tables.h

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#ifndef _IPXE_TABLES_H
#define _IPXE_TABLES_H

FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );

/** @page ifdef_harmful #ifdef considered harmful
 *
 * Overuse of @c #ifdef has long been a problem in Etherboot.
 * Etherboot provides a rich array of features, but all these features
 * take up valuable space in a ROM image.  The traditional solution to
 * this problem has been for each feature to have its own @c #ifdef
 * option, allowing the feature to be compiled in only if desired.
 *
 * The problem with this is that it becomes impossible to compile, let
 * alone test, all possible versions of Etherboot.  Code that is not
 * typically used tends to suffer from bit-rot over time.  It becomes
 * extremely difficult to predict which combinations of compile-time
 * options will result in code that can even compile and link
 * correctly.
 *
 * To solve this problem, we have adopted a new approach from
 * Etherboot 5.5 onwards.  @c #ifdef is now "considered harmful", and
 * its use should be minimised.  Separate features should be
 * implemented in separate @c .c files, and should \b always be
 * compiled (i.e. they should \b not be guarded with a @c #ifdef @c
 * MY_PET_FEATURE statement).  By making (almost) all code always
 * compile, we avoid the problem of bit-rot in rarely-used code.
 *
 * The file config.h, in combination with the @c make command line,
 * specifies the objects that will be included in any particular build
 * of Etherboot.  For example, suppose that config.h includes the line
 *
 * @code
 *
 *   #define CONSOLE_SERIAL
 *   #define DOWNLOAD_PROTO_TFTP
 *
 * @endcode
 *
 * When a particular Etherboot image (e.g. @c bin/rtl8139.zdsk) is
 * built, the options specified in config.h are used to drag in the
 * relevant objects at link-time.  For the above example, serial.o and
 * tftp.o would be linked in.
 *
 * There remains one problem to solve: how do these objects get used?
 * Traditionally, we had code such as
 *
 * @code
 *
 *    #ifdef CONSOLE_SERIAL
 *      serial_init();
 *    #endif
 *
 * @endcode
 *
 * in main.c, but this reintroduces @c #ifdef and so is a Bad Idea.
 * We cannot simply remove the @c #ifdef and make it
 *
 * @code
 *
 *   serial_init();
 *
 * @endcode
 *
 * because then serial.o would end up always being linked in.
 *
 * The solution is to use @link tables.h linker tables @endlink.
 *
 */

/** @file
 *
 * Linker tables
 *
 * Read @ref ifdef_harmful first for some background on the motivation
 * for using linker tables.
 *
 * This file provides macros for dealing with linker-generated tables
 * of fixed-size symbols.  We make fairly extensive use of these in
 * order to avoid @c #ifdef spaghetti and/or linker symbol pollution.
 * For example, instead of having code such as
 *
 * @code
 *
 *    #ifdef CONSOLE_SERIAL
 *      serial_init();
 *    #endif
 *
 * @endcode
 *
 * we make serial.c generate an entry in the initialisation function
 * table, and then have a function call_init_fns() that simply calls
 * all functions present in this table.  If and only if serial.o gets
 * linked in, then its initialisation function will be called.  We
 * avoid linker symbol pollution (i.e. always dragging in serial.o
 * just because of a call to serial_init()) and we also avoid @c
 * #ifdef spaghetti (having to conditionalise every reference to
 * functions in serial.c).
 *
 * The linker script takes care of assembling the tables for us.  All
 * our table sections have names of the format @c .tbl.NAME.NN where
 * @c NAME designates the data structure stored in the table (e.g. @c
 * init_fns) and @c NN is a two-digit decimal number used to impose an
 * ordering upon the tables if required.  @c NN=00 is reserved for the
 * symbol indicating "table start", and @c NN=99 is reserved for the
 * symbol indicating "table end".
 *
 * As an example, suppose that we want to create a "frobnicator"
 * feature framework, and allow for several independent modules to
 * provide frobnicating services.  Then we would create a frob.h
 * header file containing e.g.
 *
 * @code
 *
 *   struct frobnicator {
 *      const char *name;               // Name of the frobnicator
 *      void ( *frob ) ( void );        // The frobnicating function itself
 *   };
 *
 *   #define FROBNICATORS __table ( struct frobnicator, "frobnicators" )
 *
 *   #define __frobnicator __table_entry ( FROBNICATORS, 01 )
 *
 * @endcode
 *
 * Any module providing frobnicating services would look something
 * like
 *
 * @code
 *
 *   #include "frob.h"
 *
 *   static void my_frob ( void ) {
 *      // Do my frobnicating
 *      ...
 *   }
 *
 *   struct frob my_frobnicator __frobnicator = {
 *      .name = "my_frob",
 *      .frob = my_frob,
 *   };
 *
 * @endcode
 *
 * The central frobnicator code (frob.c) would use the frobnicating
 * modules as follows
 *
 * @code
 *
 *   #include "frob.h"
 *
 *   // Call all linked-in frobnicators
 *   void frob_all ( void ) {
 *      struct frob *frob;
 *
 *      for_each_table ( frob, FROBNICATORS ) {
 *         printf ( "Calling frobnicator \"%s\"\n", frob->name );
 *         frob->frob ();
 *      }
 *   }
 *
 * @endcode
 *
 * See init.h and init.c for a real-life example.
 *
 */

#ifdef DOXYGEN
#define __attribute__( x )
#endif

/**
 * Declare a linker table
 *
 * @v type              Data type
 * @v name              Table name
 * @ret table           Linker table
 */
#define __table( type, name ) ( type, name )

/**
 * Get linker table data type
 *
 * @v table             Linker table
 * @ret type            Data type
 */
#define __table_type( table ) __table_extract_type table
#define __table_extract_type( type, name ) type

/**
 * Get linker table name
 *
 * @v table             Linker table
 * @ret name            Table name
 */
#define __table_name( table ) __table_extract_name table
#define __table_extract_name( type, name ) name

/**
 * Get linker table section name
 *
 * @v table             Linker table
 * @v idx               Sub-table index
 * @ret section         Section name
 */
#define __table_section( table, idx ) \
        ".tbl." __table_name ( table ) "." __table_str ( idx )
#define __table_str( x ) #x

/**
 * Get linker table alignment
 *
 * @v table             Linker table
 * @ret align           Alignment
 */
#define __table_alignment( table ) __alignof__ ( __table_type ( table ) )

/**
 * Declare a linker table entry
 *
 * @v table             Linker table
 * @v idx               Sub-table index
 *
 * Example usage:
 *
 * @code
 *
 *   #define FROBNICATORS __table ( struct frobnicator, "frobnicators" )
 *
 *   #define __frobnicator __table_entry ( FROBNICATORS, 01 )
 *
 *   struct frobnicator my_frob __frobnicator = {
 *      ...
 *   };
 *
 * @endcode
 */
#define __table_entry( table, idx )                                     \
        __attribute__ (( __section__ ( __table_section ( table, idx ) ),\
                         __aligned__ ( __table_alignment ( table ) ) ))

/**
 * Get start of linker table entries
 *
 * @v table             Linker table
 * @v idx               Sub-table index
 * @ret entries         Start of entries
 */
#define __table_entries( table, idx ) ( {                               \
        static __table_type ( table ) __table_entries[0]                \
                __table_entry ( table, idx )                            \
                __attribute__ (( unused ));                             \
        __table_entries; } )

/**
 * Declare start of linker table entries
 *
 * @v entries           Start of entries
 * @v table             Linker table
 * @v idx               Sub-table index
 */
#define __TABLE_ENTRIES( entries, table, idx )                          \
        __table_type ( table ) entries[0]                               \
                __table_entry ( table, idx )

/**
 * Get start of linker table
 *
 * @v table             Linker table
 * @ret start           Start of linker table
 *
 * Example usage:
 *
 * @code
 *
 *   #define FROBNICATORS __table ( struct frobnicator, "frobnicators" )
 *
 *   struct frobnicator *frobs = table_start ( FROBNICATORS );
 *
 * @endcode
 */
#define table_start( table ) __table_entries ( table, 00 )

/**
 * Declare start of linker table
 *
 * @v start             Start of linker table
 * @v table             Linker table
 */
#define TABLE_START( start, table ) __TABLE_ENTRIES ( start, table, 00 )

/**
 * Get end of linker table
 *
 * @v table             Linker table
 * @ret end             End of linker table
 *
 * Example usage:
 *
 * @code
 *
 *   #define FROBNICATORS __table ( struct frobnicator, "frobnicators" )
 *
 *   struct frobnicator *frobs_end = table_end ( FROBNICATORS );
 *
 * @endcode
 */
#define table_end( table ) __table_entries ( table, 99 )

/**
 * Declare end of linker table
 *
 * @v end               End of linker table
 * @v table             Linker table
 */
#define TABLE_END( end, table ) __TABLE_ENTRIES ( end, table, 99 )

/**
 * Get number of entries in linker table
 *
 * @v table             Linker table
 * @ret num_entries     Number of entries in linker table
 *
 * Example usage:
 *
 * @code
 *
 *   #define FROBNICATORS __table ( struct frobnicator, "frobnicators" )
 *
 *   unsigned int num_frobs = table_num_entries ( FROBNICATORS );
 *
 * @endcode
 *
 */
#define table_num_entries( table )                                      \
        ( ( unsigned int ) ( table_end ( table ) -                      \
                             table_start ( table ) ) )

/**
 * Get index of entry within linker table
 *
 * @v table             Linker table
 * @v entry             Table entry
 *
 * Example usage:
 *
 * @code
 *
 *   #define FROBNICATORS __table ( struct frobnicator, "frobnicators" )
 *
 *   #define __frobnicator __table_entry ( FROBNICATORS, 01 )
 *
 *   struct frobnicator my_frob __frobnicator = {
 *      ...
 *   };
 *
 *   unsigned int my_frob_idx = table_index ( FROBNICATORS, &my_frob );
 *
 * @endcode
 */
#define table_index( table, entry )                                     \
        ( ( unsigned int ) ( (entry) - table_start ( table ) ) )

/**
 * Iterate through all entries within a linker table
 *
 * @v pointer           Entry pointer
 * @v table             Linker table
 *
 * Example usage:
 *
 * @code
 *
 *   #define FROBNICATORS __table ( struct frobnicator, "frobnicators" )
 *
 *   struct frobnicator *frob;
 *
 *   for_each_table_entry ( frob, FROBNICATORS ) {
 *     ...
 *   }
 *
 * @endcode
 *
 */
#define for_each_table_entry( pointer, table )                          \
        for ( (pointer) = table_start ( table ) ;                       \
              (pointer) < table_end ( table ) ;                         \
              (pointer)++ )

/**
 * Iterate through all remaining entries within a linker table
 *
 * @v pointer           Entry pointer, preset to most recent entry
 * @v table             Linker table
 *
 * Example usage:
 *
 * @code
 *
 *   #define FROBNICATORS __table ( struct frobnicator, "frobnicators" )
 *   #define __frobnicator __table_entry ( FROBNICATORS, 01 )
 *
 *   struct frob my_frobnicator __frobnicator;
 *   struct frobnicator *frob;
 *
 *   frob = &my_frobnicator;
 *   for_each_table_entry_continue ( frob, FROBNICATORS ) {
 *     ...
 *   }
 *
 * @endcode
 *
 */
#define for_each_table_entry_continue( pointer, table )                 \
        for ( (pointer)++ ;                                             \
              (pointer) < table_end ( table ) ;                         \
              (pointer)++ )

/**
 * Iterate through all entries within a linker table in reverse order
 *
 * @v pointer           Entry pointer
 * @v table             Linker table
 *
 * Example usage:
 *
 * @code
 *
 *   #define FROBNICATORS __table ( struct frobnicator, "frobnicators" )
 *
 *   struct frobnicator *frob;
 *
 *   for_each_table_entry_reverse ( frob, FROBNICATORS ) {
 *     ...
 *   }
 *
 * @endcode
 *
 */
#define for_each_table_entry_reverse( pointer, table )                  \
        for ( (pointer) = ( table_end ( table ) - 1 ) ;                 \
              (pointer) >= table_start ( table ) ;                      \
              (pointer)-- )

/**
 * Iterate through all remaining entries within a linker table in reverse order
 *
 * @v pointer           Entry pointer, preset to most recent entry
 * @v table             Linker table
 *
 * Example usage:
 *
 * @code
 *
 *   #define FROBNICATORS __table ( struct frobnicator, "frobnicators" )
 *   #define __frobnicator __table_entry ( FROBNICATORS, 01 )
 *
 *   struct frob my_frobnicator __frobnicator;
 *   struct frobnicator *frob;
 *
 *   frob = &my_frobnicator;
 *   for_each_table_entry_continue_reverse ( frob, FROBNICATORS ) {
 *     ...
 *   }
 *
 * @endcode
 *
 */
#define for_each_table_entry_continue_reverse( pointer, table )         \
        for ( (pointer)-- ;                                             \
              (pointer) >= table_start ( table ) ;                      \
              (pointer)-- )

#endif /* _IPXE_TABLES_H */