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DEC 21140 Driver

DEC 21240 Driver Introduction

One aim of our project is to port RTEMS on a standard PowerPC platform. To achieve it, we have chosen a Motorola MCP750 board. This board includes an Ethernet controller based on a DEC21140 chip. Because RTEMS has a TCP/IP stack, we will have to develop the DEC21140 related ethernet driver for the PowerPC port of RTEMS. As this controller is able to support 100Mbps network and as there is a lot of PCI card using this DEC chip, we have decided to first implement this driver on an Intel PC386 target to provide a solution for using RTEMS on PC with the 100Mbps network and then to port this code on PowerPC in a second phase.

The aim of this document is to give some PCI board generalities and to explain the software architecture of the RTEMS driver. Finally, we will see what will be done for ChorusOs and Netboot environment .

Document Revision History

Current release:

  • Current applicable release is 1.0.

Existing releases:

  • 1.0 : Released the 10/02/98. First version of this document.
  • 0.1 : First draft of this document

Planned releases:

  • None planned today.

DEC21140 PCI Board Generalities

This chapter describes rapidely the PCI interface of this Ethernet controller. The board we have chosen for our PC386 implementation is a D-Link DFE-500TX. This is a dual-speed 10/100Mbps Ethernet PCI adapter with a DEC21140AF chip. Like other PCI devices, this board has a PCI device's header containing some required configuration registers, as shown in the PCI Register Figure. By reading or writing these registers, a driver can obtain information about the type of the board, the interrupt it uses, the mapping of the chip specific registers, ...

On Intel target, the chip specific registers can be accessed via 2 methods : I/O port access or PCI address mapped access. We have chosen to implement the PCI address access to obtain compatible source code to the port the driver on a PowerPC target.


On RTEMS, a PCI API exists. We have used it to configure the board. After initializing this PCI module via the pci_initialize() function, we try to detect the DEC21140 based ethernet board. This board is characterized by its Vendor ID (0x1011) and its Device ID (0x0009). We give these arguments to the``pcib_find_by_deviceid`` function which returns , if the device is present, a pointer to the configuration header space (see PCI Registers Fgure). Once this operation performed, the driver is able to extract the information it needs to configure the board internal registers, like the interrupt line, the base address,... The board internal registers will not be detailled here. You can find them in DIGITAL Semiconductor 21140A PCI Fast Ethernet LAN Controller - Hardware Reference Manual.

RTEMS Driver Software Architecture

In this chapter will see the initialization phase, how the controller uses the host memory and the 2 threads launched at the initialization time.

Initialization phase

The DEC21140 Ethernet driver keeps the same software architecture than the other RTEMS ethernet drivers. The only API the programmer can use is the rtems_dec21140_driver_attach````(struct rtems_bsdnet_ifconfig \*config) function which detects the board and initializes the associated data structure (with registers base address, entry points to low-level initialization function,...), if the board is found.

Once the attach function executed, the driver initializes the DEC chip. Then the driver connects an interrupt handler to the interrupt line driven by the Ethernet controller (the only interrupt which will be treated is the receive interrupt) and launches 2 threads : a receiver thread and a transmitter thread. Then the driver waits for incoming frame to give to the protocol stack or outcoming frame to send on the physical link.

Memory Buffer

This DEC chip uses the host memory to store the incoming Ethernet frames and the descriptor of these frames. We have chosen to use 7 receive buffers and 1 transmit buffer to optimize memory allocation due to cache and paging problem that will be explained in the section Encountered Problems.

To reference these buffers to the DEC chip we use a buffer descriptors ring. The descriptor structure is defined in the Buffer Descriptor Figure. Each descriptor can reference one or two memory buffers. We choose to use only one buffer of 1520 bytes per descriptor.

The difference between a receive and a transmit buffer descriptor is located in the status and control bits fields. We do not give details here, please refer to the [DEC21140 Hardware Manual].


Receiver Thread

This thread is event driven. Each time a DEC PCI board interrupt occurs, the handler checks if this is a receive interrupt and send an event "reception" to the receiver thread which looks into the entire buffer descriptors ring the ones that contain a valid incoming frame (bit OWN=0 means descriptor belongs to host processor). Each valid incoming ethernet frame is sent to the protocol stack and the buffer descriptor is given back to the DEC board (the host processor reset bit OWN, which means descriptor belongs to 21140).

Transmitter Thread

This thread is also event driven. Each time an Ethernet frame is put in the transmit queue, an event is sent to the transmit thread, which empty the queue by sending each outcoming frame. Because we use only one transmit buffer, we are sure that the frame is well-sent before sending the next.

Encountered Problems

On Intel PC386 target, we were faced with a problem of memory cache management. Because the DEC chip uses the host memory to store the incoming frame and because the DEC21140 configuration registers are mapped into the PCI address space, we must ensure that the data read (or written) by the host processor are the ones written (or read) by the DEC21140 device in the host memory and not old data stored in the cache memory. Therefore, we had to provide a way to manage the cache. This module is described in the document RTEMS Cache Management For Intel. On Intel, the memory region cache management is available only if the paging unit is enabled. We have used this paging mechanism, with 4Kb page. All the buffers allocated to store the incoming or outcoming frames, buffer descriptor and also the PCI address space of the DEC board are located in a memory space with cache disable.

Concerning the buffers and their descriptors, we have tried to optimize the memory space in term of allocated page. One buffer has 1520 bytes, one descriptor has 16 bytes. We have 7 receive buffers and 1 transmit buffer, and for each, 1 descriptor : (7+1)*(1520+16) = 12288 bytes = 12Kb = 3 entire pages. This allows not to lose too much memory or not to disable cache memory for a page which contains other data than buffer, which could decrease performance.

ChorusOs DEC Driver

Because ChorusOs is used in several Canon CRF projects, we must provide such a driver on this OS to ensure compatibility between the RTEMS and ChorusOs developments. On ChorusOs, a DEC driver source code already exists but only for a PowerPC target. We plan to port this code (which uses ChorusOs API) on Intel target. This will allow us to have homogeneous developments. Moreover, the port of the development performed with ChorusOs environment to RTEMS environment will be easier for the developers.

Netboot DEC driver

We use Netboot tool to load our development from a server to the target via an ethernet network. Currently, this tool does not support the DEC board. We plan to port the DEC driver for the Netboot tool.

But concerning the port of the DEC driver into Netboot, we are faced with a problem : in RTEMS environment, the DEC driver is interrupt or event driven, in Netboot environment, it must be used in polling mode. It means that we will have to re-write some mechanisms of this driver.

List of Ethernet cards using the DEC chip

Many Ethernet adapter cards use the Tulip chip. Here is a non exhaustive list of adapters which support this driver :

  • Accton EtherDuo PCI.
  • Accton EN1207 All three media types supported.
  • Adaptec ANA6911/TX 21140-AC.
  • Cogent EM110 21140-A with DP83840 N-Way MII transceiver.
  • Cogent EM400 EM100 with 4 21140 100mbps-only ports + PCI Bridge.
  • Danpex EN-9400P3.
  • D-Link DFE500-Tx 21140-A with DP83840 transceiver.
  • Kingston EtherX KNE100TX 21140AE.
  • Netgear FX310 TX 10/100 21140AE.
  • SMC EtherPower10/100 With DEC21140 and 68836 SYM transceiver.
  • SMC EtherPower10/100 With DEC21140-AC and DP83840 MII transceiver. Note: The EtherPower II uses the EPIC chip, which requires a different driver.
  • Surecom EP-320X DEC 21140.
  • Thomas Conrad TC5048.
  • Znyx ZX345 21140-A, usually with the DP83840 N-Way MII transciever. Some ZX345 cards made in 1996 have an ICS 1890 transciver instead.
  • ZNYX ZX348 Two 21140-A chips using ICS 1890 transcievers and either a 21052 or 21152 bridge. Early versions used National 83840 transcievers, but later versions are depopulated ZX346 boards.
  • ZNYX ZX351 21140 chip with a Broadcom 100BaseT4 transciever.

Our DEC driver has not been tested with all these cards, only with the D-Link DFE500-TX.

  • [DEC21140 Hardware Manual] DIGITAL, *DIGITAL Semiconductor 21140A PCI Fast Ethernet LAN Controller - Hardware Reference Manual*.
  • [99.TA.0021.M.ER]Emmanuel Raguet,*RTEMS Cache Management For Intel*.
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