drivers: implement memory device driver

The first and only memory device mounted at /dev/null discards all data
written to it and always returns 0 bytes when read.

mentos/CMakeLists.txt

@@ -31,6 +31,7 @@ set(KERNEL_SOURCES
     ${CMAKE_SOURCE_DIR}/mentos/src/devices/pci.c
     ${CMAKE_SOURCE_DIR}/mentos/src/devices/fpu.c
     ${CMAKE_SOURCE_DIR}/mentos/src/drivers/ata.c
+    ${CMAKE_SOURCE_DIR}/mentos/src/drivers/mem.c
     ${CMAKE_SOURCE_DIR}/mentos/src/drivers/rtc.c
     ${CMAKE_SOURCE_DIR}/mentos/src/drivers/fdc.c
     ${CMAKE_SOURCE_DIR}/mentos/src/drivers/mouse.c

mentos/inc/drivers/mem.h

@@ -0,0 +1,18 @@
+/// @file mem.h
+/// @brief Drivers for memory devices
+/// @copyright (c) 2024 This file is distributed under the MIT License.
+/// See LICENSE.md for details.
+/// @addtogroup drivers Device Drivers
+/// @{
+/// @addtogroup mem Memory devices
+/// @brief Drivers for memory devices.
+/// @{
+
+#pragma once
+
+/// @brief Initializes the memory devices.
+/// @return 0 on success, 1 on error.
+int mem_devs_initialize(void);
+
+/// @}
+/// @}

mentos/src/drivers/mem.c

@@ -0,0 +1,167 @@
+/// @file mem.c
+/// @brief Memory devices.
+/// @copyright (c) 2024 This file is distributed under the MIT License.
+/// See LICENSE.md for details.
+
+// Include the kernel log levels.
+#include "sys/kernel_levels.h"
+/// Change the header.
+#define __DEBUG_HEADER__ "[MEMDEV  ]"
+/// Set the log level.
+#define __DEBUG_LEVEL__ GLOBAL_LOGLEVEL
+
+#include "assert.h"
+#include "drivers/mem.h"
+#include "io/debug.h"
+#include "fs/vfs.h"
+#include "string.h"
+#include "sys/errno.h"
+#include "system/syscall.h"
+
+struct memdev;
+
+struct memdev {
+    vfs_file_t *file;
+    struct memdev *next;
+};
+
+static struct memdev *devices;
+
+static void add_device(struct memdev *device) {
+    struct memdev *dit = devices;
+    for (;dit != NULL; dit = dit->next);
+    if (dit == NULL) {
+        devices = device;
+    } else {
+        dit->next = device;
+    }
+}
+
+static vfs_file_t* find_device_file(const char *path) {
+    for(struct memdev *dev = devices; dev != NULL; dev = dev->next) {
+        if (strcmp(dev->file->name, path) == 0) {
+            return dev->file;
+        }
+    }
+    return NULL;
+}
+
+static int mem_stat(const char* path, stat_t *stat);
+
+static vfs_sys_operations_t mem_sys_operations = {
+    .mkdir_f = NULL,
+    .rmdir_f = NULL,
+    .stat_f  = mem_stat,
+};
+
+static vfs_file_t *null_open(const char *path, int flags, mode_t mode);
+static int null_close(vfs_file_t * file);
+static ssize_t null_write(vfs_file_t * file, const void *buffer, off_t offset, size_t size);
+static ssize_t null_read(vfs_file_t * file, char *buffer, off_t offset, size_t size);
+static int null_fstat(vfs_file_t * file, stat_t *stat);
+
+static vfs_file_operations_t null_fs_operations = {
+    .open_f     = null_open,
+    .unlink_f   = NULL,
+    .close_f    = null_close,
+    .read_f     = null_read,
+    .write_f    = null_write,
+    .lseek_f    = NULL,
+    .stat_f     = null_fstat,
+    .ioctl_f    = NULL,
+    .getdents_f = NULL
+};
+
+static struct memdev *null_device_create(const char* name) {
+    // Create the device.
+    struct memdev *dev = kmalloc(sizeof(struct memdev));
+    dev->next = NULL;
+
+    // Create the file.
+    vfs_file_t *file = kmem_cache_alloc(vfs_file_cache, GFP_KERNEL);
+    if (file == NULL) {
+        pr_err("Failed to create null device.\n");
+        return NULL;
+    }
+    dev->file = file;
+
+    // Set the device name.
+    strncpy(file->name, name, NAME_MAX);
+    file->count = 0;
+    file->uid = 0;
+    file->gid = 0;
+    file->mask = 0x2000 | 0666;
+    file->atime = sys_time(NULL);
+    file->mtime = sys_time(NULL);
+    file->ctime = sys_time(NULL);
+    file->length = 0;
+    // Set the operations.
+    file->sys_operations = &mem_sys_operations;
+    file->fs_operations  = &null_fs_operations;
+    return dev;
+}
+
+static vfs_file_t* null_open(const char *path, int flags, mode_t mode) {
+    vfs_file_t* file = find_device_file(path);
+    if (file) {
+        file->count++;
+    }
+    // TODO: check permissions
+    return file;
+}
+
+static int null_close(vfs_file_t * file) {
+    assert(file && "Received null file.");
+    file->count--;
+    return 0;
+}
+
+static ssize_t null_write(vfs_file_t * file, const void *buffer, off_t offset, size_t size) {
+    return size;
+}
+
+static ssize_t null_read(vfs_file_t * file, char *buffer, off_t offset, size_t size) {
+    return 0;
+}
+
+static int null_fstat(vfs_file_t * file, stat_t *stat)
+{
+    pr_debug("null_fstat(%s, %p)\n", file->name, stat);
+    stat->st_dev   = 0;
+    stat->st_ino   = 0;
+    stat->st_mode  = file->mask;
+    stat->st_uid   = file->uid;
+    stat->st_gid   = file->gid;
+    stat->st_atime = file->atime;
+    stat->st_mtime = file->mtime;
+    stat->st_ctime = file->ctime;
+    stat->st_size  = file->length;
+    return 0;
+}
+
+static int mem_stat(const char *path, stat_t *stat) {
+    vfs_file_t* file = find_device_file(path);
+
+    if (file) {
+        return file->fs_operations->stat_f(file, stat);
+    }
+    return -ENOENT;
+}
+
+int mem_devs_initialize(void)
+{
+    struct memdev *devnull = null_device_create("/dev/null");
+    if (!devnull) {
+        pr_err("Failed to create devnull");
+        return -ENODEV;
+    }
+    
+    if (!vfs_mount("/dev/null", devnull->file)) {
+        pr_err("Failed to mount /dev/null");
+        return 1;
+    }
+
+    add_device(devnull);
+
+    return 0;
+}

mentos/src/kernel.c

@@ -18,6 +18,7 @@
 #include "drivers/keyboard/keymap.h"
 #include "drivers/ps2.h"
 #include "drivers/rtc.h"
+#include "drivers/mem.h"
 #include "fs/ext2.h"
 #include "fs/procfs.h"
 #include "fs/vfs.h"
@@ -238,6 +239,16 @@ int kmain(boot_info_t *boot_informations)
     }
     print_ok();
 
+    //==========================================================================
+    pr_notice("    Initialize memory devices...\n");
+    printf("    Initialize memory devices...");
+    if (mem_devs_initialize()) {
+        print_fail();
+        pr_emerg("Failed to initialize memory devices!\n");
+        return 1;
+    }
+    print_ok();
+
     //==========================================================================
     pr_notice("    Initialize 'procfs'...\n");
     printf("    Initialize 'procfs'...");