188 lines
6.1 KiB
ReStructuredText
188 lines
6.1 KiB
ReStructuredText
=================
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MEN Chameleon Bus
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=================
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.. Table of Contents
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=================
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1 Introduction
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1.1 Scope of this Document
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1.2 Limitations of the current implementation
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2 Architecture
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2.1 MEN Chameleon Bus
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2.2 Carrier Devices
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2.3 Parser
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3 Resource handling
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3.1 Memory Resources
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3.2 IRQs
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4 Writing an MCB driver
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4.1 The driver structure
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4.2 Probing and attaching
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4.3 Initializing the driver
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4.4 Using DMA
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Introduction
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============
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This document describes the architecture and implementation of the MEN
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Chameleon Bus (called MCB throughout this document).
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Scope of this Document
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----------------------
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This document is intended to be a short overview of the current
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implementation and does by no means describe the complete possibilities of MCB
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based devices.
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Limitations of the current implementation
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-----------------------------------------
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The current implementation is limited to PCI and PCIe based carrier devices
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that only use a single memory resource and share the PCI legacy IRQ. Not
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implemented are:
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- Multi-resource MCB devices like the VME Controller or M-Module carrier.
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- MCB devices that need another MCB device, like SRAM for a DMA Controller's
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buffer descriptors or a video controller's video memory.
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- A per-carrier IRQ domain for carrier devices that have one (or more) IRQs
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per MCB device like PCIe based carriers with MSI or MSI-X support.
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Architecture
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============
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MCB is divided into 3 functional blocks:
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- The MEN Chameleon Bus itself,
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- drivers for MCB Carrier Devices and
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- the parser for the Chameleon table.
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MEN Chameleon Bus
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-----------------
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The MEN Chameleon Bus is an artificial bus system that attaches to a so
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called Chameleon FPGA device found on some hardware produced my MEN Mikro
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Elektronik GmbH. These devices are multi-function devices implemented in a
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single FPGA and usually attached via some sort of PCI or PCIe link. Each
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FPGA contains a header section describing the content of the FPGA. The
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header lists the device id, PCI BAR, offset from the beginning of the PCI
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BAR, size in the FPGA, interrupt number and some other properties currently
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not handled by the MCB implementation.
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Carrier Devices
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---------------
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A carrier device is just an abstraction for the real world physical bus the
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Chameleon FPGA is attached to. Some IP Core drivers may need to interact with
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properties of the carrier device (like querying the IRQ number of a PCI
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device). To provide abstraction from the real hardware bus, an MCB carrier
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device provides callback methods to translate the driver's MCB function calls
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to hardware related function calls. For example a carrier device may
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implement the get_irq() method which can be translated into a hardware bus
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query for the IRQ number the device should use.
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Parser
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------
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The parser reads the first 512 bytes of a Chameleon device and parses the
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Chameleon table. Currently the parser only supports the Chameleon v2 variant
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of the Chameleon table but can easily be adopted to support an older or
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possible future variant. While parsing the table's entries new MCB devices
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are allocated and their resources are assigned according to the resource
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assignment in the Chameleon table. After resource assignment is finished, the
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MCB devices are registered at the MCB and thus at the driver core of the
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Linux kernel.
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Resource handling
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=================
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The current implementation assigns exactly one memory and one IRQ resource
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per MCB device. But this is likely going to change in the future.
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Memory Resources
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----------------
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Each MCB device has exactly one memory resource, which can be requested from
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the MCB bus. This memory resource is the physical address of the MCB device
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inside the carrier and is intended to be passed to ioremap() and friends. It
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is already requested from the kernel by calling request_mem_region().
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IRQs
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----
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Each MCB device has exactly one IRQ resource, which can be requested from the
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MCB bus. If a carrier device driver implements the ->get_irq() callback
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method, the IRQ number assigned by the carrier device will be returned,
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otherwise the IRQ number inside the Chameleon table will be returned. This
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number is suitable to be passed to request_irq().
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Writing an MCB driver
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=====================
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The driver structure
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--------------------
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Each MCB driver has a structure to identify the device driver as well as
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device ids which identify the IP Core inside the FPGA. The driver structure
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also contains callback methods which get executed on driver probe and
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removal from the system::
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static const struct mcb_device_id foo_ids[] = {
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{ .device = 0x123 },
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{ }
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};
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MODULE_DEVICE_TABLE(mcb, foo_ids);
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static struct mcb_driver foo_driver = {
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driver = {
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.name = "foo-bar",
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.owner = THIS_MODULE,
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},
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.probe = foo_probe,
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.remove = foo_remove,
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.id_table = foo_ids,
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};
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Probing and attaching
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---------------------
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When a driver is loaded and the MCB devices it services are found, the MCB
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core will call the driver's probe callback method. When the driver is removed
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from the system, the MCB core will call the driver's remove callback method::
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static init foo_probe(struct mcb_device *mdev, const struct mcb_device_id *id);
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static void foo_remove(struct mcb_device *mdev);
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Initializing the driver
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-----------------------
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When the kernel is booted or your foo driver module is inserted, you have to
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perform driver initialization. Usually it is enough to register your driver
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module at the MCB core::
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static int __init foo_init(void)
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{
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return mcb_register_driver(&foo_driver);
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}
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module_init(foo_init);
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static void __exit foo_exit(void)
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{
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mcb_unregister_driver(&foo_driver);
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}
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module_exit(foo_exit);
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The module_mcb_driver() macro can be used to reduce the above code::
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module_mcb_driver(foo_driver);
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Using DMA
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---------
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To make use of the kernel's DMA-API's function, you will need to use the
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carrier device's 'struct device'. Fortunately 'struct mcb_device' embeds a
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pointer (->dma_dev) to the carrier's device for DMA purposes::
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ret = dma_set_mask_and_coherent(&mdev->dma_dev, DMA_BIT_MASK(dma_bits));
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if (rc)
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/* Handle errors */
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