src/core/buffer.c

   1
   2 /** @file
   3  *
   4  * Buffer internals.
   5  *
   6  * A buffer consists of a single, contiguous area of memory, some of
   7  * which is "filled" and the remainder of which is "free".  The
   8  * "filled" and "free" spaces are not necessarily contiguous.
   9  *
  10  * When a buffer is initialised via init_buffer(), it consists of a
  11  * single free space.  As data is added to the buffer via
  12  * fill_buffer(), this free space decreases and can become fragmented.
  13  *
  14  * Each free block within a buffer starts with a "tail byte".  If the
  15  * tail byte is non-zero, this indicates that the free block is the
  16  * tail of the buffer, i.e. occupies all the remaining space up to the
  17  * end of the buffer.  When the tail byte is non-zero, it indicates
  18  * that a descriptor (a @c struct @c buffer_free_block) follows the
  19  * tail byte.  The descriptor describes the size of the free block and
  20  * the address of the next free block.
  21  *
  22  * We cannot simply always start a free block with a descriptor,
  23  * because it is conceivable that we will, at some point, encounter a
  24  * situation in which the final free block of a buffer is too small to
  25  * contain a descriptor.  Consider a protocol with a blocksize of 512
  26  * downloading a 1025-byte file into a 1025-byte buffer.  Suppose that
  27  * the first two blocks are received; we have now filled 1024 of the
  28  * 1025 bytes in the buffer, and our only free block consists of the
  29  * 1025th byte.  Using a "tail byte" solves this problem.
  30  *
  31  *
  32  * Note that the rather convoluted way of manipulating the buffer
  33  * descriptors (using copy_{to,from}_phys rather than straightforward
  34  * pointers) is needed to cope with operation as a PXE stack, when we
  35  * may be running in real mode or 16-bit protected mode, and therefore
  36  * cannot directly access arbitrary areas of memory using simple
  37  * pointers.
  38  *
  39  */
  40
  41 #include "stddef.h"
  42 #include "string.h"
  43 #include "io.h"
  44 #include "errno.h"
  45 #include <assert.h>
  46 #include "buffer.h"
  47
  48 /**
  49  * Initialise a buffer.
  50  *
  51  * @v buffer            The buffer to be initialised
  52  * @ret None            -
  53  * @err None            -
  54  *
  55  * Set @c buffer->start and @c buffer->end before calling init_buffer().
  56  * init_buffer() will initialise the buffer to the state of being
  57  * empty.
  58  *
  59  */
  60 void init_buffer ( struct buffer *buffer ) {
  61         char tail = 1;
  62
  63         buffer->fill = 0;
  64         if ( buffer->end != buffer->start )
  65                 copy_to_phys ( buffer->start, &tail, sizeof ( tail ) );
  66
  67         DBG ( "BUFFER [%x,%x) initialised\n", buffer->start, buffer->end );
  68 }
  69
  70 /**
  71  * Move to the next block in the free list
  72  *
  73  * @v block             The current free block
  74  * @v buffer            The buffer
  75  * @ret True            Successfully moved to the next free block
  76  * @ret False           There are no more free blocks
  77  * @ret block           The next free block
  78  * @err None            -
  79  *
  80  * Move to the next block in the free block list, filling in @c block
  81  * with the descriptor for this next block.  If the next block is the
  82  * tail block, @c block will be filled with the values calculated for
  83  * the tail block, otherwise the descriptor will be read from the free
  84  * block itself.
  85  *
  86  * If there are no more free blocks, next_free_block() returns False
  87  * and leaves @c block with invalid contents.
  88  *
  89  * Set <tt> block->next = buffer->start + buffer->fill </tt> for the
  90  * first call to next_free_block().
  91  */
  92 static inline int next_free_block ( struct buffer_free_block *block,
  93                                     struct buffer *buffer ) {
  94         /* Move to next block */
  95         block->start = block->next;
  96
  97         /* If at end of buffer, return 0 */
  98         if ( block->start >= buffer->end )
  99                 return 0;
 100
 101         /* Set up ->next and ->end as for a tail block */
 102         block->next = block->end = buffer->end;
 103
 104         /* Read tail marker from block */
 105         copy_from_phys ( &block->tail, block->start, sizeof ( block->tail ) );
 106
 107         /* If not a tail block, read whole block descriptor from block */
 108         if ( ! block->tail ) {
 109                 copy_from_phys ( block, block->start, sizeof ( *block ) );
 110         }
 111
 112         return 1;
 113 }
 114
 115 /**
 116  * Store a free block descriptor
 117  *
 118  * @v block             The free block descriptor to store
 119  * @ret None            -
 120  * @err None            -
 121  *
 122  * Writes a free block descriptor back to a free block.  If the block
 123  * is a tail block, only the tail marker will be written, otherwise
 124  * the whole block descriptor will be written.
 125  */
 126 static inline void store_free_block ( struct buffer_free_block *block ) {
 127         copy_to_phys ( block->start, block,
 128                        ( block->tail ?
 129                          sizeof ( block->tail ) : sizeof ( *block ) ) );
 130 }
 131
 132 /**
 133  * Write data into a buffer.
 134  *
 135  * @v buffer            The buffer into which to write the data
 136  * @v data              The data to be written
 137  * @v offset            Offset within the buffer at which to write the data
 138  * @v len               Length of data to be written
 139  * @ret True            Data was successfully written
 140  * @ret False           Data was not written
 141  * @err ENOMEM          Buffer is too small to contain the data
 142  *
 143  * Writes a block of data into the buffer.  The block need not be
 144  * aligned to any particular boundary, or be of any particular size,
 145  * and it may overlap blocks already in the buffer (i.e. duplicate
 146  * calls to fill_buffer() are explicitly permitted).
 147  *
 148  * @c buffer->fill will be updated to indicate the fill level of the
 149  * buffer, i.e. the offset to the first gap within the buffer.  If the
 150  * filesize is known (e.g. as with the SLAM protocol), you can test
 151  * for end-of-file by checking for @c buffer->fill==filesize.  If the
 152  * filesize is not known, but there is a well-defined end-of-file test
 153  * (e.g. as with the TFTP protocol), you can read @c buffer->fill to
 154  * determine the final filesize.  If blocks are known to be delivered
 155  * in a strictly sequential order with no packet loss or duplication,
 156  * then you can pass in @c offset==buffer->fill.
 157  *
 158  * @b NOTE: It is the caller's responsibility to ensure that the
 159  * boundaries between data blocks are more than @c sizeof(struct @c
 160  * buffer_free_block) apart.  If this condition is not satisfied, data
 161  * corruption will occur.
 162  *
 163  * In practice this is not a problem.  Callers of fill_buffer() will
 164  * be download protocols such as TFTP, and very few protocols have a
 165  * block size smaller than @c sizeof(struct @c buffer_free_block).
 166  *
 167  */
 168 int fill_buffer ( struct buffer *buffer, const void *data,
 169                   off_t offset, size_t len ) {
 170         struct buffer_free_block block, before, after;
 171         physaddr_t data_start, data_end;
 172
 173         /* Calculate start and end addresses of data */
 174         data_start = buffer->start + offset;
 175         data_end = data_start + len;
 176         DBG ( "BUFFER [%x,%x) writing portion [%x,%x)\n",
 177               buffer->start, buffer->end, data_start, data_end );
 178
 179         /* Check buffer bounds */
 180         if ( data_end > buffer->end ) {
 181                 DBG ( "BUFFER [%x,%x) too small for data!\n",
 182                       buffer->start, buffer->end );
 183                 errno = ENOMEM;
 184                 return 0;
 185         }
 186
 187         /* Find 'before' and 'after' blocks, if any */
 188         before.start = before.end = 0;
 189         after.start = after.end = buffer->end;
 190         block.next = buffer->start + buffer->fill;
 191         while ( next_free_block ( &block, buffer ) ) {
 192                 if ( ( block.start < data_start ) &&
 193                      ( block.start >= before.start ) )
 194                         memcpy ( &before, &block, sizeof ( before ) );
 195                 if ( ( block.end > data_end ) &&
 196                      ( block.end <= after.end ) )
 197                         memcpy ( &after, &block, sizeof ( after ) );
 198         }
 199
 200         /* Truncate 'before' and 'after' blocks around data. */
 201         if ( data_start < before.end )
 202                 before.end = data_start;
 203         if ( data_end > after.start )
 204                 after.start = data_end;
 205
 206         /* Link 'after' block to 'before' block */
 207         before.next = after.start;
 208
 209         /* Write back 'before' block, if any */
 210         if ( before.start ) {
 211                 before.tail = 0;
 212                 assert ( ( before.end - before.start ) >=
 213                          sizeof ( struct buffer_free_block ) );
 214                 store_free_block ( &before );
 215         } else {
 216                 buffer->fill = before.next - buffer->start;
 217         }
 218
 219         /* Write back 'after' block, if any */
 220         if ( after.start < buffer->end ) {
 221                 assert ( after.tail ||
 222                          ( ( after.end - after.start ) >=
 223                            sizeof ( struct buffer_free_block ) ) );
 224                 store_free_block ( &after );
 225         }
 226
 227         DBG ( "BUFFER [%x,%x) before [%x,%x) after [%x,%x)\n",
 228               buffer->start, buffer->end, before.start, before.end,
 229               after.start, after.end );
 230
 231         /* Copy data into buffer */
 232         copy_to_phys ( data_start, data, len );
 233
 234         DBG ( "BUFFER [%x,%x) full up to %x\n",
 235               buffer->start, buffer->end, buffer->start + buffer->fill );
 236
 237         return 1;
 238 }