+ *
+ * The pointer is initialised with NULL value and may be set by
+ * __ffs_epfile_read_data function to point to a temporary buffer.
+ *
+ * In normal operation, calls to __ffs_epfile_read_buffered will consume
+ * data from said buffer and eventually free it. Importantly, while the
+ * function is using the buffer, it sets the pointer to NULL. This is
+ * all right since __ffs_epfile_read_data and __ffs_epfile_read_buffered
+ * can never run concurrently (they are synchronised by epfile->mutex)
+ * so the latter will not assign a new value to the pointer.
+ *
+ * Meanwhile ffs_func_eps_disable frees the buffer (if the pointer is
+ * valid) and sets the pointer to READ_BUFFER_DROP value. This special
+ * value is crux of the synchronisation between ffs_func_eps_disable and
+ * __ffs_epfile_read_data.
+ *
+ * Once __ffs_epfile_read_data is about to finish it will try to set the
+ * pointer back to its old value (as described above), but seeing as the
+ * pointer is not-NULL (namely READ_BUFFER_DROP) it will instead free
+ * the buffer.
+ *
+ * == State transitions ==
+ *
+ * • ptr == NULL: (initial state)
+ * ◦ __ffs_epfile_read_buffer_free: go to ptr == DROP
+ * ◦ __ffs_epfile_read_buffered: nop
+ * ◦ __ffs_epfile_read_data allocates temp buffer: go to ptr == buf
+ * ◦ reading finishes: n/a, not in ‘and reading’ state
+ * • ptr == DROP:
+ * ◦ __ffs_epfile_read_buffer_free: nop
+ * ◦ __ffs_epfile_read_buffered: go to ptr == NULL
+ * ◦ __ffs_epfile_read_data allocates temp buffer: free buf, nop
+ * ◦ reading finishes: n/a, not in ‘and reading’ state
+ * • ptr == buf:
+ * ◦ __ffs_epfile_read_buffer_free: free buf, go to ptr == DROP
+ * ◦ __ffs_epfile_read_buffered: go to ptr == NULL and reading
+ * ◦ __ffs_epfile_read_data: n/a, __ffs_epfile_read_buffered
+ * is always called first
+ * ◦ reading finishes: n/a, not in ‘and reading’ state
+ * • ptr == NULL and reading:
+ * ◦ __ffs_epfile_read_buffer_free: go to ptr == DROP and reading
+ * ◦ __ffs_epfile_read_buffered: n/a, mutex is held
+ * ◦ __ffs_epfile_read_data: n/a, mutex is held
+ * ◦ reading finishes and …
+ * … all data read: free buf, go to ptr == NULL
+ * … otherwise: go to ptr == buf and reading
+ * • ptr == DROP and reading:
+ * ◦ __ffs_epfile_read_buffer_free: nop
+ * ◦ __ffs_epfile_read_buffered: n/a, mutex is held
+ * ◦ __ffs_epfile_read_data: n/a, mutex is held
+ * ◦ reading finishes: free buf, go to ptr == DROP