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Charles Manning

This document describes the Yaffs Direct Interface (YDI), which allows Yaffs to be

simply integrated with embedded systems, with or without an RTOS.

Table of Contents

1 Background ............................................................................................................................. 3

2 Licensing ................................................................................................................................. 3

3 What are Yaffs and Yaffs Direct Interface? ............................................................................. 3

4 Why use Yaffs? ....................................................................................................................... 4

5 Source Code and Yaffs Resources ......................................................................................... 4

6 System Requirements.............................................................................................................. 5

7 How to integrate Yaffs with an RTOS/Embedded system ...................................................... 5

7.1 Source Files ................................................................................................................. 6

7.2 Integrating the POSIX Application Interface .............................................................. 9

8 RTOS Integration Interface ................................................................................................... 11

9 Yaffs NAND Model .............................................................................................................. 13

9.1 NAND Model considerations for Yaffs1 ................................................................... 14

9.2 NAND Model for Yaffs2 ........................................................................................... 14

9.3 Overview of flash driver function ............................................................................. 15

10 Configuration and flash driver installation ......................................................................... 17

11 Using the POSIX file system interface ............................................................................... 18

11.1 Difference to Windows POSIX-like interfaces ....................................................... 18

11.2 Fundamental concepts ............................................................................................. 18

11.3 Error code ................................................................................................................ 19

11.4 Links – the hard kind (not symbolic links) ............................................................ 19

11.5 Symbolic links ......................................................................................................... 20

11.6 Handle-based file handling ...................................................................................... 21

11.7 Changing file size .................................................................................................... 23

11.8 Getting/setting information about files.................................................................... 23

11.9 Changing the directory structure and names ........................................................... 25

11.10 Searching directories ............................................................................................. 25

11.11 Searching directories (fd interface) ....................................................................... 26

11.12 Mount control ........................................................................................................ 26

11.13 Extended attributes (xattrt) .................................................................................... 27

11.14 Other ...................................................................................................................... 28

12 Example: yaffs_readdir() and yaffs_stat() .......................................................................... 30

Background

The purpose of this document is to describe the interfacing of the Yaffs Direct

Interface (YDI) as well as to provide sufficient information to allow a preliminary

evaluation of Yaffs. This document tries to focus on the issues important to the system

integrator without getting too detailed about how Yaffs works. Other documents

provide an in-depth discussion of how Yaffs works.

Licensing

Yaffs was originally released for Linux under the GNU Public License (GPL). Various

embedded developers soon identified that Yaffs would be ideal for their applications,

but were not able to use GPL based code in their systems. Aleph One has alternative

licensing arrangements to support such applications.

What are Yaffs and Yaffs Direct Interface?

Yaffs stands for Yet Another Flash File System. Yaffs was the first file system

designed, from the ground up, for NAND storage.

In 2002 Aleph One set out to identify file system options for using NAND Flash as a

file system. Various file systems available at the time were evaluated and all were

found lacking in one way or another. The need for a suitable NAND storage file

system was identified and Yaffs was designed to fill that need.

Although Yaffs was originally designed for NAND flash, it has been used successfully

with NOR flash systems and even as a RAM file system. This allows high reliability

file systems to be constructed using NOR flash, with a future migration path to

NAND flash for higher density and performance.

Yaffs was originally designed for use with the Linux operating system, but was

designed in a very modular way. The operating-system-specific code was kept

separate from the main Yaffs file system code. This allows Yaffs to be ported quite

cleanly to other operating systems through operating system personality modules. One

such personality module is the Yaffs Direct Interface (YDI) which allows Yaffs to be

simply integrated with embedded systems, with or without an RTOS.

Yaffs has been used for many products using various operating systems including

Windows CE and various RTOSs, including ThreadX, vXworks, pSOS to name just a

few.

Note that there is a native port of Yaffs to eCOS that does not use YDI. This is

supported and distributed by eCocCentric.(http://www.ecoscentric.com/).

Yaffs2, a more recent release of Yaffs, supports a wider range of NAND flash

components including 2k page devices and devices with multi-level cells (MLC), and

produces equivalent or better performance. It include Yaffs1 compatibility code so

Yaffs1 images still work and migration is quite simple. Yaffs1 or Yaffs2 mode of

operation are simply selected by a run-time parameter.

As well as providing a NAND file system, YDI also provides a RAM emulation layer

to allow Yaffs to operate as a RAM file system too. While the RAM emulation is

perhaps not as efficient as a dedicated RAM file system, this does allow one well

proven file system to use both RAM and flash.

Why use Yaffs?

Yaffs is the first, and perhaps only, file system designed specifically for NAND flash.

This means that Yaffs has been designed to work around the various limitations and

quirks of NAND flash, as well as exploit the various features of NAND to achieve an

effective file system.

Some features to consider:

• Yaffs has been well proven and has been used to ship in large volumes in several

products using many different operating systems, compilers and processors.

• Yaffs is written in portable C and is endian neutral.

• Yaffs provides bad block handling and ECC algorithms to handle deficiencies in

NAND flash.

• Yaffs is a log-structured file system which makes it very robust to corruptions

caused by power failures etc.

• Yaffs has highly optimised and predictable garbage collection strategies. This

makes it high performance and very deterministic when compared with similar file

systems.

• Yaffs has a lower memory footprint than most other log-structured flash file

systems.

• Yaffs provides a wide range of POSIX-style file system support including

directories, symbolic and hard links etc. through standard file system interface calls.

• Yaffs is highly configurable to work with various flash geometries, various ECC

options, caching options etc.

• Yaffs Direct Interface is simple to integrate in a system – only a few interface

functions are required.

• Yaffs can be used with a wide range of memory technologies.

Source Code and Yaffs Resources

Please note the licensing terms that apply to using the Yaffs code.

http://www.yaffs.net serves as the hub for all Yaffs information.

This document refers to the Yaffs source code extensively. and is available via the

download link on this site.

There are various other documents and a Yaffs mailing list for Yaffs discussions.

Consulting services are also available.

System Requirements

Determining minimum system requirements is often quite difficult. The following are

presented as a guideline only. Contact Aleph One for more detailed analysis if

required.

• Yaffs is endian-neutral and works fine with little-endian and big-endian processors.

• Yaffs code has been used successfully with many different 32-bit and 64-bit CPUs

including MIPS, 68000, ARM, ColdFire, PowerPC and x86 variants. Yaffs has

even been used with various DSP architectures. Yaffs should work with 16-bit

CPUs too, but this is generally untested and might need some tuning, depending on

compiler options.

• Because Yaffs is log structured, RAM is required to build up runtime data

structures for acceptable performance. As a rule of thumb, budget approximately 2

bytes per chunk of NAND flash, where a chunk is typically one page of NAND.

For NAND with 512byte pages, budget approximately 4kbytes of RAM per

1Mbyte of NAND. For 2kbyte page devices budget approximately 1kbyte per

1Mbyte of NAND.

How to integrate Yaffs with an RTOS/Embedded system

Yaffs Direct Interface (YDI) wraps Yaffs in a way that is simple to integrate. You need

to provide a few functions for Yaffs to use to talk to your hardware and RTOS. Yaffs

provides a set of POSIX-compliant functions for applications to use to talk to it.

In addition to the Yaffs core file system, the YDI has three parts which are each

explained in more detail in further sections:

• POSIX Application Interface: This is the POSIX-like interface that the

application code uses to access the Yaffs file system. (open, close, read, write, etc)

• RTOS Integration Interface: These are the functions that must be provided for

Yaffs to access the RTOS system resources. (initialise, lock, unlock, get time,

memory allocation, set error, etc). Note that although we use the term RTOS, the

same can be done with other OSs or even with no RTOS at all in a bare-metal

system.

• Flash Device Interface: This interface support is where the flash device and

appropriate device drivers are installed to provide an access path from Yaffs to the

flash device(s). Multiple different devices and device types can be supported

simultaneously.

Source Files

The following source files contain the core file system:

yaffs_allocator.c

Allocates Yaffs object and tnode structures.

yaffs_bitmap.c

Block and chunk bitmap handling code.

yaffs_checkpointrw.c

Streamer for writing checkpoint data

yaffs_ecc.c

1-bit Hamming ECC code

yaffs_guts.c

The major Yaffs algorithms.

yaffs_nameval.c

Name/value code for handling extended attributes (xattr).

yaffs_nand.c

Flash interfacing abstraction.

yaffs_packedtags1.c

yaffs_packedtags2.c

Tags packing code

yaffs_summary.c

Code for handling block summaries.

yaffs_tagscompat.c

Tags compatibility code to support Yaffs1 mode.

yaffs_tagsmarshall.c

Tags marshalling code.

yaffs_verify.c

Test verification code.

yaffs_yaffs1.c

Yaffs1-mode specific code.

yaffs_yaffs2.c

Yaffs2-mode specific code.

The Yaffs direct interface is in yaffsfs.c, with the interface functions and structures

defined in yaffsfd.h.

yaffs_attribs.c

Attribute handling.

yaffs_error.c

Error reporting code.

yaffsfs.c

Yaffs Direct Interface wrapper code.

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