linux_dsm_epyc7002/Documentation/fmc/fmc-write-eeprom.txt

fmc-write-eeprom
================

This module is designed to load a binary file from /lib/firmware and to
write it to the internal EEPROM of the mezzanine card. This driver uses
the `busid' generic parameter.

Overwriting the EEPROM is not something you should do daily, and it is
expected to only happen during manufacturing. For this reason, the
module makes it unlikely for the random user to change a working EEPROM.

However, since the EEPROM may include application-specific information
other than the identification, later versions of this packages added
write-support through sysfs. See *note Accessing the EEPROM::.

To avoid damaging the EEPROM content, the module takes the following
measures:

   * It accepts a `file=' argument (within /lib/firmware) and if no
     such argument is received, it doesn't write anything to EEPROM
     (i.e. there is no default file name).

   * If the file name ends with `.bin' it is written verbatim starting
     at offset 0.

   * If the file name ends with `.tlv' it is interpreted as
     type-length-value (i.e., it allows writev(2)-like operation).

   * If the file name doesn't match any of the patterns above, it is
     ignored and no write is performed.

   * Only cards listed with `busid=' are written to. If no busid is
     specified, no programming is done (and the probe function of the
     driver will fail).


Each TLV tuple is formatted in this way: the header is 5 bytes,
followed by data. The first byte is `w' for write, the next two bytes
represent the address, in little-endian byte order, and the next two
represent the data length, in little-endian order. The length does not
include the header (it is the actual number of bytes to be written).

This is a real example: that writes 5 bytes at position 0x110:

        spusa.root# od -t x1 -Ax /lib/firmware/try.tlv
        000000 77 10 01 05 00 30 31 32 33 34
        00000a
        spusa.root# insmod /tmp/fmc-write-eeprom.ko busid=0x0200 file=try.tlv
        [19983.391498] spec 0000:03:00.0: write 5 bytes at 0x0110
        [19983.414615] spec 0000:03:00.0: write_eeprom: success

Please note that you'll most likely want to use SDBFS to build your
EEPROM image, at least if your mezzanines are being used in the White
Rabbit environment. For this reason the TLV format is not expected to
be used much and is not expected to be developed further.

If you want to try reflashing fake EEPROM devices, you can use the
fmc-fakedev.ko module (see *note fmc-fakedev::).  Whenever you change
the image starting at offset 0, it will deregister and register again
after two seconds.  Please note, however, that if fmc-write-eeprom is
still loaded, the system will associate it to the new device, which
will be reprogrammed and thus will be unloaded after two seconds.  The
following example removes the module after it reflashed fakedev the
first time.

     spusa.root# insmod fmc-fakedev.ko
        [   72.984733]  fake-fmc: Manufacturer: fake-vendor
        [   72.989434]  fake-fmc: Product name: fake-design-for-testing
        spusa.root# insmod fmc-write-eeprom.ko busid=0 file=fdelay-eeprom.bin; \
            rmmod fmc-write-eeprom
        [  130.874098]  fake-fmc: Matching a generic driver (no ID)
        [  130.887845]  fake-fmc: programming 6155 bytes
        [  130.894567]  fake-fmc: write_eeprom: success
        [  132.895794]  fake-fmc: Manufacturer: CERN
        [  132.899872]  fake-fmc: Product name: FmcDelay1ns4cha


Accessing the EEPROM
=====================

The bus creates a sysfs binary file called eeprom for each mezzanine it
knows about:

        spusa.root# cd /sys/bus/fmc/devices; ls -l */eeprom
        -r--r--r-- 1 root root 8192 Feb 21 12:30 FmcAdc100m14b4cha-0800/eeprom
        -r--r--r-- 1 root root 8192 Feb 21 12:30 FmcDelay1ns4cha-0200/eeprom
        -r--r--r-- 1 root root 8192 Feb 21 12:30 FmcDio5cha-0400/eeprom

Everybody can read the files and the superuser can also modify it, but
the operation may on the carrier driver, if the carrier is unable to
access the I2C bus.  For example, the spec driver can access the bus
only with its golden gateware: after a mezzanine driver reprogrammed
the FPGA with a custom circuit, the carrier is unable to access the
EEPROM and returns ENOTSUPP.

An alternative way to write the EEPROM is the mezzanine driver
fmc-write-eeprom (See *note fmc-write-eeprom::), but the procedure is
more complex.