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-
- /** @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 "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;
- }
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