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Integrate spiffs

Girts edited this page May 31, 2017 · 11 revisions

Quick and dirty integration example

Things to check

Integrating spiffs


So, assume you're running a Cortex-M3 board with a 2 MB SPI flash on it. The SPI flash has 64kB blocks. Your project is built using gnumake, and now you want to try things out.

First, you simply copy the files in src/ to your own source folder. Exclude all files in test folder. Then you point out these files in your make script for compilation.

Also copy the spiffs_config.h over from the src/default/ folder.

Try building. This fails, nagging about inclusions and u32_t and whatnot. Open the spiffs_config.h and delete the bad inclusions. Also, add following typedefs:

  typedef signed int s32_t;
  typedef unsigned int u32_t;
  typedef signed short s16_t;
  typedef unsigned short u16_t;
  typedef signed char s8_t;
  typedef unsigned char u8_t;

Now it should build. Over to the mounting business. Assume you already implemented the read, write and erase functions to your SPI flash:

  void my_spi_read(int addr, int size, char *buf)
  void my_spi_write(int addr, int size, char *buf)
  void my_spi_erase(int addr, int size)

In your main.c or similar, include the spiffs.h and do that spiffs struct:

  #include <spiffs.h>
  static spiffs fs;

Also, toss in some of the needed buffers:

  #define LOG_PAGE_SIZE       256
  static u8_t spiffs_work_buf[LOG_PAGE_SIZE*2];
  static u8_t spiffs_fds[32*4];
  static u8_t spiffs_cache_buf[(LOG_PAGE_SIZE+32)*4];

Now, write the my_spiffs_mount function:

  void my_spiffs_mount() {
    spiffs_config cfg;
    cfg.phys_size = 2*1024*1024; // use all spi flash
    cfg.phys_addr = 0; // start spiffs at start of spi flash
    cfg.phys_erase_block = 65536; // according to datasheet
    cfg.log_block_size = 65536; // let us not complicate things
    cfg.log_page_size = LOG_PAGE_SIZE; // as we said
    cfg.hal_read_f = my_spi_read;
    cfg.hal_write_f = my_spi_write;
    cfg.hal_erase_f = my_spi_erase;
    int res = SPIFFS_mount(&fs,
    printf("mount res: %i\n", res);

Now, build warns about the my_spi_read, write and erase functions. Wrong signatures, so go wrap them:

  static s32_t my_spiffs_read(u32_t addr, u32_t size, u8_t *dst) {
    my_spi_read(addr, size, dst);
    return SPIFFS_OK;

  static s32_t my_spiffs_write(u32_t addr, u32_t size, u8_t *src) {
    my_spi_write(addr, size, src);
    return SPIFFS_OK;

  static s32_t my_spiffs_erase(u32_t addr, u32_t size) {
    my_spi_erase(addr, size);
    return SPIFFS_OK;

Redirect the config in my_spiffs_mount to the wrappers instead:

    cfg.hal_read_f = my_spiffs_read;
    cfg.hal_write_f = my_spiffs_write;
    cfg.hal_erase_f = my_spiffs_erase;

Ok, now you should be able to build and run. However, you get this output mount res: -1 but you wanted mount res: 0.

This is probably due to you having experimented with your SPI flash, so it contains rubbish from spiffs's point of view. Do a mass erase and run again.

If all is ok now, you're good to go. Try creating a file and read it back:

  static void test_spiffs() {
    char buf[12];
    // Surely, I've mounted spiffs before entering here
    spiffs_file fd = SPIFFS_open(&fs, "my_file", SPIFFS_CREAT | SPIFFS_TRUNC | SPIFFS_RDWR, 0);
    if (SPIFFS_write(&fs, fd, (u8_t *)"Hello world", 12) < 0) printf("errno %i\n", SPIFFS_errno(&fs));
    SPIFFS_close(&fs, fd); 
    fd = SPIFFS_open(&fs, "my_file", SPIFFS_RDWR, 0);
    if (SPIFFS_read(&fs, fd, (u8_t *)buf, 12) < 0) printf("errno %i\n", SPIFFS_errno(&fs));
    SPIFFS_close(&fs, fd);
    printf("--> %s <--\n", buf);

Compile, run, cross fingers hard, and you'll get the output:

  --> Hello world <-- 

Got errors? Check spiffs.h for error definitions to get a clue what went voodoo.


When you alter the spiffs_config values, make sure you also check the typedefs in spiffs_config.h:

  • spiffs_block_ix
  • spiffs_page_ix
  • spiffs_obj_id
  • spiffs_span_ix

The sizes of these typedefs must not underflow, else spiffs might end up in infinite loops. Each typedef is commented what to check for.

Also, if you alter the code or just want to verify your configuration, you can run

 > make clean && make all && make test

in the spiffs/ folder. This will run all testcases using the configuration in default/spiffs_config.h and test/params_test.h. The tests are written for linux but should run under cygwin also.


In order to integrate spiffs to your embedded target, you will basically need:

  • A SPI flash device which your processor can communicate with
  • An implementation for reading, writing and erasing the flash
  • Memory (flash or ram) for the code
  • Memory (ram) for the stack

Other stuff may be needed, threaded systems might need mutexes and so on.

Logical structure

First and foremost, one must decide how to divide up the SPI flash for spiffs. Having the datasheet for the actual SPI flash in hand will help. Spiffs can be defined to use all or only parts of the SPI flash.

If following seems arcane, check out the configuration wiki.

  • Decide the logical size of blocks. This must be a multiple of the biggest physical SPI flash block size. To be safe, use the physical block size - which in many cases is 65536 bytes.
  • Decide the logical size of pages. This must be a 2nd logarithm part of the logical block size. To be safe, use 256 bytes to start with or even better, the physical page size.
  • Decide how much of the SPI flash memory to be used for spiffs. This must be on logical block boundary. If unsure, use 1 megabyte to start with.
  • Decide where on the SPI flash memory the spiffs area should start. This must be on physical block/sector boundary. If unsure, use address 0.

SPI flash API, the hardware abstraction layer

The target must provide three functions to spiffs:

  • s32_t (*spiffs_read)(u32_t addr, u32_t size, u8_t *dst)
  • s32_t (*spiffs_write)(u32_t addr, u32_t size, u8_t *src)
  • s32_t (*spiffs_erase)(u32_t addr, u32_t size)

These functions define the only communication between the SPI flash and the spiffs stack.

If you need these callbacks to identify to which spiffs struct they belong, check the SPIFFS_HAL_CALLBACK_EXTRA switch - useful when running multiple instances of spiffs. If enabled, these function will have the extra argument struct spiffs_t *fs.

On success these must return 0 (or SPIFFS_OK). Anything else will be considered an error and propagated up to SPIFFS_errno.

spiffs_read and spiffs_write will never be called with parameters that break the logical page boundary. I.e they will always be called with address and sizes within the range (phys_offset + log_page_sz * n) <= request < (phys_offset + log_page_sz * (n+1)). If you've selected a logical page size being as big as the physical page size, you don't need to worry about wrapping buffers and such (if this is the case on your particular SPI flash).

The address and size on spiffs_erase requests will always be on physical block size boundaries. If you've selected a logical block size being, say, 4 * physical block size, spiffs_erase will simply be called four times when erasing one logical block.

Mount specification

In spiffs.h, there is a SPIFFS_mount function defined, used to mount spiffs on the SPI flash.

s32_t SPIFFS_mount(
	spiffs *fs, 
	spiffs_config *config, 
	u8_t *work,
    u8_t *fd_space, 
    u32_t fd_space_size,
    void *cache, 
    u32_t cache_size,
    spiffs_check_callback check_cb_f)
  • fs - points to a spiffs struct. This may be totally uninitialized. Do note, that there is a user_data void pointer in this struct which you can set - e.g. if you're running multiple instances of spiffs.
  • config - points to a spiffs_config struct. This struct must be initialized when mounting. See below.
  • work - a ram memory buffer being double the size of the logical page size. This buffer is used extensively by the spiffs stack. If logical page size is 256, this buffer must be 512 bytes.
  • fd_space - a ram memory buffer used for file descriptors.
  • fd_space_size - the size of the file descriptor buffer. A file descriptor normally is around 32 bytes depending on the build config - the bigger the buffer, the more file descriptors are available.
  • cache - a ram memory buffer used for cache. Ignored if cache is disabled in build config.
  • cache_size - the size of the cache buffer. Ignored if cache is disabled in build config. One cache page will be slightly larger than the logical page size. The more ram, the more cache pages, the quicker the system.
  • check_cb_f - callback function for monitoring spiffs consistency checks and mending operations. May be null.

The config struct must be initialized prior to mounting. One must always define the SPI flash access functions:

  • spiffs_config.hal_read_f - pointing to the function reading the SPI flash
  • spiffs_config.hal_write_f - pointing to the function writing the SPI flash
  • spiffs_config.hal_erase_f - pointing to the function erasing the SPI flash

Depending on the build config - if SPIFFS_SINGLETON is set to zero - following parameters must be defined:

  • spiffs_config.phys_size - the physical number of bytes accounted for spiffs on the SPI flash
  • spiffs_config.phys_addr - the physical starting address on the SPI flash
  • spiffs_config.phys_erase_block - the physical size of the largest block/sector on the SPI flash found within the spiffs usage address space
  • spiffs_config.log_block_size - the logical size of a spiffs block
  • spiffs_config.log_page_size - the logical size of a spiffs page

If SPIFFS_SINGLETON is set to one, above parameters must be set by defines in the config header file, spiffs_config.h.

If SPIFFS_FILEHDL_OFFSET is enabled, you must also define:

  • spiffs_config.fh_ix_offset - the file descriptor handle offset for this spiffs instance


  • makefile: The files needed to be compiled to your target resides in to be included in your makefile, either by cut and paste or by inclusion.

  • Types: spiffs uses the types u8_t, s8_t, u16_t, s16_t, u32_t, s32_t; these must be typedeffed.

  • spiffs_config.h: you also need to define a spiffs_config.h header. Example of this is found in the src/default/ directory. See configuration wiki for details.


Spiffs needs ram. It needs a working buffer being double the size of the logical page size. It also needs at least one file descriptor. If cache is enabled (highly recommended), it will also need a bunch of cache pages.

Say you have a logical page size of 256 bytes. You want to be able to have four files open simultaneously, and you can give spiffs four cache pages. This roughly sums up to:

256*2 (work buffer) +

32*4 (file descriptors) +

(256+32)*4 (cache pages) + 40 (cache metadata)

i.e. 1832 bytes.

This is apart from call stack usage.

To get the exact amount of bytes needed on your specific target, enable SPIFFS_BUFFER_HELP in spiffs_config.h, rebuild and call:

  • SPIFFS_buffer_bytes_for_filedescs
  • SPIFFS_buffer_bytes_for_cache

Having these figures you can disable SPIFFS_BUFFER_HELP again to save flash.

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