Fix GRUB 2.06 refusing to read filesystem due to zeroed out date field

GRUB 2.06 added support for showing file modification times using `ls -l`.
The GRUB fat module now errors with GRUB_ERR_OUT_OF_RANGE if a file's
modification date + time reads as zero. This is a valid check, since the
DOS/FAT date format uses 1-based month and day values, so no valid date
can ever be encoded as `0x0000`.

I've update the firmware to return a hard-coded date for all date fields
on directory entries, and confirmed the device is readable again from a
GRUB 2.06 shell.

Fixes #11

src/fat.c

@@ -95,4 +95,15 @@ uint8_t fat_write_dir(const struct FatDirEntry *direntry, uint8_t* output)
     memcpy(output, data, sizeof(struct FatDirEntry));
 
     return sizeof(struct FatDirEntry);
+}
+
+/**
+ * For a good explanation of the DOS date/time format used here, see:
+ * https://stackoverflow.com/a/15763512
+ */
+uint16_t fat_date(const uint16_t year, const uint8_t month, const uint8_t day)
+{
+    return ((year - 1980) << 9) | // Year is stored as an offset from 1980
+           (month << 5) |
+           (day);
 }

src/fat.h

@@ -58,4 +58,9 @@ uint8_t fat_write_lfn(const char* name, const struct FatDirEntry *direntry,  uin
  *
  * Returns the number of bytes written to the output pointer (always 32)
  */
-uint8_t fat_write_dir(const struct FatDirEntry *direntry, uint8_t* output);
+uint8_t fat_write_dir(const struct FatDirEntry *direntry, uint8_t* output);
+
+/**
+ * Return a date formatted for use in a directory entry's mdate, adate and cdate fields
+ */
+uint16_t fat_date(const uint16_t year, const uint8_t month, const uint8_t day);

src/usb.c

@@ -195,6 +195,8 @@ static struct VirtualFile _virtualFiles[] = {
  */
 static void generate_dir_sector(uint8_t* output)
 {
+    const uint16_t releaseDate = fat_date(2021, 5, 9);
+
     for (uint8_t i = 0; i < COUNT_OF(_virtualFiles); ++i)
     {
         const char* longName = _virtualFiles[i].longName;
@@ -203,6 +205,12 @@ static void generate_dir_sector(uint8_t* output)
         // Set sector automatically based on array index
         entry->start = FILEDATA_START_CLUSTER + i;
 
+        // Set the file dates to something reasonable
+        // GRUB 2.06 refuses to read the filesystem if these are set to zero
+        entry->mdate = releaseDate;
+        entry->cdate = releaseDate;
+        entry->adate = releaseDate;
+
         // Write long file name and actual directory entry
         output += fat_write_lfn(longName, entry, output);
         output += fat_write_dir(entry, output);