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Split bootsector execution code out into bootsector.c. 

Added basic El Torito ISO image boot capability
This commit is contained in:
Michael Brown 2007-01-29 04:21:38 +00:00
parent bde8878eef
commit 2cf1e33df1
7 changed files with 610 additions and 68 deletions
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111
src/arch/i386/image/bootsector.c Normal file
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@ -0,0 +1,111 @@
/*
* Copyright (C) 2007 Michael Brown <mbrown@fensystems.co.uk>.
*
* 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 any later version.
*
* This program is distributed in the hope that it will be useful, but
* WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
*/
/**
* @file
*
* x86 bootsector image format
*
*/
#include <errno.h>
#include <realmode.h>
#include <biosint.h>
#include <bootsector.h>
/** Vector for storing original INT 18 handler
*
* We do not chain to this vector, so there is no need to place it in
* .text16.
*/
static struct segoff int18_vector;
/** Vector for storing original INT 19 handler
*
* We do not chain to this vector, so there is no need to place it in
* .text16.
*/
static struct segoff int19_vector;
/** Restart point for INT 18 or 19 */
extern void bootsector_exec_fail ( void );
/**
* Jump to preloaded bootsector
*
* @v segment Real-mode segment
* @v offset Real-mode offset
* @v drive Drive number to pass to boot sector
* @ret rc Return status code
*/
int call_bootsector ( unsigned int segment, unsigned int offset,
unsigned int drive ) {
int discard_b, discard_D, discard_d;
DBG ( "Booting from boot sector at %04x:%04x\n", segment, offset );
/* Hook INTs 18 and 19 to capture failure paths */
hook_bios_interrupt ( 0x18, ( unsigned int ) bootsector_exec_fail,
&int18_vector );
hook_bios_interrupt ( 0x19, ( unsigned int ) bootsector_exec_fail,
&int19_vector );
/* Boot the loaded sector
*
* We assume that the boot sector may completely destroy our
* real-mode stack, so we preserve everything we need in
* static storage.
*/
__asm__ __volatile__ ( REAL_CODE ( /* Save return address off-stack */
"popw %%cs:saved_retaddr\n\t"
/* Save stack pointer */
"movw %%ss, %%ax\n\t"
"movw %%ax, %%cs:saved_ss\n\t"
"movw %%sp, %%cs:saved_sp\n\t"
/* Jump to boot sector */
"pushw %%bx\n\t"
"pushw %%di\n\t"
"lret\n\t"
/* Preserved variables */
"\nsaved_ss: .word 0\n\t"
"\nsaved_sp: .word 0\n\t"
"\nsaved_retaddr: .word 0\n\t"
/* Boot failure return point */
"\nbootsector_exec_fail:\n\t"
/* Restore stack pointer */
"movw %%cs:saved_ss, %%ax\n\t"
"movw %%ax, %%ss\n\t"
"movw %%cs:saved_sp, %%sp\n\t"
/* Return via saved address */
"jmp *%%cs:saved_retaddr\n\t" )
: "=b" ( discard_b ), "=D" ( discard_D ),
"=d" ( discard_d )
: "b" ( segment ), "D" ( offset ),
"d" ( drive )
: "eax", "ecx", "esi", "ebp" );
DBG ( "Booted disk returned via INT 18 or 19\n" );
/* Unhook INTs 18 and 19 */
unhook_bios_interrupt ( 0x18, ( unsigned int ) bootsector_exec_fail,
&int18_vector );
unhook_bios_interrupt ( 0x19, ( unsigned int ) bootsector_exec_fail,
&int19_vector );
return -ECANCELED;
}

334
src/arch/i386/image/eltorito.c Normal file
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@ -0,0 +1,334 @@
/*
* Copyright (C) 2007 Michael Brown <mbrown@fensystems.co.uk>.
*
* 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 any later version.
*
* This program is distributed in the hope that it will be useful, but
* WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
*/
/**
* @file
*
* El Torito bootable ISO image format
*
*/
#include <stdint.h>
#include <errno.h>
#include <assert.h>
#include <realmode.h>
#include <bootsector.h>
#include <int13.h>
#include <gpxe/uaccess.h>
#include <gpxe/image.h>
#include <gpxe/segment.h>
#include <gpxe/ramdisk.h>
#include <gpxe/shutdown.h>
#define ISO9660_BLKSIZE 2048
#define ELTORITO_VOL_DESC_OFFSET ( 17 * ISO9660_BLKSIZE )
/** An El Torito Boot Record Volume Descriptor */
struct eltorito_vol_desc {
/** Boot record indicator; must be 0 */
uint8_t record_indicator;
/** ISO-9660 identifier; must be "CD001" */
uint8_t iso9660_id[5];
/** Version, must be 1 */
uint8_t version;
/** Boot system indicator; must be "EL TORITO SPECIFICATION" */
uint8_t system_indicator[32];
/** Unused */
uint8_t unused[32];
/** Boot catalog sector */
uint32_t sector;
} __attribute__ (( packed ));
/** An El Torito Boot Catalog Validation Entry */
struct eltorito_validation_entry {
/** Header ID; must be 1 */
uint8_t header_id;
/** Platform ID
*
* 0 = 80x86
* 1 = PowerPC
* 2 = Mac
*/
uint8_t platform_id;
/** Reserved */
uint16_t reserved;
/** ID string */
uint8_t id_string[24];
/** Checksum word */
uint16_t checksum;
/** Signature; must be 0xaa55 */
uint16_t signature;
} __attribute__ (( packed ));
/** A bootable entry in the El Torito Boot Catalog */
struct eltorito_boot_entry {
/** Boot indicator
*
* Must be @c ELTORITO_BOOTABLE for a bootable ISO image
*/
uint8_t indicator;
/** Media type
*
*/
uint8_t media_type;
/** Load segment */
uint16_t load_segment;
/** System type */
uint8_t filesystem;
/** Unused */
uint8_t reserved_a;
/** Sector count */
uint16_t length;
/** Starting sector */
uint32_t start;
/** Unused */
uint8_t reserved_b[20];
} __attribute__ (( packed ));
/** Boot indicator for a bootable ISO image */
#define ELTORITO_BOOTABLE 0x88
/** El Torito media types */
enum eltorito_media_type {
/** No emulation */
ELTORITO_NO_EMULATION = 0,
};
struct image_type eltorito_image_type __image_type ( PROBE_NORMAL );
/**
* Calculate 16-bit word checksum
*
* @v data Data to checksum
* @v len Length (in bytes, must be even)
* @ret sum Checksum
*/
static unsigned int word_checksum ( void *data, size_t len ) {
uint16_t *words;
uint16_t sum = 0;
for ( words = data ; len ; words++, len -= 2 ) {
sum += *words;
}
return sum;
}
/**
* Execute El Torito image
*
* @v image El Torito image
* @ret rc Return status code
*/
static int eltorito_exec ( struct image *image ) {
struct ramdisk ramdisk;
struct int13_drive int13_drive;
unsigned int load_segment = image->priv.ul;
unsigned int load_offset = ( load_segment ? 0 : 0x7c00 );
int rc;
memset ( &ramdisk, 0, sizeof ( ramdisk ) );
init_ramdisk ( &ramdisk, image->data, image->len, ISO9660_BLKSIZE );
memset ( &int13_drive, 0, sizeof ( int13_drive ) );
int13_drive.blockdev = &ramdisk.blockdev;
register_int13_drive ( &int13_drive );
if ( ( rc = call_bootsector ( load_segment, load_offset,
int13_drive.drive ) ) != 0 ) {
DBGC ( image, "ElTorito %p boot failed: %s\n",
image, strerror ( rc ) );
goto err;
}
rc = -ECANCELED; /* -EIMPOSSIBLE */
err:
unregister_int13_drive ( &int13_drive );
return rc;
}
/**
* Read and verify El Torito Boot Record Volume Descriptor
*
* @v image El Torito file
* @ret catalog_offset Offset of Boot Catalog
* @ret rc Return status code
*/
static int eltorito_read_voldesc ( struct image *image,
unsigned long *catalog_offset ) {
static const struct eltorito_vol_desc vol_desc_signature = {
.record_indicator = 0,
.iso9660_id = "CD001",
.version = 1,
.system_indicator = "EL TORITO SPECIFICATION",
};
struct eltorito_vol_desc vol_desc;
/* Sanity check */
if ( image->len < ( ELTORITO_VOL_DESC_OFFSET + ISO9660_BLKSIZE ) ) {
DBGC ( image, "ElTorito %p too short\n", image );
return -ENOEXEC;
}
/* Read and verify Boot Record Volume Descriptor */
copy_from_user ( &vol_desc, image->data, ELTORITO_VOL_DESC_OFFSET,
sizeof ( vol_desc ) );
if ( memcmp ( &vol_desc, &vol_desc_signature,
offsetof ( typeof ( vol_desc ), sector ) ) != 0 ) {
DBGC ( image, "ElTorito %p invalid Boot Record Volume "
"Descriptor\n", image );
return -ENOEXEC;
}
*catalog_offset = ( vol_desc.sector * ISO9660_BLKSIZE );
DBGC ( image, "ElTorito %p boot catalog at offset %#lx\n",
image, *catalog_offset );
return 0;
}
/**
* Read and verify El Torito Boot Catalog
*
* @v image El Torito file
* @v catalog_offset Offset of Boot Catalog
* @ret boot_entry El Torito boot entry
* @ret rc Return status code
*/
static int eltorito_read_catalog ( struct image *image,
unsigned long catalog_offset,
struct eltorito_boot_entry *boot_entry ) {
struct eltorito_validation_entry validation_entry;
/* Sanity check */
if ( image->len < ( catalog_offset + ISO9660_BLKSIZE ) ) {
DBGC ( image, "ElTorito %p bad boot catalog offset %#lx\n",
image, catalog_offset );
return -ENOEXEC;
}
/* Read and verify the Validation Entry of the Boot Catalog */
copy_from_user ( &validation_entry, image->data, catalog_offset,
sizeof ( validation_entry ) );
if ( word_checksum ( &validation_entry,
sizeof ( validation_entry ) ) != 0 ) {
DBGC ( image, "ElTorito %p bad Validation Entry checksum\n",
image );
return -ENOEXEC;
}
/* Read and verify the Initial/Default entry */
copy_from_user ( boot_entry, image->data,
( catalog_offset + sizeof ( validation_entry ) ),
sizeof ( *boot_entry ) );
if ( boot_entry->indicator != ELTORITO_BOOTABLE ) {
DBGC ( image, "ElTorito %p not bootable\n", image );
return -ENOEXEC;
}
if ( boot_entry->media_type != ELTORITO_NO_EMULATION ) {
DBGC ( image, "ElTorito %p cannot support media type %d\n",
image, boot_entry->media_type );
return -ENOTSUP;
}
DBGC ( image, "ElTorito %p media type %d segment %04x\n",
image, boot_entry->media_type, boot_entry->load_segment );
return 0;
}
/**
* Load El Torito virtual disk image into memory
*
* @v image El Torito file
* @v boot_entry El Torito boot entry
* @ret rc Return status code
*/
static int eltorito_load_disk ( struct image *image,
struct eltorito_boot_entry *boot_entry ) {
unsigned long start = ( boot_entry->start * ISO9660_BLKSIZE );
unsigned long length = ( boot_entry->length * ISO9660_BLKSIZE );
unsigned int load_segment;
userptr_t buffer;
int rc;
/* Sanity check */
if ( image->len < ( start + length ) ) {
DBGC ( image, "ElTorito %p virtual disk lies outside image\n",
image );
return -ENOEXEC;
}
DBGC ( image, "ElTorito %p virtual disk at %#lx+%#lx\n",
image, start, length );
/* Calculate load address */
load_segment = boot_entry->load_segment;
buffer = real_to_user ( load_segment, ( load_segment ? 0 : 0x7c00 ) );
/* Verify and prepare segment */
if ( ( rc = prep_segment ( buffer, length, length ) ) != 0 ) {
DBGC ( image, "ElTorito %p could not prepare segment: %s\n",
image, strerror ( rc ) );
return rc;
}
/* Copy image to segment */
memcpy_user ( buffer, 0, image->data, start, length );
return 0;
}
/**
* Load El Torito image into memory
*
* @v image El Torito file
* @ret rc Return status code
*/
int eltorito_load ( struct image *image ) {
struct eltorito_boot_entry boot_entry;
unsigned long bootcat_offset;
int rc;
/* Read Boot Record Volume Descriptor, if present */
if ( ( rc = eltorito_read_voldesc ( image, &bootcat_offset ) ) != 0 )
return rc;
/* This is an El Torito image, valid or otherwise */
if ( ! image->type )
image->type = &eltorito_image_type;
/* Read Boot Catalog */
if ( ( rc = eltorito_read_catalog ( image, bootcat_offset,
&boot_entry ) ) != 0 )
return rc;
/* Load Virtual Disk image */
if ( ( rc = eltorito_load_disk ( image, &boot_entry ) ) != 0 )
return rc;
/* Record load segment in image private data field */
image->priv.ul = boot_entry.load_segment;
return 0;
}
/** El Torito image type */
struct image_type eltorito_image_type __image_type ( PROBE_NORMAL ) = {
.name = "El Torito",
.load = eltorito_load,
.exec = eltorito_exec,
};

12
src/arch/i386/include/bootsector.h Normal file
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@ -0,0 +1,12 @@
#ifndef _BOOTSECTOR_H
#define _BOOTSECTOR_H
/** @file
*
* x86 bootsector image format
*/
extern int call_bootsector ( unsigned int segment, unsigned int offset,
unsigned int drive );
#endif /* _BOOTSECTOR_H */

2
src/arch/i386/include/int13.h
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@ -37,6 +37,8 @@ struct block_device;
#define INT13_EXTENDED_WRITE 0x43
/** Get extended drive parameters */
#define INT13_GET_EXTENDED_PARAMETERS 0x48
/** Get CD-ROM status / terminate emulation */
#define INT13_CDROM_STATUS_TERMINATE 0x4b
/** @} */

125
src/arch/i386/interface/pcbios/int13.c
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@ -26,6 +26,7 @@
#include <realmode.h>
#include <bios.h>
#include <biosint.h>
#include <bootsector.h>
#include <int13.h>
/** @file
@ -44,23 +45,6 @@ static struct segoff __text16 ( int13_vector );
/** Assembly wrapper */
extern void int13_wrapper ( void );
/** Vector for storing original INT 18 handler
*
* We do not chain to this vector, so there is no need to place it in
* .text16.
*/
static struct segoff int18_vector;
/** Vector for storing original INT 19 handler
*
* We do not chain to this vector, so there is no need to place it in
* .text16.
*/
static struct segoff int19_vector;
/** Restart point for INT 18 or 19 */
extern void int13_exec_fail ( void );
/** List of registered emulated drives */
static LIST_HEAD ( drives );
@ -114,6 +98,13 @@ static int int13_rw_sectors ( struct int13_drive *drive,
unsigned int count;
userptr_t buffer;
/* Validate blocksize */
if ( blockdev->blksize != INT13_BLKSIZE ) {
DBG ( "Invalid blocksize (%zd) for non-extended read/write\n",
blockdev->blksize );
return -INT13_STATUS_INVALID;
}
/* Calculate parameters */
cylinder = ( ( ( ix86->regs.cl & 0xc0 ) << 2 ) | ix86->regs.ch );
assert ( cylinder < drive->cylinders );
@ -129,13 +120,6 @@ static int int13_rw_sectors ( struct int13_drive *drive,
DBG ( "C/H/S %d/%d/%d = LBA %#lx <-> %04x:%04x (count %d)\n", cylinder,
head, sector, lba, ix86->segs.es, ix86->regs.bx, count );
/* Validate blocksize */
if ( blockdev->blksize != INT13_BLKSIZE ) {
DBG ( "Invalid blocksize (%zd) for non-extended read/write\n",
blockdev->blksize );
return -INT13_STATUS_INVALID;
}
/* Read from / write to block device */
if ( io ( blockdev, lba, count, buffer ) != 0 )
return -INT13_STATUS_READ_ERROR;
@ -333,6 +317,38 @@ static int int13_get_extended_parameters ( struct int13_drive *drive,
return 0;
}
struct int13_cdrom_specification {
/** Size of packet in bytes */
uint8_t size;
/** Boot media type */
uint8_t media_type;
/** Drive number */
uint8_t drive;
};
/**
* INT 13, 4b - Get CD-ROM status / terminate emulation
*
* @v drive Emulated drive
* @v ds:si El Torito specification packet to fill in
* @ret status Status code
*/
static int int13_cdrom_status_terminate ( struct int13_drive *drive,
struct i386_all_regs *ix86 ) {
struct int13_cdrom_specification specification;
DBG ( "Get CD-ROM emulation parameters to %04x:%04x\n",
ix86->segs.ds, ix86->regs.di );
memset ( &specification, 0, sizeof ( specification ) );
specification.size = sizeof ( specification );
specification.drive = drive->drive;
copy_to_real ( ix86->segs.ds, ix86->regs.si, &specification,
sizeof ( specification ) );
return 0;
}
/**
* INT 13 handler
*
@ -346,7 +362,7 @@ static void int13 ( struct i386_all_regs *ix86 ) {
if ( drive->drive != ix86->regs.dl )
continue;
DBG ( "INT 13,%02x (%02x): ", command, drive->drive );
DBG ( "INT 13,%04x (%02x): ", ix86->regs.ax, drive->drive );
switch ( command ) {
case INT13_RESET:
@ -379,6 +395,9 @@ static void int13 ( struct i386_all_regs *ix86 ) {
case INT13_GET_EXTENDED_PARAMETERS:
status = int13_get_extended_parameters ( drive, ix86 );
break;
case INT13_CDROM_STATUS_TERMINATE:
status = int13_cdrom_status_terminate ( drive, ix86 );
break;
default:
DBG ( "*** Unrecognised INT 13 ***\n" );
status = -INT13_STATUS_INVALID;
@ -398,6 +417,8 @@ static void int13 ( struct i386_all_regs *ix86 ) {
/* Set OF to indicate to wrapper not to chain this call */
ix86->flags |= OF;
break;
}
}
@ -453,6 +474,10 @@ static void guess_int13_geometry ( struct int13_drive *drive ) {
unsigned long blocks_per_cyl;
unsigned int i;
/* Don't even try when the blksize is invalid for C/H/S access */
if ( drive->blockdev->blksize != INT13_BLKSIZE )
return;
/* Scan through partition table and modify guesses for heads
* and sectors_per_track if we find any used partitions.
*/
@ -564,6 +589,7 @@ void unregister_int13_drive ( struct int13_drive *drive ) {
int int13_boot ( unsigned int drive ) {
int status, signature;
int discard_c, discard_d;
int rc;
DBG ( "Booting from INT 13 drive %02x\n", drive );
@ -593,48 +619,11 @@ int int13_boot ( unsigned int drive ) {
return -ENOEXEC;
}
/* Hook INTs 18 and 19 to capture failure paths */
hook_bios_interrupt ( 0x18, ( unsigned int ) int13_exec_fail,
&int18_vector );
hook_bios_interrupt ( 0x19, ( unsigned int ) int13_exec_fail,
&int19_vector );
/* Jump to boot sector */
if ( ( rc = call_bootsector ( 0x0, 0x7c00, drive ) ) != 0 ) {
DBG ( "INT 13 drive %02x boot returned\n", drive );
return rc;
}
/* Boot the loaded sector
*
* We assume that the boot sector may completely destroy our
* real-mode stack, so we preserve everything we need in
* static storage.
*/
__asm__ __volatile__ ( REAL_CODE ( /* Save return address off-stack */
"popw %%cs:int13_saved_retaddr\n\t"
/* Save stack pointer */
"movw %%ss, %%ax\n\t"
"movw %%ax, %%cs:int13_saved_ss\n\t"
"movw %%sp, %%cs:int13_saved_sp\n\t"
/* Jump to boot sector */
"ljmp $0, $0x7c00\n\t"
/* Preserved variables */
"\nint13_saved_ss: .word 0\n\t"
"\nint13_saved_sp: .word 0\n\t"
"\nint13_saved_retaddr: .word 0\n\t"
/* Boot failure return point */
"\nint13_exec_fail:\n\t"
/* Restore stack pointer */
"movw %%cs:int13_saved_ss, %%ax\n\t"
"movw %%ax, %%ss\n\t"
"movw %%cs:int13_saved_sp, %%sp\n\t"
/* Return via saved address */
"jmp *%%cs:int13_saved_retaddr\n\t")
: "=d" ( discard_d ) : "d" ( drive )
: "eax", "ebx", "ecx", "esi", "edi", "ebp" );
DBG ( "Booted disk returned via INT 18 or 19\n" );
/* Unhook INTs 18 and 19 */
unhook_bios_interrupt ( 0x18, ( unsigned int ) int13_exec_fail,
&int18_vector );
unhook_bios_interrupt ( 0x19, ( unsigned int ) int13_exec_fail,
&int19_vector );
return -ECANCELED;
return -ECANCELED; /* -EIMPOSSIBLE */
}

3
src/core/config.c
View File

@ -143,6 +143,9 @@ REQUIRE_OBJECT ( script );
#ifdef IMAGE_BZIMAGE
REQUIRE_OBJECT ( bzimage );
#endif
#ifdef IMAGE_ELTORITO
REQUIRE_OBJECT ( eltorito );
#endif
/*
* Drag in all requested commands

91
src/drivers/block/ramdisk.c Normal file
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@ -0,0 +1,91 @@
/*
* Copyright (C) 2007 Michael Brown <mbrown@fensystems.co.uk>.
*
* 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 any later version.
*
* This program is distributed in the hope that it will be useful, but
* WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
*/
#include <gpxe/blockdev.h>
#include <gpxe/ramdisk.h>
/**
* @file
*
* RAM disks
*
*/
static inline __attribute__ (( always_inline )) struct ramdisk *
block_to_ramdisk ( struct block_device *blockdev ) {
return container_of ( blockdev, struct ramdisk, blockdev );
}
/**
* Read block
*
* @v blockdev Block device
* @v block Block number
* @v count Block count
* @v buffer Data buffer
* @ret rc Return status code
*/
static int ramdisk_read ( struct block_device *blockdev, uint64_t block,
unsigned long count, userptr_t buffer ) {
struct ramdisk *ramdisk = block_to_ramdisk ( blockdev );
unsigned long offset = ( block * blockdev->blksize );
unsigned long length = ( count * blockdev->blksize );
DBGC ( ramdisk, "RAMDISK %p reading [%lx,%lx)\n",
ramdisk, offset, length );
memcpy_user ( buffer, 0, ramdisk->data, offset, length );
return 0;
}
/**
* Write block
*
* @v blockdev Block device
* @v block Block number
* @v count Block count
* @v buffer Data buffer
* @ret rc Return status code
*/
static int ramdisk_write ( struct block_device *blockdev, uint64_t block,
unsigned long count, userptr_t buffer ) {
struct ramdisk *ramdisk = block_to_ramdisk ( blockdev );
unsigned long offset = ( block * blockdev->blksize );
unsigned long length = ( count * blockdev->blksize );
DBGC ( ramdisk, "RAMDISK %p writing [%lx,%lx)\n",
ramdisk, offset, length );
memcpy_user ( ramdisk->data, offset, buffer, 0, length );
return 0;
}
int init_ramdisk ( struct ramdisk *ramdisk, userptr_t data, size_t len,
unsigned int blksize ) {
if ( ! blksize )
blksize = 512;
ramdisk->data = data;
ramdisk->blockdev.read = ramdisk_read;
ramdisk->blockdev.write = ramdisk_write;
ramdisk->blockdev.blksize = blksize;
ramdisk->blockdev.blocks = ( len / blksize );
return 0;
}