Add UIDs to all Linux brightness methods

The UID here is based on the I2C path of the display, which is easily exposed for I2C and DDCutil.
For SysFiles and XRandr, the process is a bit more complicated. It relies on the idea that we can
link the entries in `/sys/class/backlight` to `/sys/class/drm` which should contain a link to the
I2C system.

For SysFiles, we expand the `/sys/class/backlight/*/device` symlink to expose the path to the
underlying display connection (in this case, usually eDP). We can follow this link to the
equivalent path in `/sys/class/drm` which will usually have an `i2c-X` or `ddc/i2c-dev` folder
inside that maps to a relevant I2C connection.

On XRandr, it's a similar process, except we start by looking in `/sys/class/drm` and matching the
connection names to our interface name. For example: HDMI-1 -> card0-HDMI-A-1 or DP-1 -> card0-DP-1.
We can then find the I2C connection by browsing the folder tree.

I have tested this on multiple devices and in all cases, both laptop and desktop displays were able
to be uniquely identified in this manner across brightness methods. This means that displays can
avoid being duplicated across all methods even if there is no EDID information available.

There are a few cases in which this doesn't work. On my Pinebook Pro, the edp-backlight subsystem
seems to be a completely different subsystem to the actual display, with no tangible connection
between the two. In this case, a UID cannot be found.
On a laptop running a Fedora Wayland live CD, there was no `intel_backlight` folder, which meant
that no UID was found. On the same laptop running X11 mint, `intel_backlight` was present and the
UID was found so I believe this is a driver/kernel module issue.

Author: Crozzers

screen_brightness_control/linux.py

@@ -42,6 +42,11 @@ def get_display_info(cls, display: Optional[DisplayIdentifier] = None) -> List[d
         displays_by_edid = {}
         index = 0
 
+        # map drm devices to their pci device paths
+        drm_paths = {}
+        for folder in glob.glob('/sys/class/drm/card*-*'):
+            drm_paths[os.path.realpath(folder)] = folder
+
         for subsystem in subsystems:
 
             device: dict = {
@@ -54,7 +59,8 @@ def get_display_info(cls, display: Optional[DisplayIdentifier] = None) -> List[d
                 'manufacturer': None,
                 'manufacturer_id': None,
                 'edid': None,
-                'scale': None
+                'scale': None,
+                'uid': None
             }
 
             for folder in subsystem:
@@ -77,6 +83,12 @@ def get_display_info(cls, display: Optional[DisplayIdentifier] = None) -> List[d
                     )
                     continue
 
+                # check if backlight subsystem device matches any of the PCI devices discovered earlier
+                # if so, extract the i2c bus from the drm device folder
+                pci_path = os.path.realpath(f'/sys/class/backlight/{folder}/device')
+                if pci_path in drm_paths:
+                    device['uid'] = device['uid'] or i2c_bus_from_drm_device(drm_paths[pci_path])
+
             if os.path.isfile('%s/device/edid' % device['path']):
                 device['edid'] = EDID.hexdump('%s/device/edid' % device['path'])
 
@@ -371,7 +383,8 @@ def get_display_info(cls, display: Optional[DisplayIdentifier] = None) -> List[d
                         'index': index,
                         # convert edid to hex string
                         'edid': ''.join(f'{i:02x}' for i in edid),
-                        'i2c_bus': i2c_path
+                        'i2c_bus': i2c_path,
+                        'uid': i2c_path.split('-')[-1]
                     }
                 )
                 index += 1
@@ -439,6 +452,37 @@ class XRandr(BrightnessMethodAdv):
     executable: str = 'xrandr'
     '''the xrandr executable to be called'''
 
+    @staticmethod
+    def _get_uid(interface: str) -> Optional[str]:
+        '''
+        Attempts to find a UID (I2C bus path) for a given display interface.
+
+        This works by parsing the interface name and matching it up to the entries in `/sys/class/drm`.
+        `i2c_bus_from_drm_device` is then used to extract the bus number
+
+        Args:
+            interface: the interface in question. EG: `eDP-1`, `eDP1`, `HDMI-1`...
+
+        Returns:
+            The bus number as a string if found. Otherwise, none.
+        '''
+        if not os.path.isdir('/sys/class/drm'):
+            return None
+
+        # use regex because sometimes it can be `eDP-1` and sometimes it's `eDP1`
+        if interface_match := re.match(r'([a-z]+)-?(\d+)', interface, re.I):
+            interface, count = interface_match.groups()
+        else:
+            return None
+
+        for dir in os.listdir('/sys/class/drm/'):
+            # use regex here for case insensitivity on the interface
+            if not re.match(r'card\d+-%s(?:-[A-Z])?-%s' % (interface, count), dir, re.I):
+                continue
+            dir = f'/sys/class/drm/{dir}'
+            if bus := i2c_bus_from_drm_device(dir):
+                return bus
+
     @classmethod
     def _gdi(cls):
         '''
@@ -472,7 +516,8 @@ def _gdi(cls):
                     'manufacturer': None,
                     'manufacturer_id': None,
                     'edid': None,
-                    'unsupported': line.startswith('XWAYLAND') or 'WAYLAND_DISPLAY' in os.environ
+                    'unsupported': line.startswith('XWAYLAND') or 'WAYLAND_DISPLAY' in os.environ,
+                    'uid': cls._get_uid(line.split(' ')[0])
                 }
                 display_count += 1
 
@@ -606,14 +651,16 @@ def _gdi(cls):
                     'manufacturer': None,
                     'manufacturer_id': None,
                     'edid': None,
-                    'unsupported': 'invalid display' in line.lower()
+                    'unsupported': 'invalid display' in line.lower(),
+                    'uid': None
                 }
                 display_count += 1
 
             elif 'I2C bus' in line:
                 tmp_display['i2c_bus'] = line[line.index('/'):]
                 tmp_display['bus_number'] = int(
                     tmp_display['i2c_bus'].replace('/dev/i2c-', ''))
+                tmp_display['uid'] = tmp_display['i2c_bus'].split('-')[-1]
 
             elif 'Mfg id' in line:
                 # Recently ddcutil has started reporting manufacturer IDs like
@@ -739,6 +786,38 @@ def set_brightness(cls, value: IntPercentage, display: Optional[int] = None):
             )
 
 
+def i2c_bus_from_drm_device(dir: str) -> Optional[str]:
+    '''
+    Extract the relevant I2C bus number from a device in `/sys/class/drm`.
+
+    This function works by searching the directory for `i2c-*` and `ddc/i2c-dev/i2c-*` folders.
+
+    Args:
+        dir: the DRM directory, in the format `/sys/class/drm/<device>`
+
+    Returns:
+        Returns the I2C bus number as a string if found. Otherwise, returns None
+    '''
+    # check for enabled file and skip device if monitor inactive
+    if os.path.isfile(f'{dir}/enabled'):
+        with open(f'{dir}/enabled') as f:
+            if f.read().strip().lower() != 'enabled':
+                return
+
+    # sometimes the i2c path is in /sys/class/drm/*/i2c-*
+    # do this first because, in my testing, sometimes a device can have both `.../i2c-X` and `.../ddc/i2c-dev/...`
+    # and the latter is usually the wrong i2c path
+    paths = glob.glob(f'{dir}/i2c-*')
+    if paths:
+        return paths[0].split('-')[-1]
+
+    # sometimes the i2c path is in /sys/class/drm/*/ddc/i2c-dev
+    if os.path.isdir(f'{dir}/ddc/i2c-dev'):
+        paths = os.listdir(f'{dir}/ddc/i2c-dev')
+        if paths:
+            return paths[0].replace('i2c-', '')
+
+
 def list_monitors_info(
     method: Optional[str] = None, allow_duplicates: bool = False, unsupported: bool = False
 ) -> List[dict]: