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src/core/buffer.c

@@ -49,15 +49,44 @@
  *
  * @endcode
  *
+ * For a description of the internal operation, see \ref buffer_int.
+ *
  */
 
-/** @package Internals
+/** @page buffer_int Buffer internals
  *
+ * A buffer consists of a single, contiguous area of memory, some of
+ * which is "filled" and the remainder of which is "free".  The
+ * "filled" and "free" spaces are not necessarily contiguous.
+ *
+ * When a buffer is initialised via init_buffer(), it consists of a
+ * single free space.  As data is added to the buffer via
+ * fill_buffer(), this free space decreases and can become fragmented.
+ *
+ * Each free block within a buffer starts with a "tail byte".  If the
+ * tail byte is non-zero, this indicates that the free block is the
+ * tail of the buffer, i.e. occupies all the remaining space up to the
+ * end of the buffer.  When the tail byte is non-zero, it indicates
+ * that a descriptor (a @c struct @c buffer_free_block) follows the
+ * tail byte.  The descriptor describes the size of the free block and
+ * the address of the next free block.
+ *
+ * We cannot simply always start a free block with a descriptor,
+ * because it is conceivable that we will, at some point, encounter a
+ * situation in which the final free block of a buffer is too small to
+ * contain a descriptor.  Consider a protocol with a blocksize of 512
+ * downloading a 1025-byte file into a 1025-byte buffer.  Suppose that
+ * the first two blocks are received; we have now filled 1024 of the
+ * 1025 bytes in the buffer, and our only free block consists of the
+ * 1025th byte.  Using a "tail byte" solves this problem.
+ *
+ * 
  * Note that the rather convoluted way of manipulating the buffer
  * descriptors (using copy_{to,from}_phys rather than straightforward
  * pointers) is needed to cope with operation as a PXE stack, when we
  * may be running in real mode or 16-bit protected mode, and therefore
- * cannot directly access arbitrary areas of memory.
+ * cannot directly access arbitrary areas of memory using simple
+ * pointers.
  *
  */
 
@@ -203,10 +232,9 @@ static inline void unfree_block ( struct buffer *buffer,
  * buffer_free_block) apart.  If this condition is not satisfied, data
  * corruption will occur.  (See split_free_block() for details.)
  *
- * @att In practice this is not a problem.  Callers of fill_buffer()
- * will be download protocols such as TFTP, and very few protocols
- * have a block size smaller than @c sizeof(struct @c
- * buffer_free_block).
+ * In practice this is not a problem.  Callers of fill_buffer() will
+ * be download protocols such as TFTP, and very few protocols have a
+ * block size smaller than @c sizeof(struct @c buffer_free_block).
  *
  */
 int fill_buffer ( struct buffer *buffer, const void *data,

src/include/buffer.h

@@ -3,34 +3,33 @@
 
 #include "stdint.h"
 
-/*
- * "start" and "end" denote the real boundaries of the buffer.  "fill"
- * denotes the offset to the first free block in the buffer.  (If the
- * buffer is full, "fill" will equal ( end - start ) ).
+/* @file */
+
+/**
+ * A buffer
+ *
+ * @c start and @c end denote the real boundaries of the buffer, and
+ * are physical addresses.  @c fill denotes the offset to the first
+ * free block in the buffer.  (If the buffer is full, @c fill will
+ * equal @c end-start.)
  *
  */
 struct buffer {
-	physaddr_t	start;
-	physaddr_t	end;
-	off_t		fill;
+	physaddr_t	start;		/**< Start of buffer in memory */
+	physaddr_t	end;		/**< End of buffer in memory */
+	off_t		fill;		/**< Offset to first gap in buffer */
 };
 
-/*
- * Free blocks in the buffer start with a "tail byte".  If non-zero,
- * this byte indicates that the free block is the tail of the buffer,
- * i.e. occupies all the remaining space up to the end of the buffer.
- * When the tail byte is non-zero, it indicates that the remainder of
- * the descriptor (the struct buffer_free_block) follows the tail
- * byte.
+/**
+ * A free block descriptor.
  *
- * This scheme is necessary because we may end up with a tail that is
- * smaller than a struct buffer_free_block.
+ * See \ref buffer_int for a full description of the fields.
  *
  */
 struct buffer_free_block {
-	char		tail;
-	physaddr_t	next_free;
-	physaddr_t	end;
+	char		tail;		/**< Tail byte marker */
+	physaddr_t	next_free;	/**< Address of next free block */
+	physaddr_t	end;		/**< End of this block */
 } __attribute__ (( packed ));
 
 /* Functions in buffer.c */