diff --git a/phpcs.xml b/phpcs.xml
index 7ba6b304..b98b14a9 100644
--- a/phpcs.xml
+++ b/phpcs.xml
@@ -26,13 +26,6 @@
     <exclude name="Drupal.NamingConventions.ValidVariableName.LowerCamelName"/>
     <exclude name="Drupal.NamingConventions.ValidFunctionName.ScopeNotCamelCaps"/>
 
-    <!-- Fix doc comments later. -->
-    <exclude name="Drupal.Commenting.ClassComment"/>
-    <exclude name="Drupal.Commenting.DataTypeNamespace"/>
-    <exclude name="Drupal.Commenting.DocComment"/>
-    <exclude name="Drupal.Commenting.FunctionComment"/>
-    <exclude name="Drupal.Commenting.VariableComment"/>
-
     <!-- Fix translatable strings later. -->
     <exclude name="Drupal.Semantics.FunctionT.WhiteSpace"/>
   </rule>
diff --git a/src/Controller/ViewerController.php b/src/Controller/ViewerController.php
index 5835bb3f..083b9166 100644
--- a/src/Controller/ViewerController.php
+++ b/src/Controller/ViewerController.php
@@ -24,10 +24,18 @@
  */
 class ViewerController extends ControllerBase {
 
+    /**
+     * The renderer service.
+     *
+     * @var \Drupal\Core\Render\RendererInterface
+     */
     private $renderer;
 
     /**
      * The controller constructor.
+     *
+     * @param \Drupal\Core\Render\RendererInterface $renderer
+     *   The renderer service.
      */
     public function __construct(RendererInterface $renderer) {
         $this->renderer = $renderer;
@@ -45,9 +53,14 @@ public static function create(ContainerInterface $container): self {
     /**
      * Returns a raw page for the iframe embed.
      *
-     * Set edit to true for an editor.
+     * @param \Drupal\media\Entity\Media $media
+     *   Media entity.
+     * @param bool $edit
+     *   TRUE to open Collabora Online in edit mode.
+     *   FALSE to open Collabora Online in readonly mode.
      *
-     * @return Response
+     * @return \Symfony\Component\HttpFoundation\Response
+     *   Response suitable for iframe, without the usual page decorations.
      */
     public function editor(Media $media, $edit = FALSE) {
         $options = [
diff --git a/src/Controller/WopiController.php b/src/Controller/WopiController.php
index 9ca7d088..4f22dd4a 100644
--- a/src/Controller/WopiController.php
+++ b/src/Controller/WopiController.php
@@ -40,6 +40,17 @@ public static function permissionDenied(): Response {
         );
     }
 
+    /**
+     * Handles the WOPI 'info' request for a media entity.
+     *
+     * @param string $id
+     *   Media id from url.
+     * @param \Symfony\Component\HttpFoundation\Request $request
+     *   Request object with query parameters.
+     *
+     * @return \Symfony\Component\HttpFoundation\Response
+     *   The response with file contents.
+     */
     public function wopiCheckFileInfo(string $id, Request $request) {
         $token = $request->query->get('access_token');
 
@@ -97,6 +108,17 @@ public function wopiCheckFileInfo(string $id, Request $request) {
         return $response;
     }
 
+    /**
+     * Handles the wopi "content" request for a media entity.
+     *
+     * @param string $id
+     *   Media id from url.
+     * @param \Symfony\Component\HttpFoundation\Request $request
+     *   Request object with query parameters.
+     *
+     * @return \Symfony\Component\HttpFoundation\Response
+     *   The response with file contents.
+     */
     public function wopiGetFile(string $id, Request $request) {
         $token = $request->query->get('access_token');
 
@@ -121,6 +143,17 @@ public function wopiGetFile(string $id, Request $request) {
         return $response;
     }
 
+    /**
+     * Handles the wopi "save" request for a media entity..
+     *
+     * @param string $id
+     *   Media id from url.
+     * @param \Symfony\Component\HttpFoundation\Request $request
+     *   Request object with headers, query parameters and payload.
+     *
+     * @return \Symfony\Component\HttpFoundation\Response
+     *   The response.
+     */
     public function wopiPutFile(string $id, Request $request) {
         $token = $request->query->get('access_token');
         $timestamp = $request->headers->get('x-cool-wopi-timestamp');
@@ -214,11 +247,15 @@ public function wopiPutFile(string $id, Request $request) {
     /**
      * The WOPI entry point.
      *
-     * action: 'info', 'content' or 'save'.
-     * id: the ID of the media.
-     * request: The request as originating.
+     * @param string $action
+     *   One of 'info', 'content' or 'save', depending with path is visited.
+     * @param string $id
+     *   Media id from url.
+     * @param \Symfony\Component\HttpFoundation\Request $request
+     *   Request object for headers and query parameters.
      *
-     * @return Response
+     * @return \Symfony\Component\HttpFoundation\Response
+     *   Response to be consumed by Collabora Online.
      */
     public function wopi(string $action, string $id, Request $request) {
         $returnCode = Response::HTTP_BAD_REQUEST;
diff --git a/src/Cool/CoolRequest.php b/src/Cool/CoolRequest.php
index 394530ac..631457aa 100644
--- a/src/Cool/CoolRequest.php
+++ b/src/Cool/CoolRequest.php
@@ -13,9 +13,13 @@
 namespace Drupal\collabora_online\Cool;
 
 /**
- * Get the discovery XML content
+ * Gets the contents of discovery.xml from the Collabora server.
  *
- * Return `false` in case of error.
+ * @param string $server
+ *   Url of the Collabora Online server.
+ *
+ * @return string|false
+ *   The full contents of discovery.xml, or FALSE on failure.
  */
 function getDiscovery($server) {
     $discovery_url = $server . '/hosting/discovery';
@@ -36,6 +40,19 @@ function getDiscovery($server) {
     return $res;
 }
 
+/**
+ * Extracts a WOPI url from the parsed discovery.xml.
+ *
+ * @param \SimpleXMLElement|null|false $discovery_parsed
+ *   Parsed contents from discovery.xml from the Collabora server.
+ *   Currently, NULL or FALSE are supported too, but lead to NULL return value.
+ * @param string $mimetype
+ *   MIME type for which to fetch the WOPI url. E.g. 'text/plain'.
+ *
+ * @return mixed|null
+ *   WOPI url as configured for this MIME type in discovery.xml, or NULL if none
+ *   was found for the given MIME type.
+ */
 function getWopiSrcUrl($discovery_parsed, $mimetype) {
     if ($discovery_parsed === NULL || $discovery_parsed == FALSE) {
         return NULL;
@@ -47,13 +64,34 @@ function getWopiSrcUrl($discovery_parsed, $mimetype) {
     return NULL;
 }
 
+/**
+ * Checks if a string starts with another string.
+ *
+ * @param string $s
+ *   Haystack.
+ * @param string $ss
+ *   Needle.
+ *
+ * @return bool
+ *   TRUE if $ss is a prefix of $s.
+ *
+ * @see str_starts_with()
+ */
 function strStartsWith($s, $ss) {
     $res = strrpos($s, $ss);
     return !is_bool($res) && $res == 0;
 }
 
+/**
+ * Helper class to fetch a WOPI client url.
+ */
 class CoolRequest {
 
+    /**
+     * Error code from last attempt to fetch the client WOPI url.
+     *
+     * @var int
+     */
     private $error_code;
 
     const ERROR_MSG = [
@@ -67,18 +105,37 @@ class CoolRequest {
         204 => 'Warning! You have to specify the scheme protocol too (http|https) for the server address.',
     ];
 
+    /**
+     * The WOPI url that was last fetched, or '' as initial value.
+     *
+     * @var int
+     */
     private $wopi_src;
 
+    /**
+     * Constructor.
+     */
     public function __construct() {
         $this->error_code = 0;
         $this->wopi_src = '';
     }
 
+    /**
+     * Gets an error string from the last attempt to fetch the WOPI url.
+     *
+     * @return string
+     *   Error string containing int error code and a message.
+     */
     public function errorString() {
         return $this->error_code . ': ' . static::ERROR_MSG[$this->error_code];
     }
 
-    /** Return the wopi client URL */
+    /**
+     * Gets the URL for the WOPI client.
+     *
+     * @return string|null
+     *   The WOPI client url, or NULL on failure.
+     */
     public function getWopiClientURL() {
         $_HOST_SCHEME = isset($_SERVER['HTTPS']) ? 'https' : 'http';
         $default_config = \Drupal::config('collabora_online.settings');
diff --git a/src/Cool/CoolUtils.php b/src/Cool/CoolUtils.php
index 1db446f7..f8c4b8e3 100644
--- a/src/Cool/CoolUtils.php
+++ b/src/Cool/CoolUtils.php
@@ -18,9 +18,20 @@
 use Firebase\JWT\JWT;
 use Firebase\JWT\Key;
 
+/**
+ * Class with various static methods.
+ */
 class CoolUtils {
 
-    /** Get the file from the Media entity */
+    /**
+     * Gets the file referenced by a media entity.
+     *
+     * @param \Drupal\media\Entity\Media $media
+     *   The media entity.
+     *
+     * @return \Drupal\file\FileInterface|null
+     *   The file entity, or NULL if not found.
+     */
     public static function getFile(Media $media) {
         $fid = $media->getSource()->getSourceFieldValue($media);
         $file = File::load($fid);
@@ -28,19 +39,40 @@ public static function getFile(Media $media) {
         return $file;
     }
 
-    /** Get the file based on the entity id */
+    /**
+     * Gets a file based on the media id.
+     *
+     * @param int|string $id
+     *   Media id which might be in strong form like '123'.
+     *
+     * @return \Drupal\file\FileInterface|null
+     *   File referenced by the media entity, or NULL if not found.
+     */
     public static function getFileById($id) {
         /** @var \Drupal\media\MediaInterface|null $media */
         $media = \Drupal::entityTypeManager()->getStorage('media')->load($id);
         return CoolUtils::getFile($media);
     }
 
+    /**
+     * Sets the file entity reference for a media entity.
+     *
+     * @param \Drupal\media\Entity\Media $media
+     *   Media entity to be modified.
+     * @param \Drupal\file\Entity\File $source
+     *   File entity to reference.
+     */
     public static function setMediaSource(Media $media, File $source) {
         $name = $media->getSource()->getSourceFieldDefinition($media->bundle->entity)->getName();
         $media->set($name, $source);
     }
 
-    /** Obtain the signing key from the key storage */
+    /**
+     * Obtains the signing key from the key storage.
+     *
+     * @return string
+     *   The key value.
+     */
     public static function getKey() {
         $default_config = \Drupal::config('collabora_online.settings');
         $key_id = $default_config->get('cool')['key_id'];
@@ -49,11 +81,22 @@ public static function getKey() {
         return $key;
     }
 
-    /** Verify JWT token
+    /**
+     * Decodes and verifies a JWT token.
      *
-     *  Verification include:
+     * Verification include:
      *  - matching $id with fid in the payload
-     *  - verifying the expiration
+     *  - verifying the expiration.
+     *
+     * @param string $token
+     *   The token to verify.
+     * @param int|string $id
+     *   Media id for which the token was created.
+     *   This could be in string form like '123'.
+     *
+     * @return \stdClass|null
+     *   Data decoded from the token, or NULL on failure or if the token has
+     *   expired.
      */
     public static function verifyTokenForId(
         #[\SensitiveParameter]
@@ -75,7 +118,10 @@ public static function verifyTokenForId(
     }
 
     /**
-     * Return the TTL of the token in seconds, from the EPOCH.
+     * Gets the TTL of the token in seconds, from the EPOCH.
+     *
+     * @return int
+     *   Token TTL in seconds.
      */
     public static function getAccessTokenTtl() {
         $default_config = \Drupal::config('collabora_online.settings');
@@ -85,8 +131,7 @@ public static function getAccessTokenTtl() {
     }
 
     /**
-     * Create a JWT token for the Media with id $id, a $ttl, and an
-     * eventual write permission.
+     * Creates a JWT token for a media entity.
      *
      * The token will carry the following:
      *
@@ -97,6 +142,16 @@ public static function getAccessTokenTtl() {
      * - wri: if true, then this token has write permissions.
      *
      * The signing key is stored in Drupal key management.
+     *
+     * @param int|string $id
+     *   Media id, which could be in string form like '123'.
+     * @param int $ttl
+     *   Access token TTL in seconds.
+     * @param bool $can_write
+     *   TRUE if the token is for an editor in write/edit mode.
+     *
+     * @return string
+     *   The access token.
      */
     public static function tokenForFileId($id, $ttl, $can_write = FALSE) {
         $payload = [
@@ -112,8 +167,7 @@ public static function tokenForFileId($id, $ttl, $can_write = FALSE) {
     }
 
     /**
-     *  List of read only formats. Currently limited to the one Drupal
-     *  accept.
+     * List of read only formats. Currently limited to the one Drupal accept.
      */
     const READ_ONLY = [
         'application/x-iwork-keynote-sffkey' => TRUE,
@@ -122,24 +176,48 @@ public static function tokenForFileId($id, $ttl, $can_write = FALSE) {
     ];
 
     /**
-     * Return if we can edit that media file.
+     * Determines if we can edit that media file.
      *
      * There are few types that Collabora Online only views.
+     *
+     * @param \Drupal\file\Entity\File $file
+     *   File entity.
+     *
+     * @return bool
+     *   TRUE if the file has a file type that is supported for editing.
+     *   FALSE if the file can only be opened as read-only.
      */
     public static function canEdit(File $file) {
         $mimetype = $file->getMimeType();
         return !array_key_exists($mimetype, static::READ_ONLY);
     }
 
-    /** Return the mime type for the document.
+    /**
+     * Gets the mime type for the document.
+     *
+     * Drupal will figure it out for us.
+     *
+     * @param \Drupal\file\Entity\File $file
+     *   File entity.
      *
-     *  Drupal will figure it out for us.
+     * @return string|null
+     *   The mime type, or NULL if it cannot be determined.
      */
     public static function getDocumentType(File $file) {
         return $file->getMimeType();
     }
 
-    /** Return the editor / viewer Drupal URL from the routes configured. */
+    /**
+     * Gets the editor / viewer Drupal URL from the routes configured.
+     *
+     * @param \Drupal\media\Entity\Media $media
+     *   Media entity that holds the file to open in the editor.
+     * @param bool $can_write
+     *   TRUE for an edit url, FALSE for a read-only preview url.
+     *
+     * @return \Drupal\Core\Url
+     *   Editor url to visit as full-page, or to embed in an iframe.
+     */
     public static function getEditorUrl(Media $media, $can_write = FALSE) {
         if ($can_write) {
             return Url::fromRoute('collabora-online.edit', ['media' => $media->id()]);
@@ -150,18 +228,19 @@ public static function getEditorUrl(Media $media, $can_write = FALSE) {
     }
 
     /**
-     * Get a render array for a cool viewer.
-     *
-     * @param Media $media
-     *   The media entity to view / edit
+     * Gets a render array for a cool viewer.
      *
+     * @param \Drupal\media\Entity\Media $media
+     *   The media entity to view / edit.
      * @param bool $can_write
      *   Whether this is a viewer (false) or an edit (true). Permissions will
      *   also be checked.
-     *
-     * @param array $options
+     * @param array{closebutton: bool} $options
      *   Options for the renderer. Current values:
      *     - "closebutton" if "true" will add a close box. (see COOL SDK)
+     *
+     * @return array|array{error: string}
+     *   A stub render element array, or an array with an error on failure.
      */
     public static function getViewerRender(Media $media, bool $can_write, $options = NULL) {
         $default_config = \Drupal::config('collabora_online.settings');
diff --git a/src/Form/ConfigForm.php b/src/Form/ConfigForm.php
index 9831e30b..9d559fd9 100644
--- a/src/Form/ConfigForm.php
+++ b/src/Form/ConfigForm.php
@@ -15,6 +15,9 @@
 use Drupal\Core\Form\ConfigFormBase;
 use Drupal\Core\Form\FormStateInterface;
 
+/**
+ * Form to configure module settings for Collabora.
+ */
 class ConfigForm extends ConfigFormBase {
 
     const SETTINGS = 'collabora_online.settings';