Merge pull request #516 from PanRe/tusb_fifo_unmaksed_pointers

Tusb fifo unmaksed pointers

2 files changed, 468 insertions, 143 deletions

diff --git a/src/common/tusb_fifo.c b/src/common/tusb_fifo.c
index 6ab158cca..f41cf9f6c 100644
--- a/src/common/tusb_fifo.c
+++ b/src/common/tusb_fifo.c
@@ -2,6 +2,7 @@
  * The MIT License (MIT)
  *
  * Copyright (c) 2019 Ha Thach (tinyusb.org)
+ * Copyright (c) 2020 Reinhard Panhuber - rework to unmasked pointers
  *
  * Permission is hereby granted, free of charge, to any person obtaining a copy
  * of this software and associated documentation files (the "Software"), to deal
@@ -57,6 +58,8 @@ static void tu_fifo_unlock(tu_fifo_t *f)
 
 bool tu_fifo_config(tu_fifo_t *f, void* buffer, uint16_t depth, uint16_t item_size, bool overwritable)
 {
+  if (depth > 0x8000) return false;               // Maximum depth is 2^15 items
+
   tu_fifo_lock(f);
 
   f->buffer = (uint8_t*) buffer;
@@ -64,55 +67,328 @@ bool tu_fifo_config(tu_fifo_t *f, void* buffer, uint16_t depth, uint16_t item_si
   f->item_size = item_size;
   f->overwritable = overwritable;
 
-  f->rd_idx = f->wr_idx = f->count = 0;
+  f->max_pointer_idx = 2*depth - 1;               // Limit index space to 2*depth - this allows for a fast "modulo" calculation but limits the maximum depth to 2^16/2 = 2^15 and buffer overflows are detectable only if overflow happens once (important for unsupervised DMA applications)
+  f->non_used_index_space = 0xFFFF - f->max_pointer_idx;
+
+  f->rd_idx = f->wr_idx = 0;
 
   tu_fifo_unlock(f);
 
   return true;
 }
 
+// Static functions are intended to work on local variables
+
 static inline uint16_t _ff_mod(uint16_t idx, uint16_t depth)
 {
-  return (idx < depth) ? idx : (idx-depth);
+  while ( idx >= depth) idx -= depth;
+  return idx;
 }
 
-// retrieve data from fifo
-static inline void _ff_pull(tu_fifo_t* f, void * buffer, uint16_t n)
+// send one item to FIFO WITHOUT updating write pointer
+static inline void _ff_push(tu_fifo_t* f, void const * data, uint16_t wRel)
 {
-  memcpy(buffer,
-         f->buffer + (f->rd_idx * f->item_size),
-         f->item_size*n);
+  memcpy(f->buffer + (wRel * f->item_size), data, f->item_size);
+}
+
+// send n items to FIFO WITHOUT updating write pointer
+static void _ff_push_n(tu_fifo_t* f, void const * data, uint16_t n, uint16_t wRel)
+{
+  if(wRel + n <= f->depth)  // Linear mode only
+  {
+    memcpy(f->buffer + (wRel * f->item_size), data, n*f->item_size);
+  }
+  else      // Wrap around
+  {
+    uint16_t nLin = f->depth - wRel;
 
-  f->rd_idx = _ff_mod(f->rd_idx + n, f->depth);
-  f->count -= n;
+    // Write data to linear part of buffer
+    memcpy(f->buffer + (wRel * f->item_size), data, nLin*f->item_size);
+
+    // Write data wrapped around
+    memcpy(f->buffer, data + nLin*f->item_size, (n - nLin) * f->item_size);
+  }
 }
 
-// send data to fifo
-static inline void _ff_push(tu_fifo_t* f, void const * data, uint16_t n)
+// get one item from FIFO WITHOUT updating read pointer
+static inline void _ff_pull(tu_fifo_t* f, void * p_buffer, uint16_t rRel)
 {
-  memcpy(f->buffer + (f->wr_idx * f->item_size),
-         data,
-         f->item_size*n);
+  memcpy(p_buffer, f->buffer + (rRel * f->item_size), f->item_size);
+}
 
-  f->wr_idx = _ff_mod(f->wr_idx + n, f->depth);
+// get n items from FIFO WITHOUT updating read pointer
+static void _ff_pull_n(tu_fifo_t* f, void * p_buffer, uint16_t n, uint16_t rRel)
+{
+  if(rRel + n <= f->depth)       // Linear mode only
+  {
+    memcpy(p_buffer, f->buffer + (rRel * f->item_size), n*f->item_size);
+  }
+  else      // Wrap around
+  {
+    uint16_t nLin = f->depth - rRel;
 
-  if (tu_fifo_full(f))
+    // Read data from linear part of buffer
+    memcpy(p_buffer, f->buffer + (rRel * f->item_size), nLin*f->item_size);
+
+    // Read data wrapped part
+    memcpy(p_buffer + nLin*f->item_size, f->buffer, (n - nLin) * f->item_size);
+  }
+}
+
+// Advance an absolute pointer
+static uint16_t advance_pointer(tu_fifo_t* f, uint16_t p, uint16_t offset)
+{
+  // We limit the index space of p such that a correct wrap around happens
+  // Check for a wrap around or if we are in unused index space - This has to be checked first!! We are exploiting the wrap around to the correct index
+  if ((p > p + offset) || (p + offset > f->max_pointer_idx))
   {
-    f->rd_idx = f->wr_idx; // keep the full state (rd == wr && count = depth)
+    p = (p + offset) + f->non_used_index_space;
   }
   else
   {
-    f->count += n;
+    p += offset;
   }
+  return p;
+}
+
+// Backward an absolute pointer
+static uint16_t backward_pointer(tu_fifo_t* f, uint16_t p, uint16_t offset)
+{
+  // We limit the index space of p such that a correct wrap around happens
+  // Check for a wrap around or if we are in unused index space - This has to be checked first!! We are exploiting the wrap around to the correct index
+  if ((p < p - offset) || (p - offset > f->max_pointer_idx))
+  {
+    p = (p - offset) - f->non_used_index_space;
+  }
+  else
+  {
+    p -= offset;
+  }
+  return p;
+}
+
+// get relative from absolute pointer
+static uint16_t get_relative_pointer(tu_fifo_t* f, uint16_t p, uint16_t offset)
+{
+  return _ff_mod(advance_pointer(f, p, offset), f->depth);
+}
+
+// Works on local copies of w and r
+static inline uint16_t _tu_fifo_count(tu_fifo_t* f, uint16_t wAbs, uint16_t rAbs)
+{
+  uint16_t cnt = wAbs-rAbs;
+
+  // In case we have non-power of two depth we need a further modification
+  if (rAbs > wAbs) cnt -= f->non_used_index_space;
+
+  return cnt;
+}
+
+// Works on local copies of w and r
+static inline bool _tu_fifo_empty(uint16_t wAbs, uint16_t rAbs)
+{
+  return wAbs == rAbs;
+}
+
+// Works on local copies of w and r
+static inline bool _tu_fifo_full(tu_fifo_t* f, uint16_t wAbs, uint16_t rAbs)
+{
+  return (_tu_fifo_count(f, wAbs, rAbs) == f->depth);
+}
+
+// Works on local copies of w and r
+// BE AWARE - THIS FUNCTION MIGHT NOT GIVE A CORRECT ANSWERE IN CASE WRITE POINTER "OVERFLOWS"
+// Only one overflow is allowed for this function to work e.g. if depth = 100, you must not
+// write more than 2*depth-1 items in one rush without updating write pointer. Otherwise
+// write pointer wraps and you pointer states are messed up. This can only happen if you
+// use DMAs, write functions do not allow such an error.
+static inline bool _tu_fifo_overflowed(tu_fifo_t* f, uint16_t wAbs, uint16_t rAbs)
+{
+  return (_tu_fifo_count(f, wAbs, rAbs) > f->depth);
+}
+
+// Works on local copies of w
+// For more details see _tu_fifo_overflow()!
+static inline void _tu_fifo_correct_read_pointer(tu_fifo_t* f, uint16_t wAbs)
+{
+  f->rd_idx = backward_pointer(f, wAbs, f->depth);
+}
+
+// Works on local copies of w and r
+// Must be protected by mutexes since in case of an overflow read pointer gets modified
+static bool _tu_fifo_peek_at(tu_fifo_t* f, uint16_t offset, void * p_buffer, uint16_t wAbs, uint16_t rAbs)
+{
+  uint16_t cnt = _tu_fifo_count(f, wAbs, rAbs);
+
+  // Check overflow and correct if required
+  if (cnt > f->depth)
+  {
+    _tu_fifo_correct_read_pointer(f, wAbs);
+    cnt = f->depth;
+  }
+
+  // Skip beginning of buffer
+  if (cnt == 0 || offset >= cnt) return false;
+
+  uint16_t rRel = get_relative_pointer(f, rAbs, offset);
+
+  // Peek data
+  _ff_pull(f, p_buffer, rRel);
+
+  return true;
+}
+
+// Works on local copies of w and r
+// Must be protected by mutexes since in case of an overflow read pointer gets modified
+static uint16_t _tu_fifo_peek_at_n(tu_fifo_t* f, uint16_t offset, void * p_buffer, uint16_t n, uint16_t wAbs, uint16_t rAbs)
+{
+  uint16_t cnt = _tu_fifo_count(f, wAbs, rAbs);
+
+  // Check overflow and correct if required
+  if (cnt > f->depth)
+  {
+    _tu_fifo_correct_read_pointer(f, wAbs);
+    rAbs = f->rd_idx;
+    cnt = f->depth;
+  }
+
+  // Skip beginning of buffer
+  if (cnt == 0 || offset >= cnt) return 0;
+
+  // Check if we can read something at and after offset - if too less is available we read what remains
+  cnt -= offset;
+  if (cnt < n) {
+    if (cnt == 0) return 0;
+    n = cnt;
+  }
+
+  uint16_t rRel = get_relative_pointer(f, rAbs, offset);
+
+  // Peek data
+  _ff_pull_n(f, p_buffer, n, rRel);
+
+  return n;
+}
+
+// Works on local copies of w and r
+static inline uint16_t _tu_fifo_remaining(tu_fifo_t* f, uint16_t wAbs, uint16_t rAbs)
+{
+  return f->depth - _tu_fifo_count(f, wAbs, rAbs);
+}
+
+/******************************************************************************/
+/*!
+    @brief Get number of items in FIFO.
+
+    As this function only reads the read and write pointers once, this function is
+    reentrant and thus thread and ISR save without any mutexes.
+
+    @param[in]  f
+                Pointer to the FIFO buffer to manipulate
+
+    @returns Number of items in FIFO
+ */
+/******************************************************************************/
+uint16_t tu_fifo_count(tu_fifo_t* f)
+{
+  return _tu_fifo_count(f, f->wr_idx, f->rd_idx);
+}
+
+/******************************************************************************/
+/*!
+    @brief Check if FIFO is empty.
+
+    As this function only reads the read and write pointers once, this function is
+    reentrant and thus thread and ISR save without any mutexes.
+
+    @param[in]  f
+                Pointer to the FIFO buffer to manipulate
+
+    @returns Number of items in FIFO
+ */
+/******************************************************************************/
+bool tu_fifo_empty(tu_fifo_t* f)
+{
+  return _tu_fifo_empty(f->wr_idx, f->rd_idx);
+}
+
+/******************************************************************************/
+/*!
+    @brief Check if FIFO is full.
+
+    As this function only reads the read and write pointers once, this function is
+    reentrant and thus thread and ISR save without any mutexes.
+
+    @param[in]  f
+                Pointer to the FIFO buffer to manipulate
+
+    @returns Number of items in FIFO
+ */
+/******************************************************************************/
+bool tu_fifo_full(tu_fifo_t* f)
+{
+  return _tu_fifo_full(f, f->wr_idx, f->rd_idx);
 }
 
 /******************************************************************************/
 /*!
-    @brief Read one element out of the RX buffer.
+    @brief Get remaining space in FIFO.
+
+    As this function only reads the read and write pointers once, this function is
+    reentrant and thus thread and ISR save without any mutexes.
+
+    @param[in]  f
+                Pointer to the FIFO buffer to manipulate
+
+    @returns Number of items in FIFO
+ */
+/******************************************************************************/
+uint16_t tu_fifo_remaining(tu_fifo_t* f)
+{
+  return _tu_fifo_remaining(f, f->wr_idx, f->rd_idx);
+}
+
+/******************************************************************************/
+/*!
+    @brief Check if overflow happened.
+
+     BE AWARE - THIS FUNCTION MIGHT NOT GIVE A CORRECT ANSWERE IN CASE WRITE POINTER "OVERFLOWS"
+     Only one overflow is allowed for this function to work e.g. if depth = 100, you must not
+     write more than 2*depth-1 items in one rush without updating write pointer. Otherwise
+     write pointer wraps and you pointer states are messed up. This can only happen if you
+     use DMAs, write functions do not allow such an error. Avoid such nasty things!
+
+     All reading functions (read, peek) check for overflows and correct read pointer on their own such
+     that latest items are read.
+     If required (e.g. for DMA use) you can also correct the read pointer by
+     tu_fifo_correct_read_pointer().
+
+    @param[in]  f
+                Pointer to the FIFO buffer to manipulate
+
+    @returns True if overflow happened
+ */
+/******************************************************************************/
+bool tu_fifo_overflowed(tu_fifo_t* f)
+{
+  return _tu_fifo_overflowed(f, f->wr_idx, f->rd_idx);
+}
+
+// Only use in case tu_fifo_overflow() returned true!
+void tu_fifo_correct_read_pointer(tu_fifo_t* f)
+{
+  tu_fifo_lock(f);
+  _tu_fifo_correct_read_pointer(f, f->wr_idx);
+  tu_fifo_unlock(f);
+}
+
+/******************************************************************************/
+/*!
+    @brief Read one element out of the buffer.
 
     This function will return the element located at the array index of the
-    read pointer, and then increment the read pointer index.  If the read
-    pointer exceeds the maximum buffer size, it will roll over to zero.
+    read pointer, and then increment the read pointer index.
+    This function checks for an overflow and corrects read pointer if required.
 
     @param[in]  f
                 Pointer to the FIFO buffer to manipulate
@@ -120,26 +396,27 @@ static inline void _ff_push(tu_fifo_t* f, void const * data, uint16_t n)
                 Pointer to the place holder for data read from the buffer
 
     @returns TRUE if the queue is not empty
-*/
+ */
 /******************************************************************************/
 bool tu_fifo_read(tu_fifo_t* f, void * buffer)
 {
-  if( tu_fifo_empty(f) ) return false;
+  tu_fifo_lock(f);                                          // TODO: Here we may distinguish for read and write pointer mutexes!
 
-  tu_fifo_lock(f);
+  // Peek the data
+  bool ret = _tu_fifo_peek_at(f, 0, buffer, f->wr_idx, f->rd_idx);    // f->rd_idx might get modified in case of an overflow so we can not use a local variable
 
-  _ff_pull(f, buffer, 1);
+  // Advance pointer
+  f->rd_idx = advance_pointer(f, f->rd_idx, ret);
 
   tu_fifo_unlock(f);
-
-  return true;
+  return ret;
 }
 
 /******************************************************************************/
 /*!
     @brief This function will read n elements from the array index specified by
-    the read pointer and increment the read index. If the read index
-    exceeds the max buffer size, then it will roll over to zero.
+    the read pointer and increment the read index.
+    This function checks for an overflow and corrects read pointer if required.
 
     @param[in]  f
                 Pointer to the FIFO buffer to manipulate
@@ -149,76 +426,76 @@ bool tu_fifo_read(tu_fifo_t* f, void * buffer)
                 Number of element that buffer can afford
 
     @returns number of items read from the FIFO
-*/
+ */
 /******************************************************************************/
-uint16_t tu_fifo_read_n (tu_fifo_t* f, void * buffer, uint16_t count)
+uint16_t tu_fifo_read_n(tu_fifo_t* f, void * buffer, uint16_t count)
 {
-  if(tu_fifo_empty(f)) return 0;
-
-  tu_fifo_lock(f);
+  tu_fifo_lock(f);                                          // TODO: Here we may distinguish for read and write pointer mutexes!
 
-  // Limit up to fifo's count
-  if(count > f->count) count = f->count;
+  // Peek the data
+  count = _tu_fifo_peek_at_n(f, 0, buffer, count, f->wr_idx, f->rd_idx);        // f->rd_idx might get modified in case of an overflow so we can not use a local variable
 
-  if(count + f->rd_idx <= f->depth)
-  {
-    _ff_pull(f, buffer, count);
-  }
-  else
-  {
-    uint16_t const part1 = f->depth - f->rd_idx;
-
-    // Part 1: from rd_idx to end
-    _ff_pull(f, buffer, part1);
-    buffer = ((uint8_t*) buffer) + part1*f->item_size;
-
-    // Part 2: start to remaining
-    _ff_pull(f, buffer, count-part1);
-  }
+  // Advance read pointer
+  f->rd_idx = advance_pointer(f, f->rd_idx, count);
 
   tu_fifo_unlock(f);
-
   return count;
 }
 
 /******************************************************************************/
 /*!
-    @brief Read one item without removing it from the FIFO
+    @brief Read one item without removing it from the FIFO.
+    This function checks for an overflow and corrects read pointer if required.
 
     @param[in]  f
                 Pointer to the FIFO buffer to manipulate
-    @param[in]  pos
-                Position to read from in the FIFO buffer
+    @param[in]  offset
+                Position to read from in the FIFO buffer with respect to read pointer
     @param[in]  p_buffer
                 Pointer to the place holder for data read from the buffer
 
     @returns TRUE if the queue is not empty
-*/
+ */
 /******************************************************************************/
-bool tu_fifo_peek_at(tu_fifo_t* f, uint16_t pos, void * p_buffer)
+bool tu_fifo_peek_at(tu_fifo_t* f, uint16_t offset, void * p_buffer)
 {
-  if ( pos >= f->count ) return false;
+  tu_fifo_lock(f);                                          // TODO: Here we may distinguish for read and write pointer mutexes!
+  bool ret = _tu_fifo_peek_at(f, offset, p_buffer, f->wr_idx, f->rd_idx);
+  tu_fifo_unlock(f);
+  return ret;
+}
 
-  tu_fifo_lock(f);
+/******************************************************************************/
+/*!
+    @brief Read n items without removing it from the FIFO
+    This function checks for an overflow and corrects read pointer if required.
 
-  // rd_idx is pos=0
-  uint16_t index = _ff_mod(f->rd_idx + pos, f->depth);
-  memcpy(p_buffer,
-         f->buffer + (index * f->item_size),
-         f->item_size);
+    @param[in]  f
+                Pointer to the FIFO buffer to manipulate
+    @param[in]  offset
+                Position to read from in the FIFO buffer with respect to read pointer
+    @param[in]  p_buffer
+                Pointer to the place holder for data read from the buffer
+    @param[in]  n
+                Number of items to peek
 
+    @returns Number of bytes written to p_buffer
+ */
+/******************************************************************************/
+uint16_t tu_fifo_peek_at_n(tu_fifo_t* f, uint16_t offset, void * p_buffer, uint16_t n)
+{
+  tu_fifo_lock(f);                                          // TODO: Here we may distinguish for read and write pointer mutexes!
+  bool ret = _tu_fifo_peek_at_n(f, offset, p_buffer, n, f->wr_idx, f->rd_idx);
   tu_fifo_unlock(f);
-
-  return true;
+  return ret;
 }
 
 /******************************************************************************/
 /*!
-    @brief Write one element into the RX buffer.
+    @brief Write one element into the buffer.
 
     This function will write one element into the array index specified by
-    the write pointer and increment the write index. If the write index
-    exceeds the max buffer size, then it will roll over to zero.
+    the write pointer and increment the write index.
 
     @param[in]  f
                 Pointer to the FIFO buffer to manipulate
@@ -227,15 +504,23 @@ bool tu_fifo_peek_at(tu_fifo_t* f, uint16_t pos, void * p_buffer)
 
     @returns TRUE if the data was written to the FIFO (overwrittable
              FIFO will always return TRUE)
-*/
+ */
 /******************************************************************************/
-bool tu_fifo_write (tu_fifo_t* f, const void * data)
+bool tu_fifo_write(tu_fifo_t* f, const void * data)
 {
-  if ( tu_fifo_full(f) && !f->overwritable ) return false;
-
   tu_fifo_lock(f);
 
-  _ff_push(f, data, 1);
+  uint16_t w = f->wr_idx;
+
+  if ( _tu_fifo_full(f, w, f->rd_idx) && !f->overwritable ) return false;
+
+  uint16_t wRel = get_relative_pointer(f, w, 0);
+
+  // Write data
+  _ff_push(f, data, wRel);
+
+  // Advance pointer
+  f->wr_idx = advance_pointer(f, w, 1);
 
   tu_fifo_unlock(f);
 
@@ -245,8 +530,7 @@ bool tu_fifo_write (tu_fifo_t* f, const void * data)
 /******************************************************************************/
 /*!
     @brief This function will write n elements into the array index specified by
-    the write pointer and increment the write index. If the write index
-    exceeds the max buffer size, then it will roll over to zero.
+    the write pointer and increment the write index.
 
     @param[in]  f
                 Pointer to the FIFO buffer to manipulate
@@ -255,47 +539,42 @@ bool tu_fifo_write (tu_fifo_t* f, const void * data)
     @param[in]  count
                 Number of element
     @return Number of written elements
-*/
+ */
 /******************************************************************************/
-uint16_t tu_fifo_write_n (tu_fifo_t* f, const void * data, uint16_t count)
+uint16_t tu_fifo_write_n(tu_fifo_t* f, const void * data, uint16_t count)
 {
   if ( count == 0 ) return 0;
 
   tu_fifo_lock(f);
 
+  uint16_t w = f->wr_idx, r = f->rd_idx;
   uint8_t const* buf8 = (uint8_t const*) data;
 
   if (!f->overwritable)
   {
     // Not overwritable limit up to full
-    count = tu_min16(count, tu_fifo_remaining(f));
+    count = tu_min16(count, _tu_fifo_remaining(f, w, r));
   }
   else if (count > f->depth)
   {
     // Only copy last part
     buf8 = buf8 + (count - f->depth) * f->item_size;
     count = f->depth;
-    f->wr_idx = 0;
-    f->rd_idx = 0;
-    f->count = 0;
-  }
 
-  if (count + f->wr_idx <= f->depth )
-  {
-    _ff_push(f, buf8, count);
+    // We start writing at the read pointer's position since we fill the complete
+    // buffer and we do not want to modify the read pointer within a write function!
+    // This would end up in a race condition with read functions!
+    f->wr_idx = r;
   }
-  else
-  {
-    uint16_t const part1 = f->depth - f->wr_idx;
 
-    // Part 1: from wr_idx to end
-    _ff_push(f, buf8, part1);
-    buf8 += part1*f->item_size;
+  uint16_t wRel = get_relative_pointer(f, w, 0);
+
+  // Write data
+  _ff_push_n(f, buf8, count, wRel);
+
+  // Advance pointer
+  f->wr_idx = advance_pointer(f, w, count);
 
-    // Part 2: start to remaining
-    _ff_push(f, buf8, count-part1);
-  }
-  
   tu_fifo_unlock(f);
 
   return count;
@@ -303,19 +582,59 @@ uint16_t tu_fifo_write_n (tu_fifo_t* f, const void * data, uint16_t count)
 
 /******************************************************************************/
 /*!
-    @brief Clear the fifo read and write pointers and set length to zero
+    @brief Clear the fifo read and write pointers
 
     @param[in]  f
                 Pointer to the FIFO buffer to manipulate
-*/
+ */
 /******************************************************************************/
 bool tu_fifo_clear(tu_fifo_t *f)
 {
   tu_fifo_lock(f);
-
-  f->rd_idx = f->wr_idx = f->count = 0;
-
+  f->rd_idx = f->wr_idx = 0;
   tu_fifo_unlock(f);
 
   return true;
 }
+
+/******************************************************************************/
+/*!
+    @brief Advance write pointer - intended to be used in combination with DMA.
+    It is possible to fill the FIFO by use of a DMA in circular mode. Within
+    DMA ISRs you may update the write pointer to be able to read from the FIFO.
+    As long as the DMA is the only process writing into the FIFO this is safe
+    to use.
+
+    USE WITH CARE - WE DO NOT CONDUCT SAFTY CHECKS HERE!
+
+    @param[in]  f
+                Pointer to the FIFO buffer to manipulate
+    @param[in]  n
+                Number of items the write pointer moves forward
+ */
+/******************************************************************************/
+void tu_fifo_advance_write_pointer(tu_fifo_t *f, uint16_t n)
+{
+  f->wr_idx = advance_pointer(f, f->wr_idx, n);
+}
+
+/******************************************************************************/
+/*!
+    @brief Advance read pointer - intended to be used in combination with DMA.
+    It is possible to read from the FIFO by use of a DMA in linear mode. Within
+    DMA ISRs you may update the read pointer to be able to again write into the
+    FIFO. As long as the DMA is the only process reading from the FIFO this is
+    safe to use.
+
+    USE WITH CARE - WE DO NOT CONDUCT SAFTY CHECKS HERE!
+
+    @param[in]  f
+                Pointer to the FIFO buffer to manipulate
+    @param[in]  n
+                Number of items the read pointer moves forward
+ */
+/******************************************************************************/
+void tu_fifo_advance_read_pointer(tu_fifo_t *f, uint16_t n)
+{
+  f->rd_idx = advance_pointer(f, f->rd_idx, n);
+}
diff --git a/src/common/tusb_fifo.h b/src/common/tusb_fifo.h
index fb0c896f3..b87695743 100644
--- a/src/common/tusb_fifo.h
+++ b/src/common/tusb_fifo.h
@@ -31,6 +31,15 @@
 #ifndef _TUSB_FIFO_H_
 #define _TUSB_FIFO_H_
 
+// Due to the use of unmasked pointers, this FIFO does not suffer from loosing
+// one item slice. Furthermore, write and read operations are completely
+// decoupled as write and read functions do not modify a common state. Henceforth,
+// writing or reading from the FIFO within an ISR is safe as long as no other
+// process (thread or ISR) interferes.
+// Also, this FIFO is ready to be used in combination with a DMA as the write and
+// read pointers can be updated from within a DMA ISR. Overflows are detectable
+// within a certain number (see tu_fifo_overflow()).
+
 // mutex is only needed for RTOS
 // for OS None, we don't get preempted
 #define CFG_FIFO_MUTEX      (CFG_TUSB_OS != OPT_OS_NONE)
@@ -39,7 +48,7 @@
 #include <stdbool.h>
 
 #ifdef __cplusplus
- extern "C" {
+extern "C" {
 #endif
 
 #if CFG_FIFO_MUTEX
@@ -52,14 +61,16 @@
  */
 typedef struct
 {
-           uint8_t* buffer    ; ///< buffer pointer
-           uint16_t depth     ; ///< max items
-           uint16_t item_size ; ///< size of each item
-           bool overwritable  ;
+  uint8_t* buffer                        ; ///< buffer pointer
+  uint16_t depth                         ; ///< max items
+  uint16_t item_size                     ; ///< size of each item
+  bool overwritable                      ;
+
+  uint16_t non_used_index_space          ; ///< required for non-power-of-two buffer length
+  uint16_t max_pointer_idx               ; ///< maximum absolute pointer index
 
-  volatile uint16_t count     ; ///< number of items in queue
-  volatile uint16_t wr_idx    ; ///< write pointer
-  volatile uint16_t rd_idx    ; ///< read pointer
+  volatile uint16_t wr_idx               ; ///< write pointer
+  volatile uint16_t rd_idx               ; ///< read pointer
 
 #if CFG_FIFO_MUTEX
   tu_fifo_mutex_t mutex;
@@ -67,14 +78,16 @@ typedef struct
 
 } tu_fifo_t;
 
-#define TU_FIFO_DEF(_name, _depth, _type, _overwritable) \
-  uint8_t _name##_buf[_depth*sizeof(_type)]; \
-  tu_fifo_t _name = {                        \
-      .buffer       = _name##_buf,           \
-      .depth        = _depth,                \
-      .item_size    = sizeof(_type),         \
-      .overwritable = _overwritable,         \
-  }
+#define TU_FIFO_DEF(_name, _depth, _type, _overwritable)                \
+    uint8_t _name##_buf[_depth*sizeof(_type)];                          \
+    tu_fifo_t _name = {                                                 \
+        .buffer                 = _name##_buf,                          \
+        .depth                  = _depth,                               \
+        .item_size              = sizeof(_type),                        \
+        .overwritable           = _overwritable,                        \
+        .max_pointer_idx        = 2*_depth-1,                           \
+        .non_used_index_space   = 0xFFFF - 2*_depth-1,                  \
+    }
 
 bool tu_fifo_clear(tu_fifo_t *f);
 bool tu_fifo_config(tu_fifo_t *f, void* buffer, uint16_t depth, uint16_t item_size, bool overwritable);
@@ -86,37 +99,30 @@ static inline void tu_fifo_config_mutex(tu_fifo_t *f, tu_fifo_mutex_t mutex_hdl)
 }
 #endif
 
-bool     tu_fifo_write   (tu_fifo_t* f, void const * p_data);
-uint16_t tu_fifo_write_n (tu_fifo_t* f, void const * p_data, uint16_t count);
+bool     tu_fifo_write                  (tu_fifo_t* f, void const * p_data);
+uint16_t tu_fifo_write_n                (tu_fifo_t* f, void const * p_data, uint16_t count);
 
-bool     tu_fifo_read    (tu_fifo_t* f, void * p_buffer);
-uint16_t tu_fifo_read_n  (tu_fifo_t* f, void * p_buffer, uint16_t count);
+bool     tu_fifo_read                   (tu_fifo_t* f, void * p_buffer);
+uint16_t tu_fifo_read_n                 (tu_fifo_t* f, void * p_buffer, uint16_t count);
 
-bool     tu_fifo_peek_at (tu_fifo_t* f, uint16_t pos, void * p_buffer);
+bool     tu_fifo_peek_at                (tu_fifo_t* f, uint16_t pos, void * p_buffer);
+uint16_t tu_fifo_peek_at_n              (tu_fifo_t* f, uint16_t pos, void * p_buffer, uint16_t n);
 
-static inline bool tu_fifo_peek(tu_fifo_t* f, void * p_buffer)
-{
-  return tu_fifo_peek_at(f, 0, p_buffer);
-}
+uint16_t tu_fifo_count                  (tu_fifo_t* f);
+bool     tu_fifo_empty                  (tu_fifo_t* f);
+bool     tu_fifo_full                   (tu_fifo_t* f);
+uint16_t tu_fifo_remaining              (tu_fifo_t* f);
+bool     tu_fifo_overflowed               (tu_fifo_t* f);
+void     tu_fifo_correct_read_pointer   (tu_fifo_t* f);
 
-static inline bool tu_fifo_empty(tu_fifo_t* f)
-{
-  return (f->count == 0);
-}
+// Pointer modifications intended to be used in combinations with DMAs.
+// USE WITH CARE - NO SAFTY CHECKS CONDUCTED HERE! NOT MUTEX PROTECTED!
+void     tu_fifo_advance_write_pointer  (tu_fifo_t *f, uint16_t n);
+void     tu_fifo_advance_read_pointer   (tu_fifo_t *f, uint16_t n);
 
-static inline bool tu_fifo_full(tu_fifo_t* f)
-{
-  return (f->count == f->depth);
-}
-
-static inline uint16_t tu_fifo_count(tu_fifo_t* f)
-{
-  return f->count;
-}
-
-static inline uint16_t tu_fifo_remaining(tu_fifo_t* f)
+static inline bool tu_fifo_peek(tu_fifo_t* f, void * p_buffer)
 {
-  return f->depth - f->count;
+  return tu_fifo_peek_at(f, 0, p_buffer);
 }
 
 static inline uint16_t tu_fifo_depth(tu_fifo_t* f)
@@ -125,7 +131,7 @@ static inline uint16_t tu_fifo_depth(tu_fifo_t* f)
 }
 
 #ifdef __cplusplus
- }
+}
 #endif
 
 #endif /* _TUSB_FIFO_H_ */