Abstract

This document is a collection of extracts, observations and notes pertaining to the S/390 port of Linux.

Contents

Linux/390

On December 18, 1999, IBM published its modifications and additions to the Linux 2.2.13 code base for the support of the S/390 architecture. This port is designed to run under VM/ESA and natively. The code has subsequently been rolled into the 2.2.15 level.

This document contains information specific to the S/390 port of Linux. In it I have reproduced documentation found within the distribution that describes the I/O facilities and DASD handling. In addition, information that I have come across as I have looked at the port such as new source files, system calls, and register conventions have been included.

Common Device Support

The following section was copied from the Documentation/390 directory of the Linux distribution. It was written by Indo Adlung and is copyright IBM 1999, under the GNU Public License.

This chapter describes the common device support routines for Linux/390. Different than other hardware architectures, ESA/390 has defined a unified I/O access method. This gives relief to the device drivers as they don't have to deal with different bus types, polling versus interrupt processing, shared versus non-shared interrupt processing, DMA versus port I/O (PIO), and other hardware features more. i However, this implies that either every single device driver needs to implement the hardware I/O attachment functionality itself, or the operating system provides for a unified method to access the hardware, providing all the functionality that every single device driver would have to provide itself.

The document does not intend to explain the ESA/390 hardware architecture in every detail. This information can be obtained from the ESA/390 Principles of Operation manual (IBM Form. No. SA22-7201).

In order to build common device support for ESA/390 I/O interfaces, a functional layer was introduced that provides generic I/O access methods to the hardware. The following figure shows the usage of the common device support of Linux/390 using a TCP/IP driven device access an example. Similar figures could be drawn for other access methods, e.g. file system access to disk devices.

The common device support layer shown above comprises the I/O support routines defined below. Some of them implement common Linux device driver interfaces, while some of them are ESA/390 platform specific.

• get_dev_info_by_IRQ() / get_dev_info_by_devno()

allow a device driver to determine the devices attached (visible) to the system and their current status.

• get_IRQ_by_devno() / get_devno_by_IRQ()

get IRQ (subchannel) from device number and vice versa.

• read_dev_chars()

read device characteristics

• request_IRQ()

obtain ownership for a specific device.

• free_IRQ()

release ownership for a specific device.

• disable_IRQ()

disable a device from presenting interrupts.

• enable_IRQ()

enable a device, allowing for I/O interrupts.

• do_IO()

initiate an I/O request.

• halt_IO()

terminate the current I/O request processed on the device.

• do_IRQ()

generic interrupt routine. This function is called by the interrupt entry routine whenever an I/O interrupt is presented to the system. The do_IRQ() routine determines the interrupt status and calls the device specific interrupt handler according to the rules (flags) defined during I/O request initiation with do_IO().

The next sections describe the functions, other than do_IRQ() in more details. The do_IRQ() interface is not described, as it is called from the Linux/390 first level interrupt handler only and does not comprise a device driver callable interface. Instead, the functional description of do_IO() also describes the