file in cli.args.files)
+    # If -a is specified
+    elif cli.args.all_files:
+        all_files = qmk.path.c_source_files(core_dirs)
+        # The following statement checks each file to see if the file path is in the ignored directories.
+        files.extend(file for file in all_files if not any(i in str(file) for i in ignores))
+    # No files specified & no -a flag
+    else:
+        base_args = ['git', 'diff', '--name-only', cli.args.base_branch]
+        out = subprocess.run(base_args + core_dirs, check=True, stdout=subprocess.PIPE)
+        changed_files = filter(None, out.stdout.decode('UTF-8').split('\n'))
+        filtered_files = [qmk.path.normpath(file) for file in changed_files if not any(i in file for i in ignores)]
+        files.extend(file for file in filtered_files if file.exists() and file.suffix in ['.c', '.h', '.cpp'])
+
+    # Run clang-format on the files we've found
+    cformat_run(files, cli.args.all_files)
diff --git a/lib/python/qmk/path.py b/lib/python/qmk/path.py
index d16928afb..bfaa43924 100644
--- a/lib/python/qmk/path.py
+++ b/lib/python/qmk/path.py
@@ -68,3 +68,17 @@ def normpath(path):
         return Path(path)
 
     return Path(os.environ['ORIG_CWD']) / path
+
+
+def c_source_files(dir_names):
+    """Returns a list of all *.c, *.h, and *.cpp files for a given list of directories
+
+    Args:
+
+        dir_names
+            List of directories, relative pathing starts at qmk's cwd
+    """
+    files = []
+    for dir in dir_names:
+        files.extend(file for file in Path(dir).glob('**/*') if file.suffix in ['.c', '.h', '.cpp'])
+    return files
diff --git a/lib/python/qmk/tests/test_cli_commands.py b/lib/python/qmk/tests/test_cli_commands.py
index bb77952fa..a2595eb78 100644
--- a/lib/python/qmk/tests/test_cli_commands.py
+++ b/lib/python/qmk/tests/test_cli_commands.py
@@ -7,7 +7,8 @@ def check_subcommand(command, *args):
 
 
 def test_cformat():
-    assert check_subcommand('cformat', 'tmk_core/common/keyboard.c').returncode == 0
+    result = check_subcommand('cformat', 'quantum/matrix.c')
+    assert result.returncode == 0
 
 
 def test_compile():