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Jean-Michel Picod
2020-01-28 15:09:10 +01:00
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src/embedded_flash/store/format.rs Normal file
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// Copyright 2019 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
use super::super::{Index, Storage};
use super::{bitfield, StoreConfig, StoreEntry, StoreError};
use alloc::vec::Vec;
/// Whether a user entry is a replace entry.
pub enum IsReplace {
/// This is a replace entry.
Replace,
/// This is an insert entry.
Insert,
}
/// Helpers to parse the store format.
///
/// See the store module-level documentation for information about the format.
pub struct Format {
pub word_size: usize,
pub page_size: usize,
pub num_pages: usize,
pub max_page_erases: usize,
pub num_tags: usize,
/// Whether an entry is present.
///
/// - 0 for entries (user entries or internal entries).
/// - 1 for free space until the end of the page.
present_bit: usize,
/// Whether an entry is deleted.
///
/// - 0 for deleted entries.
/// - 1 for alive entries.
deleted_bit: usize,
/// Whether an entry is internal.
///
/// - 0 for internal entries.
/// - 1 for user entries.
internal_bit: usize,
/// Whether a user entry is a replace entry.
///
/// - 0 for replace entries.
/// - 1 for insert entries.
replace_bit: usize,
/// The data length of a user entry.
length_range: bitfield::BitRange,
/// The tag of a user entry.
tag_range: bitfield::BitRange,
/// The page index of a replace entry.
replace_page_range: bitfield::BitRange,
/// The byte index of a replace entry.
replace_byte_range: bitfield::BitRange,
/// The index of the page to erase.
///
/// This is only present for internal entries.
old_page_range: bitfield::BitRange,
/// The current erase count of the page to erase.
///
/// This is only present for internal entries.
saved_erase_count_range: bitfield::BitRange,
/// Whether a page is initialized.
///
/// - 0 for initialized pages.
/// - 1 for uninitialized pages.
initialized_bit: usize,
/// The erase count of a page.
erase_count_range: bitfield::BitRange,
/// Whether a page is being compacted.
///
/// - 0 for pages being compacted.
/// - 1 otherwise.
compacting_bit: usize,
/// The page index to which a page is being compacted.
new_page_range: bitfield::BitRange,
}
impl Format {
/// Returns a helper to parse the store format for a given storage and config.
///
/// # Errors
///
/// Returns `None` if any of the following conditions does not hold:
/// - The word size must be a power of two.
/// - The page size must be a power of two.
/// - There should be at least 2 pages in the storage.
/// - It should be possible to write a word at least twice.
/// - It should be possible to erase a page at least once.
/// - There should be at least 1 tag.
pub fn new<S: Storage, C: StoreConfig>(storage: &S, config: &C) -> Option<Format> {
let word_size = storage.word_size();
let page_size = storage.page_size();
let num_pages = storage.num_pages();
let max_word_writes = storage.max_word_writes();
let max_page_erases = storage.max_page_erases();
let num_tags = config.num_tags();
if !(word_size.is_power_of_two()
&& page_size.is_power_of_two()
&& num_pages > 1
&& max_word_writes >= 2
&& max_page_erases > 0
&& num_tags > 0)
{
return None;
}
// Compute how many bits we need to store the fields.
let page_bits = num_bits(num_pages);
let byte_bits = num_bits(page_size);
let tag_bits = num_bits(num_tags);
let erase_bits = num_bits(max_page_erases + 1);
// Compute the bit position of the fields.
let present_bit = 0;
let deleted_bit = present_bit + 1;
let internal_bit = deleted_bit + 1;
let replace_bit = internal_bit + 1;
let length_range = bitfield::BitRange {
start: replace_bit + 1,
length: byte_bits,
};
let tag_range = bitfield::BitRange {
start: length_range.end(),
length: tag_bits,
};
let replace_page_range = bitfield::BitRange {
start: tag_range.end(),
length: page_bits,
};
let replace_byte_range = bitfield::BitRange {
start: replace_page_range.end(),
length: byte_bits,
};
let old_page_range = bitfield::BitRange {
start: internal_bit + 1,
length: page_bits,
};
let saved_erase_count_range = bitfield::BitRange {
start: old_page_range.end(),
length: erase_bits,
};
let initialized_bit = 0;
let erase_count_range = bitfield::BitRange {
start: initialized_bit + 1,
length: erase_bits,
};
let compacting_bit = erase_count_range.end();
let new_page_range = bitfield::BitRange {
start: compacting_bit + 1,
length: page_bits,
};
let format = Format {
word_size,
page_size,
num_pages,
max_page_erases,
num_tags,
present_bit,
deleted_bit,
internal_bit,
replace_bit,
length_range,
tag_range,
replace_page_range,
replace_byte_range,
old_page_range,
saved_erase_count_range,
initialized_bit,
erase_count_range,
compacting_bit,
new_page_range,
};
// Make sure all the following conditions hold:
// - The page header is one word.
// - The internal entry is one word.
// - The entry header fits in one word.
if format.page_header_size() != word_size
|| format.internal_entry_size() != word_size
|| format.header_size() > word_size
{
return None;
}
Some(format)
}
/// Ensures a user entry is valid.
pub fn validate_entry(&self, entry: StoreEntry) -> Result<(), StoreError> {
if entry.tag >= self.num_tags {
return Err(StoreError::InvalidTag);
}
if entry.data.len() >= self.page_size {
return Err(StoreError::StoreFull);
}
Ok(())
}
/// Returns the entry header length in bytes.
///
/// This is the smallest number of bytes necessary to store all fields of the entry info up to
/// and including `length`.
pub fn header_size(&self) -> usize {
self.bits_to_bytes(self.length_range.end())
}
/// Returns the entry info length in bytes.
///
/// This is the number of bytes necessary to store all fields of the entry info. This also
/// includes the internal padding to protect the `committed` bit from the `deleted` bit.
fn info_size(&self, is_replace: IsReplace) -> usize {
let suffix_bits = 2; // committed + complete
let info_bits = match is_replace {
IsReplace::Replace => self.replace_byte_range.end() + suffix_bits,
IsReplace::Insert => self.tag_range.end() + suffix_bits,
};
let info_size = self.bits_to_bytes(info_bits);
// If the suffix bits would end up in the header, we need to add one byte for them.
if info_size == self.header_size() {
info_size + 1
} else {
info_size
}
}
/// Returns the length in bytes of an entry.
///
/// This depends on the length of the user data and whether the entry replaces an old entry or
/// is an insertion. This also includes the internal padding to protect the `committed` bit from
/// the `deleted` bit.
pub fn entry_size(&self, is_replace: IsReplace, length: usize) -> usize {
let mut entry_size = length + self.info_size(is_replace);
let word_size = self.word_size;
entry_size = self.align_word(entry_size);
// The entry must be at least 2 words such that the `committed` and `deleted` bits are on
// different words.
if entry_size == word_size {
entry_size += word_size;
}
entry_size
}
/// Returns the length in bytes of an internal entry.
pub fn internal_entry_size(&self) -> usize {
let length = self.bits_to_bytes(self.saved_erase_count_range.end());
self.align_word(length)
}
pub fn is_present(&self, header: &[u8]) -> bool {
bitfield::is_zero(self.present_bit, header, bitfield::NO_GAP)
}
pub fn set_present(&self, header: &mut [u8]) {
bitfield::set_zero(self.present_bit, header, bitfield::NO_GAP)
}
pub fn is_deleted(&self, header: &[u8]) -> bool {
bitfield::is_zero(self.deleted_bit, header, bitfield::NO_GAP)
}
/// Returns whether an entry is present and not deleted.
pub fn is_alive(&self, header: &[u8]) -> bool {
self.is_present(header) && !self.is_deleted(header)
}
pub fn set_deleted(&self, header: &mut [u8]) {
bitfield::set_zero(self.deleted_bit, header, bitfield::NO_GAP)
}
pub fn is_internal(&self, header: &[u8]) -> bool {
bitfield::is_zero(self.internal_bit, header, bitfield::NO_GAP)
}
pub fn set_internal(&self, header: &mut [u8]) {
bitfield::set_zero(self.internal_bit, header, bitfield::NO_GAP)
}
pub fn is_replace(&self, header: &[u8]) -> IsReplace {
if bitfield::is_zero(self.replace_bit, header, bitfield::NO_GAP) {
IsReplace::Replace
} else {
IsReplace::Insert
}
}
fn set_replace(&self, header: &mut [u8]) {
bitfield::set_zero(self.replace_bit, header, bitfield::NO_GAP)
}
pub fn get_length(&self, header: &[u8]) -> usize {
bitfield::get_range(self.length_range, header, bitfield::NO_GAP)
}
fn set_length(&self, header: &mut [u8], length: usize) {
bitfield::set_range(self.length_range, header, bitfield::NO_GAP, length)
}
pub fn get_data<'a>(&self, entry: &'a [u8]) -> &'a [u8] {
&entry[self.header_size()..][..self.get_length(entry)]
}
/// Returns the span of user data in an entry.
///
/// The complement of this gap in the entry is exactly the entry info. The header is before the
/// gap and the footer is after the gap.
fn entry_gap(&self, entry: &[u8]) -> bitfield::ByteGap {
let start = self.header_size();
let length = self.get_length(entry);
bitfield::ByteGap { start, length }
}
pub fn get_tag(&self, entry: &[u8]) -> usize {
bitfield::get_range(self.tag_range, entry, self.entry_gap(entry))
}
fn set_tag(&self, entry: &mut [u8], tag: usize) {
bitfield::set_range(self.tag_range, entry, self.entry_gap(entry), tag)
}
pub fn get_replace_index(&self, entry: &[u8]) -> Index {
let gap = self.entry_gap(entry);
let page = bitfield::get_range(self.replace_page_range, entry, gap);
let byte = bitfield::get_range(self.replace_byte_range, entry, gap);
Index { page, byte }
}
fn set_replace_page(&self, entry: &mut [u8], page: usize) {
bitfield::set_range(self.replace_page_range, entry, self.entry_gap(entry), page)
}
fn set_replace_byte(&self, entry: &mut [u8], byte: usize) {
bitfield::set_range(self.replace_byte_range, entry, self.entry_gap(entry), byte)
}
/// Returns the bit position of the `committed` bit.
///
/// This cannot be precomputed like other fields since it depends on the length of the entry.
fn committed_bit(&self, entry: &[u8]) -> usize {
8 * entry.len() - 2
}
/// Returns the bit position of the `complete` bit.
///
/// This cannot be precomputed like other fields since it depends on the length of the entry.
fn complete_bit(&self, entry: &[u8]) -> usize {
8 * entry.len() - 1
}
pub fn is_committed(&self, entry: &[u8]) -> bool {
bitfield::is_zero(self.committed_bit(entry), entry, bitfield::NO_GAP)
}
pub fn set_committed(&self, entry: &mut [u8]) {
bitfield::set_zero(self.committed_bit(entry), entry, bitfield::NO_GAP)
}
pub fn is_complete(&self, entry: &[u8]) -> bool {
bitfield::is_zero(self.complete_bit(entry), entry, bitfield::NO_GAP)
}
fn set_complete(&self, entry: &mut [u8]) {
bitfield::set_zero(self.complete_bit(entry), entry, bitfield::NO_GAP)
}
pub fn get_old_page(&self, header: &[u8]) -> usize {
bitfield::get_range(self.old_page_range, header, bitfield::NO_GAP)
}
pub fn set_old_page(&self, header: &mut [u8], old_page: usize) {
bitfield::set_range(self.old_page_range, header, bitfield::NO_GAP, old_page)
}
pub fn get_saved_erase_count(&self, header: &[u8]) -> usize {
bitfield::get_range(self.saved_erase_count_range, header, bitfield::NO_GAP)
}
pub fn set_saved_erase_count(&self, header: &mut [u8], erase_count: usize) {
bitfield::set_range(
self.saved_erase_count_range,
header,
bitfield::NO_GAP,
erase_count,
)
}
/// Builds an entry for replace or insert operations.
pub fn build_entry(&self, replace: Option<Index>, user_entry: StoreEntry) -> Vec<u8> {
let StoreEntry { tag, data } = user_entry;
let is_replace = match replace {
None => IsReplace::Insert,
Some(_) => IsReplace::Replace,
};
let entry_len = self.entry_size(is_replace, data.len());
let mut entry = Vec::with_capacity(entry_len);
// Build the header.
entry.resize(self.header_size(), 0xff);
self.set_present(&mut entry[..]);
self.set_length(&mut entry[..], data.len());
// Add the data.
entry.extend_from_slice(data);
// Build the footer.
entry.resize(entry_len, 0xff);
self.set_tag(&mut entry[..], tag);
self.set_complete(&mut entry[..]);
match replace {
None => self.set_committed(&mut entry[..]),
Some(Index { page, byte }) => {
self.set_replace(&mut entry[..]);
self.set_replace_page(&mut entry[..], page);
self.set_replace_byte(&mut entry[..], byte);
}
}
entry
}
/// Builds an entry for replace or insert operations.
pub fn build_erase_entry(&self, old_page: usize, saved_erase_count: usize) -> Vec<u8> {
let mut entry = vec![0xff; self.internal_entry_size()];
self.set_present(&mut entry[..]);
self.set_internal(&mut entry[..]);
self.set_old_page(&mut entry[..], old_page);
self.set_saved_erase_count(&mut entry[..], saved_erase_count);
entry
}
/// Returns the length in bytes of a page header entry.
///
/// This includes the word padding.
pub fn page_header_size(&self) -> usize {
self.align_word(self.bits_to_bytes(self.erase_count_range.end()))
}
pub fn is_initialized(&self, header: &[u8]) -> bool {
bitfield::is_zero(self.initialized_bit, header, bitfield::NO_GAP)
}
pub fn set_initialized(&self, header: &mut [u8]) {
bitfield::set_zero(self.initialized_bit, header, bitfield::NO_GAP)
}
pub fn get_erase_count(&self, header: &[u8]) -> usize {
bitfield::get_range(self.erase_count_range, header, bitfield::NO_GAP)
}
pub fn set_erase_count(&self, header: &mut [u8], count: usize) {
bitfield::set_range(self.erase_count_range, header, bitfield::NO_GAP, count)
}
pub fn is_compacting(&self, header: &[u8]) -> bool {
bitfield::is_zero(self.compacting_bit, header, bitfield::NO_GAP)
}
pub fn set_compacting(&self, header: &mut [u8]) {
bitfield::set_zero(self.compacting_bit, header, bitfield::NO_GAP)
}
pub fn get_new_page(&self, header: &[u8]) -> usize {
bitfield::get_range(self.new_page_range, header, bitfield::NO_GAP)
}
pub fn set_new_page(&self, header: &mut [u8], new_page: usize) {
bitfield::set_range(self.new_page_range, header, bitfield::NO_GAP, new_page)
}
/// Returns the smallest word boundary greater or equal to a value.
fn align_word(&self, value: usize) -> usize {
let word_size = self.word_size;
(value + word_size - 1) / word_size * word_size
}
/// Returns the minimum number of bytes to represent a given number of bits.
fn bits_to_bytes(&self, bits: usize) -> usize {
(bits + 7) / 8
}
}
/// Returns the number of bits necessary to write numbers smaller than `x`.
fn num_bits(x: usize) -> usize {
x.next_power_of_two().trailing_zeros() as usize
}
#[test]
fn num_bits_ok() {
assert_eq!(num_bits(0), 0);
assert_eq!(num_bits(1), 0);
assert_eq!(num_bits(2), 1);
assert_eq!(num_bits(3), 2);
assert_eq!(num_bits(4), 2);
assert_eq!(num_bits(5), 3);
assert_eq!(num_bits(8), 3);
assert_eq!(num_bits(9), 4);
assert_eq!(num_bits(16), 4);
}