null_device.cpp

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/**
 * @file    zero_device.cpp
 *
 * @date Feb 12, 2021
 * @author Matthew Kennedy, (c) 2021
 *
 * This file implements a special block device that simply reports "no media"
 * Use it when you need to mount *something* but don't have an SD card available.
 */
 
#include "pch.h"
 
#include <cstring>
 
struct NullDevice {
    const struct BaseBlockDeviceVMT *vmt;
    _base_block_device_data
};
 
static bool nd_is_inserted(void*) {
    // This function is the whole point - we have no media!
    return false;
}
 
static bool nd_is_protected(void*) {
    return false;
}
 
static bool nd_return_success(void*) {
    return HAL_SUCCESS;
}
 
static bool nd_return_success_read(void*, uint32_t, uint8_t* buffer, uint32_t n) {
    // write zeroes to the buffer to prevent somebody reading random memory
    memset(buffer, 0, n);
 
    return HAL_SUCCESS;
}
 
static bool nd_return_success_write(void*, uint32_t, const uint8_t*, uint32_t) {
    return HAL_SUCCESS;
}
 
static bool nd_get_info(void*, BlockDeviceInfo* bdip) {
    // We have to report non-zero size here because Windows
    // will query the size of the block device even if we indicate
    // that the device has no media
    // If we report zeroes, it breaks USB until you unplug this device
    bdip->blk_num = 1000;
    bdip->blk_size = 512;
    return HAL_SUCCESS;
}
 
static const struct BaseBlockDeviceVMT ndVmt = {
    (size_t)0, // instanceOffset
    nd_is_inserted,
    nd_is_protected,
 
    // These functions just claim success to make the host happy
    nd_return_success,
    nd_return_success,
    nd_return_success_read,
    nd_return_success_write,
    nd_return_success,
    nd_get_info
};
 
// This device is always ready and has no state
NullDevice ND1 = { &ndVmt, BLK_READY };