Add root flag to CardReader (#15275)

Marlin/src/lcd/extensible_ui/ui_api.cpp

 
   bool FileList::isAtRootDir() {
     #if ENABLED(SDSUPPORT)
-      card.getWorkDirName();
-      return card.filename[0] == '/';
+      card.flag.workDirIsRoot;
     #else
       return true;
     #endif

Marlin/src/lcd/menu/menu_media.cpp

 
   #if HAS_GRAPHICAL_LCD
     static uint16_t fileCnt;
-    static bool at_root;
-    if (ui.first_page) {
-      fileCnt = card.get_num_Files();
-      card.getWorkDirName();
-      at_root = card.filename[0] == '/';
-    }
+    if (ui.first_page) fileCnt = card.get_num_Files();
   #else
     const uint16_t fileCnt = card.get_num_Files();
-    card.getWorkDirName();
-    const bool at_root = card.filename[0] == '/';
   #endif
 
   START_MENU();
   MENU_BACK(MSG_MAIN);
-  if (at_root) {
+  if (card.flag.workDirIsRoot) {
     #if !PIN_EXISTS(SD_DETECT)
       MENU_ITEM(function, LCD_STR_REFRESH MSG_REFRESH, lcd_sd_refresh);
     #endif

Marlin/src/sd/cardreader.cpp

   lsDive(nullptr, workDir, match);
 }
 
-uint16_t CardReader::getnrfilenames() {
+uint16_t CardReader::countFilesInWorkDir() {
   lsAction = LS_Count;
   nrFiles = 0;
   workDir.rewind();
 
   if (newDir.open(parent, relpath, O_READ)) {
     workDir = newDir;
+    flag.workDirIsRoot = false;
     if (workDirDepth < MAX_DIR_DEPTH)
       workDirParents[workDirDepth++] = workDir;
     #if ENABLED(SDCARD_SORT_ALPHA)
       presort();
     #endif
   }
+  if (!workDirDepth) flag.workDirIsRoot = true;
   return workDirDepth;
 }
 
 void CardReader::setroot() {
-  /*if (!workDir.openRoot(&volume)) {
-    SERIAL_ECHOLNPGM(MSG_SD_WORKDIR_FAIL);
-  }*/
   workDir = root;
+  flag.workDirIsRoot = true;
   #if ENABLED(SDCARD_SORT_ALPHA)
     presort();
   #endif
     #endif
 
     // If there are files, sort up to the limit
-    uint16_t fileCnt = getnrfilenames();
+    uint16_t fileCnt = countFilesInWorkDir();
     if (fileCnt > 0) {
 
       // Never sort more than the max allowed
     #if ENABLED(SDCARD_SORT_ALPHA) && SDSORT_USES_RAM && SDSORT_CACHE_NAMES
       nrFiles // no need to access the SD card for filenames
     #else
-      getnrfilenames()
+      countFilesInWorkDir()
     #endif
   ;
 }

Marlin/src/sd/cardreader.h

        sdprinting:1,
        mounted:1,
        filenameIsDir:1,
+       workDirIsRoot:1,
        abort_sd_printing:1
        #if ENABLED(BINARY_FILE_TRANSFER)
          , binary_mode:1
   #endif
 
   static void getfilename(uint16_t nr, const char* const match=nullptr);
-  static uint16_t getnrfilenames();
+  static uint16_t countFilesInWorkDir();
 
   static void getAbsFilename(char *t);