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Linux for S/390 and zSeries

Common Device Support (CDS)

Device Driver I/O Support Routines

Authors : Ingo Adlung

          Cornelia Huck

Copyright, IBM Corp. 1999-2002

Introduction

This document 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. 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 common device support layer 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.

Note:

In order to write a driver for S/390, you also need to look into the interface

described in Documentation/s390/driver-model.txt.

Note for porting drivers from 2.4:

The major changes are:

* The functions use a ccw_device instead of an irq (subchannel).

* All drivers must define a ccw_driver (see driver-model.txt) and the associated

  functions.

* request_irq() and free_irq() are no longer done by the driver.

* The oper_handler is (kindof) replaced by the probe() and set_online() functions

  of the ccw_driver.

* The not_oper_handler is (kindof) replaced by the remove() and set_offline()

  functions of the ccw_driver.

* The channel device layer is gone.

* The interrupt handlers must be adapted to use a ccw_device as argument.

  Moreover, they don't return a devstat, but an irb.

* Before initiating an io, the options must be set via ccw_device_set_options().

* Instead of calling read_dev_chars()/read_conf_data(), the driver issues

  the channel program and handles the interrupt itself.

ccw_device_get_ciw()

   get commands from extended sense data.

ccw_device_start()

ccw_device_start_timeout()

ccw_device_start_key()

ccw_device_start_key_timeout()

   initiate an I/O request.

ccw_device_resume()

   resume channel program execution.

ccw_device_halt()

   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 chapters 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 input to the device specific interrupt handler.

Note: All explanations apply also to the 64 bit architecture s390x.

Common Device Support (CDS) for Linux/390 Device Drivers

General Information

The following chapters describe the I/O related interface routines the

Linux/390 common device support (CDS) provides to allow for device specific

driver implementations on the IBM ESA/390 hardware platform. Those interfaces

intend to provide the functionality required by every device driver

implementation to allow to drive a specific hardware device on the ESA/390

platform. Some of the interface routines are specific to Linux/390 and some

of them can be found on other Linux platforms implementations too.

Miscellaneous function prototypes, data declarations, and macro definitions

can be found in the architecture specific C header file

linux/include/asm-s390/irq.h.

Overview of CDS interface concepts

Different to other hardware platforms, the ESA/390 architecture doesn't define

interrupt lines managed by a specific interrupt controller and bus systems

that may or may not allow for shared interrupts, DMA processing, etc.. Instead,

the ESA/390 architecture has implemented a so called channel subsystem, that

provides a unified view of the devices physically attached to the systems.

Though the ESA/390 hardware platform knows about a huge variety of different

peripheral attachments like disk devices (aka. DASDs), tapes, communication

controllers, etc. they can all be accessed by a well defined access method and

they are presenting I/O completion a unified way : I/O interruptions. Every

single device is uniquely identified to the system by a so called subchannel,

where the ESA/390 architecture allows for 64k devices be attached.

Linux, however, was first built on the Intel PC architecture, with its two

cascaded 8259 programmable interrupt controllers (PICs), that allow for a

maximum of 15 different interrupt lines. All devices attached to such a system

share those 15 interrupt levels. Devices attached to the ISA bus system must

not share interrupt levels (aka. IRQs), as the ISA bus bases on edge triggered

interrupts. MCA, EISA, PCI and other bus systems base on level triggered

interrupts, and therewith allow for shared IRQs. However, if multiple devices

present their hardware status by the same (shared) IRQ, the operating system

has to call every single device driver registered on this IRQ in order to

determine the device driver owning the device that raised the interrupt.

Up to kernel 2.4, Linux/390 used to provide interfaces via the IRQ (subchannel).

For internal use of the common I/O layer, these are still there. However,

device drivers should use the new calling interface via the ccw_device only.

During its startup the Linux/390 system checks for peripheral devices. Each

of those devices is uniquely defined by a so called subchannel by the ESA/390

channel subsystem. While the subchannel numbers are system generated, each

subchannel also takes a user defined attribute, the so called device number.

Both subchannel number and device number cannot exceed 65535. During driverfs

initialisation, the information about control unit type and device types that

imply specific I/O commands (channel command words - CCWs) in order to operate

the device are gathered. Device drivers can retrieve this set of hardware

information during their initialization step to recognize the devices they

support using the information saved in the struct ccw_device given to them.

This methods implies that Linux/390 doesn't require to probe for free (not

armed) interrupt request lines (IRQs) to drive its devices with. Where

applicable, the device drivers can use issue the READ DEVICE CHARACTERISTICS

ccw to retrieve device characteristics in its online routine.

In order to allow for easy I/O initiation the CDS layer provides a

ccw_device_start() interface that takes a device specific channel program (one

or more CCWs) as input sets up the required architecture specific control blocks

and initiates an I/O request on behalf of the device driver. The

ccw_device_start() routine allows to specify whether it expects the CDS layer

to notify the device driver for every interrupt it observes, or with final status

only. See ccw_device_start() for more details. A device driver must never issue

ESA/390 I/O commands itself, but must use the Linux/390 CDS interfaces instead.

For long running I/O request to be canceled, the CDS layer provides the

ccw_device_halt() function. Some devices require to initially issue a HALT

SUBCHANNEL (HSCH) command without having pending I/O requests. This function is

also covered by ccw_device_halt().

get_ciw() - get command information word

This call enables a device driver to get information about supported commands

from the extended SenseID data.

struct ciw *

ccw_device_get_ciw(struct ccw_device *cdev, __u32 cmd);

cdev - The ccw_device for which the command is to be retrieved.

cmd  - The command type to be retrieved.

ccw_device_get_ciw() returns:

NULL    - No extended data available, invalid device or command not found.

!NULL   - The command requested.

ccw_device_start() - Initiate I/O Request

The ccw_device_start() routines is the I/O request front-end processor. All

device driver I/O requests must be issued using this routine. A device driver

must not issue ESA/390 I/O commands itself. Instead the ccw_device_start()

routine provides all interfaces required to drive arbitrary devices.

This description also covers the status information passed to the device

driver's interrupt handler as this is related to the rules (flags) defined

with the associated I/O request when calling ccw_device_start().

int ccw_device_start(struct ccw_device *cdev,

                     struct ccw1 *cpa,

                     unsigned long intparm,

                     __u8 lpm,

                     unsigned long flags);

int ccw_device_start_timeout(struct ccw_device *cdev,

                             struct ccw1 *cpa,

                             unsigned long intparm,

                             __u8 lpm,

                             unsigned long flags,

                             int expires);

int ccw_device_start_key(struct ccw_device *cdev,

                         struct ccw1 *cpa,

                         unsigned long intparm,

                         __u8 lpm,

                         __u8 key,

                         unsigned long flags);

int ccw_device_start_key_timeout(struct ccw_device *cdev,

                                 struct ccw1 *cpa,

                                 unsigned long intparm,

                                 __u8 lpm,

                                 __u8 key,

                                 unsigned long flags,

                                 int expires);

cdev         : ccw_device the I/O is destined for

cpa          : logical start address of channel program

user_intparm : user specific interrupt information; will be presented

               back to the device driver's interrupt handler. Allows a

               device driver to associate the interrupt with a

               particular I/O request.

lpm          : defines the channel path to be used for a specific I/O

               request. A value of 0 will make cio use the opm.

key          : the storage key to use for the I/O (useful for operating on a

               storage with a storage key != default key)

flag         : defines the action to be performed for I/O processing

expires      : timeout value in jiffies. The common I/O layer will terminate

               the running program after this and call the interrupt handler

               with ERR_PTR(-ETIMEDOUT) as irb.

Possible flag values are :

DOIO_ALLOW_SUSPEND       - channel program may become suspended

DOIO_DENY_PREFETCH       - don't allow for CCW prefetch; usually

                           this implies the channel program might

                           become modified

DOIO_SUPPRESS_INTER     - don't call the handler on intermediate status

The cpa parameter points to the first format 1 CCW of a channel program :

struct ccw1 {

      __u8  cmd_code;/* command code */

      __u8  flags;   /* flags, like IDA addressing, etc. */

      __u16 count;   /* byte count */

      __u32 cda;     /* data address */

} __attribute__ ((packed,aligned(8)));

with the following CCW flags values defined :

CCW_FLAG_DC        - data chaining

CCW_FLAG_CC        - command chaining

CCW_FLAG_SLI       - suppress incorrect length

CCW_FLAG_SKIP      - skip

CCW_FLAG_PCI       - PCI

CCW_FLAG_IDA       - indirect addressing

CCW_FLAG_SUSPEND   - suspend

Via ccw_device_set_options(), the device driver may specify the following

options for the device:

DOIO_EARLY_NOTIFICATION  - allow for early interrupt notification

DOIO_REPORT_ALL          - report all interrupt conditions

The ccw_device_start() function returns :

-EBUSY  - The device is currently processing a previous I/O request, or there is

          a status pending at the device.

-ENODEV - cdev is invalid, the device is not operational or the ccw_device is

          not online.

When the I/O request completes, the CDS first level interrupt handler will

accumulate the status in a struct irb and then call the device interrupt handler.

The intparm field will contain the value the device driver has associated with a

particular I/O request. If a pending device status was recognized,

intparm will be set to 0 (zero). This may happen during I/O initiation or delayed

by an alert status notification. In any case this status is not related to the

current (last) I/O request. In case of a delayed status notification no special

interrupt will be presented to indicate I/O completion as the I/O request was

never started, even though ccw_device_start() returned with successful completion.

The irb may contain an error value, and the device driver should check for this

first:

-ETIMEDOUT: the common I/O layer terminated the request after the specified

            timeout value

-EIO:       the common I/O layer terminated the request due to an error state

If the concurrent sense flag in the extended status word (esw) in the irb is

set, the field erw.scnt in the esw describes the number of device specific

sense bytes available in the extended control word irb->scsw.ecw[]. No device

sensing by the device driver itself is required.

The device interrupt handler can use the following definitions to investigate

the primary unit check source coded in sense byte 0 :

SNS0_CMD_REJECT         0x80

SNS0_INTERVENTION_REQ   0x40

SNS0_BUS_OUT_CHECK      0x20

SNS0_EQUIPMENT_CHECK    0x10

SNS0_DATA_CHECK         0x08

SNS0_OVERRUN            0x04

SNS0_INCOMPL_DOMAIN     0x01

Depending on the device status, multiple of those values may be set together.

Please refer to the device specific documentation for details.

The irb->scsw.cstat field provides the (accumulated) subchannel status :

SCHN_STAT_PCI            - program controlled interrupt

SCHN_STAT_INCORR_LEN     - incorrect length

SCHN_STAT_PROG_CHECK     - program check

SCHN_STAT_PROT_CHECK     - protection check

SCHN_STAT_CHN_DATA_CHK   - channel data check

SCHN_STAT_CHN_CTRL_CHK   - channel control check

SCHN_STAT_INTF_CTRL_CHK  - interface control check

SCHN_STAT_CHAIN_CHECK    - chaining check

The irb->scsw.dstat field provides the (accumulated) device status :

DEV_STAT_ATTENTION   - attention

DEV_STAT_STAT_MOD    - status modifier

DEV_STAT_CU_END      - control unit end

DEV_STAT_BUSY        - busy

DEV_STAT_CHN_END     - channel end

DEV_STAT_DEV_END     - device end

DEV_STAT_UNIT_CHECK  - unit check

DEV_STAT_UNIT_EXCEP  - unit exception

Please see the ESA/390 Principles of Operation manual for details on the

individual flag meanings.

Usage Notes :

ccw_device_start() must be called disabled and with the ccw device lock held.

The device driver is allowed to issue the next ccw_device_start() call from

within its interrupt handler already. It is not required to schedule a

bottom-half, unless a non deterministically long running error recovery procedure

or similar needs to be scheduled. During I/O processing the Linux/390 generic

I/O device driver support has already obtained the IRQ lock, i.e. the handler

must not try to obtain it again when calling ccw_device_start() or we end in a

deadlock situation!

If a device driver relies on an I/O request to be completed prior to start the

next it can reduce I/O processing overhead by chaining a NoOp I/O command

CCW_CMD_NOOP to the end of the submitted CCW chain. This will force Channel-End

and Device-End status to be presented together, with a single interrupt.

However, this should be used with care as it implies the channel will remain

busy, not being able to process I/O requests for other devices on the same

channel. Therefore e.g. read commands should never use this technique, as the

result will be presented by a single interrupt anyway.

In order to minimize I/O overhead, a device driver should use the

DOIO_REPORT_ALL  only if the device can report intermediate interrupt

information prior to device-end the device driver urgently relies on. In this

case all I/O interruptions are presented to the device driver until final

status is recognized.

If a device is able to recover from asynchronously presented I/O errors, it can

perform overlapping I/O using the DOIO_EARLY_NOTIFICATION flag. While some

devices always report channel-end and device-end together, with a single

interrupt, others present primary status (channel-end) when the channel is

ready for the next I/O request and secondary status (device-end) when the data

transmission has been completed at the device.

Above flag allows to exploit this feature, e.g. for communication devices that

can handle lost data on the network to allow for enhanced I/O processing.

Unless the channel subsystem at any time presents a secondary status interrupt,

exploiting this feature will cause only primary status interrupts to be

presented to the device driver while overlapping I/O is performed. When a

secondary status without error (alert status) is presented, this indicates

successful completion for all overlapping ccw_device_start() requests that have

been issued since the last secondary (final) status.

Channel programs that intend to set the suspend flag on a channel command word

(CCW)  must start the I/O operation with the DOIO_ALLOW_SUSPEND option or the

suspend flag will cause a channel program check. At the time the channel program

becomes suspended an intermediate interrupt will be generated by the channel

subsystem.

ccw_device_resume() - Resume Channel Program Execution

If a device driver chooses to suspend the current channel program execution by

setting the CCW suspend flag on a particular CCW, the channel program execution

is suspended. In order to resume channel program execution the CIO layer

provides the ccw_device_resume() routine.

int ccw_device_resume(struct ccw_device *cdev);

cdev - ccw_device the resume operation is requested for

The ccw_device_resume() function returns:

-EBUSY     - status pending

-ENODEV    - cdev invalid or not-operational subchannel

-EINVAL    - resume function not applicable

-ENOTCONN  - there is no I/O request pending for completion

Usage Notes:

Please have a look at the ccw_device_start() usage notes for more details on

suspended channel programs.

ccw_device_halt() - Halt I/O Request Processing

Sometimes a device driver might need a possibility to stop the processing of

a long-running channel program or the device might require to initially issue

a halt subchannel (HSCH) I/O command. For those purposes the ccw_device_halt()

command is provided.

ccw_device_halt() must be called disabled and with the ccw device lock held.

int ccw_device_halt(struct ccw_device *cdev,

                    unsigned long intparm);

cdev    : ccw_device the halt operation is requested for

intparm : interruption parameter; value is only used if no I/O

          is outstanding, otherwise the intparm associated with

          the I/O request is returned

The ccw_device_halt() function returns :

-EBUSY  - the device is currently busy, or status pending.

-ENODEV - cdev invalid.

-EINVAL - The device is not operational or the ccw device is not online.

Usage Notes :

A device driver may write a never-ending channel program by writing a channel

program that at its end loops back to its beginning by means of a transfer in

channel (TIC)   command (CCW_CMD_TIC). Usually this is performed by network

device drivers by setting the PCI CCW flag (CCW_FLAG_PCI). Once this CCW is

executed a program controlled interrupt (PCI) is generated. The device driver

can then perform an appropriate action. Prior to interrupt of an outstanding

read to a network device (with or without PCI flag) a ccw_device_halt()

is required to end the pending operation.

ccw_device_clear() - Terminage I/O Request Processing

In order to terminate all I/O processing at the subchannel, the clear subchannel

(CSCH) command is used. It can be issued via ccw_device_clear().

ccw_device_clear() must be called disabled and with the ccw device lock held.

int ccw_device_clear(struct ccw_device *cdev, unsigned long intparm);

cdev:    ccw_device the clear operation is requested for

intparm: interruption parameter (see ccw_device_halt())

The ccw_device_clear() function returns:

-ENODEV - cdev invalid

-EINVAL - The device is not operational or the ccw device is not online.

Miscellaneous Support Routines

This chapter describes various routines to be used in a Linux/390 device

driver programming environment.

get_ccwdev_lock()

Get the address of the device specific lock. This is then used in

spin_lock() / spin_unlock() calls.

__u8 ccw_device_get_path_mask(struct ccw_device *cdev);

Get the mask of the path currently available for cdev.