/** @file
*
* 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 using simple
* pointers.
*
*/
#include "stdio.h"
#include "stddef.h"
#include "string.h"
#include "io.h"
#include "errno.h"
#include <assert.h>
#include "buffer.h"
/**
* Initialise a buffer.
*
* @v buffer The buffer to be initialised
* @ret None -
* @err None -
*
* Set @c buffer->start and @c buffer->end before calling init_buffer().
* init_buffer() will initialise the buffer to the state of being
* empty.
*
*/
void init_buffer ( struct buffer *buffer ) {
char tail = 1;
buffer->fill = 0;
if ( buffer->end != buffer->start )
copy_to_phys ( buffer->start, &tail, sizeof ( tail ) );
DBG ( "BUFFER [%x,%x) initialised\n", buffer->start, buffer->end );
}
/**
* Move to the next block in the free list
*
* @v block The current free block
* @v buffer The buffer
* @ret True Successfully moved to the next free block
* @ret False There are no more free blocks
* @ret block The next free block
* @err None -
*
* Move to the next block in the free block list, filling in @c block
* with the descriptor for this next block. If the next block is the
* tail block, @c block will be filled with the values calculated for
* the tail block, otherwise the descriptor will be read from the free
* block itself.
*
* If there are no more free blocks, next_free_block() returns False
* and leaves @c block with invalid contents.
*
* Set <tt> block->next = buffer->start + buffer->fill </tt> for the
* first call to next_free_block().
*/
static inline int next_free_block ( struct buffer_free_block *block,
struct buffer *buffer ) {
/* Move to next block */
block->start = block->next;
/* If at end of buffer, return 0 */
if ( block->start >= buffer->end )
return 0;
/* Set up ->next and ->end as for a tail block */
block->next = block->end = buffer->end;
/* Read tail marker from block */
copy_from_phys ( &block->tail, block->start, sizeof ( block->tail ) );
/* If not a tail block, read whole block descriptor from block */
if ( ! block->tail ) {
copy_from_phys ( block, block->start, sizeof ( *block ) );
}
return 1;
}
/**
* Store a free block descriptor
*
* @v block The free block descriptor to store
* @ret None -
* @err None -
*
* Writes a free block descriptor back to a free block. If the block
* is a tail block, only the tail marker will be written, otherwise
* the whole block descriptor will be written.
*/
static inline void store_free_block ( struct buffer_free_block *block ) {
copy_to_phys ( block->start, block,
( block->tail ?
sizeof ( block->tail ) : sizeof ( *block ) ) );
}
/**
* Write data into a buffer.
*
* @v buffer The buffer into which to write the data
* @v data The data to be written
* @v offset Offset within the buffer at which to write the data
* @v len Length of data to be written
* @ret True Data was successfully written
* @ret False Data was not written
* @err ENOMEM Buffer is too small to contain the data
*
* Writes a block of data into the buffer. The block need not be
* aligned to any particular boundary, or be of any particular size,
* and it may overlap blocks already in the buffer (i.e. duplicate
* calls to fill_buffer() are explicitly permitted).
*
* @c buffer->fill will be updated to indicate the fill level of the
* buffer, i.e. the offset to the first gap within the buffer. If the
* filesize is known (e.g. as with the SLAM protocol), you can test
* for end-of-file by checking for @c buffer->fill==filesize. If the
* filesize is not known, but there is a well-defined end-of-file test
* (e.g. as with the TFTP protocol), you can read @c buffer->fill to
* determine the final filesize. If blocks are known to be delivered
* in a strictly sequential order with no packet loss or duplication,
* then you can pass in @c offset==buffer->fill.
*
* @b NOTE: It is the caller's responsibility to ensure that the
* boundaries between data blocks are more than @c sizeof(struct @c
* buffer_free_block) apart. If this condition is not satisfied, data
* corruption will occur.
*
* 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,
off_t offset, size_t len ) {
struct buffer_free_block block, before, after;
physaddr_t data_start, data_end;
/* Calculate start and end addresses of data */
data_start = buffer->start + offset;
data_end = data_start + len;
DBG ( "BUFFER [%x,%x) writing portion [%x,%x)\n",
buffer->start, buffer->end, data_start, data_end );
/* Check buffer bounds */
if ( data_end > buffer->end ) {
DBG ( "BUFFER [%x,%x) too small for data!\n",
buffer->start, buffer->end );
errno = ENOMEM;
return 0;
}
/* Find 'before' and 'after' blocks, if any */
before.start = before.end = 0;
after.start = after.end = buffer->end;
block.next = buffer->start + buffer->fill;
while ( next_free_block ( &block, buffer ) ) {
if ( ( block.start < data_start ) &&
( block.start >= before.start ) )
memcpy ( &before, &block, sizeof ( before ) );
if ( ( block.end > data_end ) &&
( block.end <= after.end ) )
memcpy ( &after, &block, sizeof ( after ) );
}
/* Truncate 'before' and 'after' blocks around data. */
if ( data_start < before.end )
before.end = data_start;
if ( data_end > after.start )
after.start = data_end;
/* Link 'after' block to 'before' block */
before.next = after.start;
/* Write back 'before' block, if any */
if ( before.start ) {
before.tail = 0;
assert ( ( before.end - before.start ) >=
sizeof ( struct buffer_free_block ) );
store_free_block ( &before );
} else {
buffer->fill = before.next - buffer->start;
}
/* Write back 'after' block, if any */
if ( after.start < buffer->end ) {
assert ( after.tail ||
( ( after.end - after.start ) >=
sizeof ( struct buffer_free_block ) ) );
store_free_block ( &after );
}
DBG ( "BUFFER [%x,%x) before [%x,%x) after [%x,%x)\n",
buffer->start, buffer->end, before.start, before.end,
after.start, after.end );
/* Copy data into buffer */
copy_to_phys ( data_start, data, len );
DBG ( "BUFFER [%x,%x) full up to %x\n",
buffer->start, buffer->end, buffer->start + buffer->fill );
return 1;
}
