[U-Boot] [PATCH 2/3] common: Implement support for linker-generated arrays
Joe Hershberger
joe.hershberger at gmail.com
Mon Sep 24 18:30:11 CEST 2012
Hi Marek,
On Sun, Sep 23, 2012 at 8:21 PM, Marek Vasut <marex at denx.de> wrote:
> This patch adds support for linker-generated array. These arrays
> are a generalization of the U-Boot command declaration approach.
>
> Basically, the idea is to generate an array, where elements of the
> array are statically initialized at compiler time and each element
> is declared separatelly at different place. Such array though can
> later be accessed and used via special accessor.
>
> The actual implementation relies on placing any variable that is to
> represent an element of LG-array into subsection of .u_boot_list
> linker section . Once compiled, it is possible to dump all symbols
> placed in .u_boot_list section and generate appropriate bounds for
> each subsection of the .u_boot_list section. Each such subsection
> this contains .__start and .__end entries at the begining and end
> respecitively.
>
> This allows for simple run-time traversing of the array, since the
> symbols are properly defined.
>
> Signed-off-by: Marek Vasut <marex at denx.de>
> Cc: Joe Hershberger <joe.hershberger at gmail.com>
> Cc: Mike Frysinger <vapier at gentoo.org>
> ---
> include/linker_lists.h | 125 ++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 125 insertions(+)
> create mode 100644 include/linker_lists.h
>
> diff --git a/include/linker_lists.h b/include/linker_lists.h
> new file mode 100644
> index 0000000..ffa95f6
> --- /dev/null
> +++ b/include/linker_lists.h
> @@ -0,0 +1,125 @@
> +/*
> + * include/linker_lists.h
> + *
> + * Implementation of linker-generated arrays
> + *
> + * Copyright (C) 2012 Marek Vasut <marex at denx.de>
> + *
> + * See file CREDITS for list of people who contributed to this
> + * project.
> + *
> + * This program is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU General Public License as
> + * published by the Free Software Foundation; either version 2 of
> + * the License, or (at your option) any later version.
> + */
> +#ifndef __LINKER_LISTS_H__
> +#define __LINKER_LISTS_H__
> +
> +/**
> + * ll_entry_declare() - Declare linker-generated array entry
> + * _type: Data type of the entry
> + * _name: Name of the entry
> + * _section_u: Subsection of u_boot_list in which this entry is placed
> + * (with underscores instead of dots, for name concatenation)
> + * _section_d: Subsection of u_boot_list in which this entry is placed
> + * (with dots, for section concatenation)
> + *
> + * This macro declares a variable that is placed into a linker-generated
> + * array. This is a basic building block for more advanced use of linker-
> + * generated arrays. The user is expected to build their own macro wrapper
> + * around this one.
> + *
> + * A variable declared using this macro must be compile-time initialized
> + * and is as such placed into subsection of special section, .u_boot_list.
> + * The subsection is specified by the _section_[u,d] parameter, see below.
> + * The base name of the variable is _name, yet the actual variable is
> + * declared as concatenation of:
> + *
> + * _u_boot_list_ + _section_u + _ + _name
> + *
> + * which ensures name uniqueness. This variable shall never be refered
> + * directly though.
> + *
> + * Special precaution must be made when using this macro:
> + *
> + * 1) The _type must not contain the "static" keyword, otherwise the entry
> + * is not generated.
> + * 2) The _section_u and _section_d variables must match, the only difference
> + * is that in _section_u is every dot "." character present in _section_d
> + * replaced by a single underscore "_" character in _section_u. The actual
> + * purpose of these parameters is to select proper subsection in the global
> + * .u_boot_list section.
> + * 3) In case a section is declared that contains some array elements AND a
> + * subsection of this section is declared and contains some elements, it is
> + * imperative that the elements are of the same type.
> + * 4) In case an outer section is declared that contains some array elements
> + * AND am inner subsection of this section is declared and contains some
> + * elements, then when traversing the outer section, even the elements of
> + * the inner sections are present in the array.
> + *
> + * Example of usage:
> + *
> + * ll_entry_declare(struct my_sub_cmd, my_sub_cmd, cmd_sub, cmd.sub) = {
> + * .x = 3,
> + * .y = 4,
> + * };
> + */
> +#define ll_entry_declare(_type, _name, _section_u, _section_d) \
> + _type _u_boot_list_##_section_u##_##_name __attribute__(( \
> + unused, aligned(4), \
> + section(".u_boot_list."#_section_d"."#_name))); \
> + _type _u_boot_list_##_section_u##_##_name
Why repeat the "_type _u_boot_list_##_section_u##_##_name" instead of
just leaving the semicolon off after the __attribute__()?
> +
> +/**
> + * ll_entry_start() - Point to first entry of linker-generated array
> + * _type: Data type of the entry
> + * _section_u: Subsection of u_boot_list in which this entry is placed
> + * (with underscores instead of dots)
> + *
> + * This function returns (_type *) pointer to the very first entry of a
> + * linker-generated array placed into subsection of .u_boot_list section
> + * specified by _section_u argument.
> + *
> + * Example of usage:
> + *
> + * struct my_sub_cmd *msc = ll_entry_start(struct my_sub_cmd, cmd_sub);
> + */
> +#define ll_entry_start(_type, _section_u) \
> + ({ \
> + extern _type _u_boot_list_##_section_u##__start; \
> + _type *_ll_result = &_u_boot_list_##_section_u##__start;\
> + _ll_result; \
> + })
Tricky. I didn't realize this gcc extension existed.
> +
> +/**
> + * ll_entry_count() - Return the number of elements in linker-generated array
> + * _type: Data type of the entry
> + * _section_u: Subsection of u_boot_list in which this entry is placed
> + * (with underscores instead of dots)
> + *
> + * This function returns the number of elements of a linker-generated array
> + * placed into subsection of .u_boot_list section specified by _section_u
> + * argument. The result is of an unsigned int type.
> + *
> + * Example of usage:
> + *
> + * int i;
> + * const unsigned int count = ll_entry_count(struct my_sub_cmd, cmd_sub);
> + * struct my_sub_cmd *msc = ll_entry_start(struct my_sub_cmd, cmd_sub);
> + * for (i = 0; i < count; i++) {
> + * printf("Entry %i, x=%i y=%i\n", i, msc->x, msc->y);
> + * msc++;
> + * }
> + */
> +#define ll_entry_count(_type, _section_u) \
> + ({ \
> + extern _type _u_boot_list_##_section_u##__start; \
> + extern _type _u_boot_list_##_section_u##__end; \
> + unsigned int _ll_result = \
> + &_u_boot_list_##_section_u##__end - \
> + &_u_boot_list_##_section_u##__start; \
> + _ll_result; \
> + })
> +
> +#endif /* __LINKER_LISTS_H__ */
> --
> 1.7.10.4
>
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