xref: /openbmc/linux/Documentation/driver-api/men-chameleon-bus.rst (revision 7594bb08)

=================
MEN Chameleon Bus
=================

.. Table of Contents
   =================
   1 Introduction
       1.1 Scope of this Document
       1.2 Limitations of the current implementation
   2 Architecture
       2.1 MEN Chameleon Bus
       2.2 Carrier Devices
       2.3 Parser
   3 Resource handling
       3.1 Memory Resources
       3.2 IRQs
   4 Writing an MCB driver
       4.1 The driver structure
       4.2 Probing and attaching
       4.3 Initializing the driver
       4.4 Using DMA


Introduction
============

This document describes the architecture and implementation of the MEN
Chameleon Bus (called MCB throughout this document).

Scope of this Document
----------------------

This document is intended to be a short overview of the current
implementation and does by no means describe the complete possibilities of MCB
based devices.

Limitations of the current implementation
-----------------------------------------

The current implementation is limited to PCI and PCIe based carrier devices
that only use a single memory resource and share the PCI legacy IRQ.  Not
implemented are:

- Multi-resource MCB devices like the VME Controller or M-Module carrier.
- MCB devices that need another MCB device, like SRAM for a DMA Controller's
  buffer descriptors or a video controller's video memory.
- A per-carrier IRQ domain for carrier devices that have one (or more) IRQs
  per MCB device like PCIe based carriers with MSI or MSI-X support.

Architecture
============

MCB is divided into 3 functional blocks:

- The MEN Chameleon Bus itself,
- drivers for MCB Carrier Devices and
- the parser for the Chameleon table.

MEN Chameleon Bus
-----------------

The MEN Chameleon Bus is an artificial bus system that attaches to a so
called Chameleon FPGA device found on some hardware produced my MEN Mikro
Elektronik GmbH. These devices are multi-function devices implemented in a
single FPGA and usually attached via some sort of PCI or PCIe link. Each
FPGA contains a header section describing the content of the FPGA. The
header lists the device id, PCI BAR, offset from the beginning of the PCI
BAR, size in the FPGA, interrupt number and some other properties currently
not handled by the MCB implementation.

Carrier Devices
---------------

A carrier device is just an abstraction for the real world physical bus the
Chameleon FPGA is attached to. Some IP Core drivers may need to interact with
properties of the carrier device (like querying the IRQ number of a PCI
device). To provide abstraction from the real hardware bus, an MCB carrier
device provides callback methods to translate the driver's MCB function calls
to hardware related function calls. For example a carrier device may
implement the get_irq() method which can be translated into a hardware bus
query for the IRQ number the device should use.

Parser
------

The parser reads the first 512 bytes of a Chameleon device and parses the
Chameleon table. Currently the parser only supports the Chameleon v2 variant
of the Chameleon table but can easily be adopted to support an older or
possible future variant. While parsing the table's entries new MCB devices
are allocated and their resources are assigned according to the resource
assignment in the Chameleon table. After resource assignment is finished, the
MCB devices are registered at the MCB and thus at the driver core of the
Linux kernel.

Resource handling
=================

The current implementation assigns exactly one memory and one IRQ resource
per MCB device. But this is likely going to change in the future.

Memory Resources
----------------

Each MCB device has exactly one memory resource, which can be requested from
the MCB bus. This memory resource is the physical address of the MCB device
inside the carrier and is intended to be passed to ioremap() and friends. It
is already requested from the kernel by calling request_mem_region().

IRQs
----

Each MCB device has exactly one IRQ resource, which can be requested from the
MCB bus. If a carrier device driver implements the ->get_irq() callback
method, the IRQ number assigned by the carrier device will be returned,
otherwise the IRQ number inside the Chameleon table will be returned. This
number is suitable to be passed to request_irq().

Writing an MCB driver
=====================

The driver structure
--------------------

Each MCB driver has a structure to identify the device driver as well as
device ids which identify the IP Core inside the FPGA. The driver structure
also contains callback methods which get executed on driver probe and
removal from the system::

	static const struct mcb_device_id foo_ids[] = {
		{ .device = 0x123 },
		{ }
	};
	MODULE_DEVICE_TABLE(mcb, foo_ids);

	static struct mcb_driver foo_driver = {
	driver = {
		.name = "foo-bar",
		.owner = THIS_MODULE,
	},
		.probe = foo_probe,
		.remove = foo_remove,
		.id_table = foo_ids,
	};

Probing and attaching
---------------------

When a driver is loaded and the MCB devices it services are found, the MCB
core will call the driver's probe callback method. When the driver is removed
from the system, the MCB core will call the driver's remove callback method::

	static init foo_probe(struct mcb_device *mdev, const struct mcb_device_id *id);
	static void foo_remove(struct mcb_device *mdev);

Initializing the driver
-----------------------

When the kernel is booted or your foo driver module is inserted, you have to
perform driver initialization. Usually it is enough to register your driver
module at the MCB core::

	static int __init foo_init(void)
	{
		return mcb_register_driver(&foo_driver);
	}
	module_init(foo_init);

	static void __exit foo_exit(void)
	{
		mcb_unregister_driver(&foo_driver);
	}
	module_exit(foo_exit);

The module_mcb_driver() macro can be used to reduce the above code::

	module_mcb_driver(foo_driver);

Using DMA
---------

To make use of the kernel's DMA-API's function, you will need to use the
carrier device's 'struct device'. Fortunately 'struct mcb_device' embeds a
pointer (->dma_dev) to the carrier's device for DMA purposes::

        ret = dma_set_mask_and_coherent(&mdev->dma_dev, DMA_BIT_MASK(dma_bits));
        if (rc)
                /* Handle errors */