src/kernel/linux/v4.19/Documentation/fmc/mezzanine.txt

FMC Driver
**********

An FMC driver is concerned with the specific mezzanine and associated
gateware. As such, it is expected to be independent of the carrier
being used: it will perform I/O accesses only by means of
carrier-provided functions.

The matching between device and driver is based on the content of the
EEPROM (as mandated by the FMC standard) or by the actual cores
configured in the FPGA; the latter technique is used when the FPGA is
already programmed when the device is registered to the bus core.

In some special cases it is possible for a driver to directly access
FPGA registers, by means of the `fpga_base' field of the device
structure. This may be needed for high-bandwidth peripherals like fast
ADC cards. If the device module registered a remote device (for example
by means of Etherbone), the `fpga_base' pointer will be NULL.
Therefore, drivers must be ready to deal with NULL base pointers, and
fail gracefully.  Most driver, however, are not expected to access the
pointer directly but run fmc_readl and fmc_writel instead, which will
work in any case.

In even more special cases, the driver may access carrier-specific
functionality: the `carrier_name' string allows the driver to check
which is the current carrier and make use of the `carrier_data'
pointer.  We chose to use carrier names rather than numeric identifiers
for greater flexibility, but also to avoid a central registry within
the `fmc.h' file - we hope other users will exploit our framework with
their own carriers.  An example use of carrier names is in GPIO setup
(see *note The GPIO Abstraction::), although the name match is not
expected to be performed by the driver.  If you depend on specific
carriers, please check the carrier name and fail gracefully if your
driver finds it is running in a yet-unknown-to-it environment.


ID Table
========

Like most other Linux drivers, and FMC driver must list all the devices
which it is able to drive.  This is usually done by means of a device
table, but in FMC we can match hardware based either on the contents of
their EEPROM or on the actual FPGA cores that can be enumerated.
Therefore, we have two tables of identifiers.

Matching of FRU information depends on two names, the manufacturer (or
vendor) and the device (see *note FMC Identification::); for
flexibility during production (i.e. before writing to the EEPROM) the
bus supports a catch-all driver that specifies NULL strings. For this
reason, the table is specified as pointer-and-length, not a a
null-terminated array - the entry with NULL names can be a valid entry.

Matching on FPGA cores depends on two numeric fields: the 64-bit vendor
number and the 32-bit device number. Support for matching based on
class is not yet implemented.  Each device is expected to be uniquely
identified by an array of cores (it matches if all of the cores are
instantiated), and for consistency the list is passed as
pointer-and-length.  Several similar devices can be driven by the same
driver, and thus the driver specifies and array of such arrays.

The complete set of involved data structures is thus the following:

        struct fmc_fru_id { char *manufacturer; char *product_name; };
        struct fmc_sdb_one_id { uint64_t vendor; uint32_t device; };
        struct fmc_sdb_id { struct fmc_sdb_one_id *cores; int cores_nr; };

        struct fmc_device_id {
                struct fmc_fru_id *fru_id; int fru_id_nr;
                struct fmc_sdb_id *sdb_id; int sdb_id_nr;
        };

A better reference, with full explanation, is the <linux/fmc.h> header.


Module Parameters
=================

Most of the FMC drivers need the same set of kernel parameters. This
package includes support to implement common parameters by means of
fields in the `fmc_driver' structure and simple macro definitions.

The parameters are carrier-specific, in that they rely on the busid
concept, that varies among carriers. For the SPEC, the identifier is a
PCI bus and devfn number, 16 bits wide in total; drivers for other
carriers will most likely offer something similar but not identical,
and some code duplication is unavoidable.

This is the list of parameters that are common to several modules to
see how they are actually used, please look at spec-trivial.c.

`busid='
     This is an array of integers, listing carrier-specific
     identification numbers. For PIC, for example, `0x0400' represents
     bus 4, slot 0.  If any such ID is specified, the driver will only
     accept to drive cards that appear in the list (even if the FMC ID
     matches). This is accomplished by the validate carrier method.

`gateware='
     The argument is an array of strings. If no busid= is specified,
     the first string of gateware= is used for all cards; otherwise the
     identifiers and gateware names are paired one by one, in the order
     specified.

`show_sdb='
     For modules supporting it, this parameter asks to show the SDB
     internal structure by means of kernel messages. It is disabled by
     default because those lines tend to hide more important messages,
     if you look at the system console while loading the drivers.
     Note: the parameter is being obsoleted, because fmc.ko itself now
     supports dump_sdb= that applies to every client driver.


For example, if you are using the trivial driver to load two different
gateware files to two different cards, you can use the following
parameters to load different binaries to the cards, after looking up
the PCI identifiers. This has been tested with a SPEC carrier.

        insmod fmc-trivial.ko \
                              busid=0x0200,0x0400 \
                              gateware=fmc/fine-delay.bin,fmc/simple-dio.bin

Please note that not all sub-modules support all of those parameters.
You can use modinfo to check what is supported by each module.