cds.txt

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().

read_dev_chars()	
   read device characteristics
   
read_conf_data()
   read configuration data.

ccw_device_get_ciw()
   get commands from extended sense data.

ccw_device_start()	
   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
implementaion 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 by 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.

In order not to introduce a new I/O concept to the common Linux code,
Linux/390 preserves the IRQ concept and semantically maps the ESA/390
subchannels to Linux as IRQs. This allows Linux/390 to support up to 64k
different IRQs, uniquely representig a single device each.

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 can not 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 the read_dev_chars() to retrieve device
characteristics. This can be done without having to request device ownership
previously.

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().


read_dev_chars() - Read Device Characteristics

This routine returns the characteristics for the device specified.

The function is meant to be called with an irq handler in place; that is,
at earliest during set_online() processing.

While the request is procesed synchronously, the device interrupt
handler is called for final ending status. In case of error situations the
interrupt handler may recover appropriately. The device irq handler can
recognize the corresponding interrupts by the interruption parameter be
0x00524443.The ccw_device must not be locked prior to calling read_dev_chars().

The function may be called enabled or disabled.

int read_dev_chars(struct ccw_device *cdev, void **buffer, int length );

cdev   - the ccw_device the information is requested for.
buffer - pointer to a buffer pointer. The buffer pointer itself
         must contain a valid buffer area.
length - length of the buffer provided.

The read_dev_chars() function returns :

      0 - successful completion
-ENODEV - cdev invalid
-EINVAL - an invalid parameter was detected, or the function was called early.
-EBUSY  - an irrecoverable I/O error occurred or the device is not
          operational.


read_conf_data() - Read Configuration Data

Retrieve the device dependent configuration data. Please have a look at your 
device dependent I/O commands for the device specific layout of the node 
descriptor elements. 

The function is meant to be called with an irq handler in place; that is,
at earliest during set_online() processing.

The function may be called enabled or disabled, but the device must not be
locked

int read_conf_data(struct ccw_device, void **buffer, int *length, __u8 lpm);

cdev   - the ccw_device the data is requested for.
buffer - Pointer to a buffer pointer. The read_conf_data() routine
         will allocate a buffer and initialize the buffer pointer
         accordingly. It's the device driver's responsibility to
         release the kernel memory if no longer needed. 
length - Length of the buffer allocated and retrieved.
lpm    - Logical path mask to be used for retrieving the data. If
         zero the data is retrieved on the next path available.

The read_conf_data() function returns :
          0 - Successful completion
-ENODEV     - cdev invalid.
-EINVAL     - An invalid parameter was detected, or the function was called early.
-EIO        - An irrecoverable I/O error occurred or the device is
              not operational.
-ENOMEM     - The read_conf_data() routine couldn't obtain storage.
-EOPNOTSUPP - The device doesn't support the read configuration 
              data command.


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);

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

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 incorrct 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 :

      0 - successful completion or request successfully initiated
-EBUSY  - The device is currently processing a previous I/O request, or ther 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
accumalate 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.

If the concurrent sense flag in the extended status word in the irb is set, the
field irb->scsw.count describes the numer of device specific sense bytes
available in the extended control word irb->scsw.ecw[0]. 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 :

Prior to call ccw_device_start() the device driver must assure disabled state,
i.e. the I/O mask value in the PSW must be disabled. This can be accomplished
by calling local_save_flags( flags). The current PSW flags are preserved and
can be restored by local_irq_restore( flags) at a later time.

If the device driver violates this rule while running in a uni-processor
environment an interrupt might be presented prior to the ccw_device_start()
routine returning to the device driver main path. In this case we will end in a
deadlock situation as the interrupt handler will try to obtain the irq
lock the device driver still owns (see below) !

The driver must assure to hold the device specific lock. This can be
accomplished by

(i)  spin_lock(get_ccwdev_lock(cdev)), or
(ii) spin_lock_irqsave(get_ccwdev_lock(cdev), flags)

Option (i) should be used if the calling routine is running disabled for
I/O interrupts (see above) already. Option (ii) obtains the device gate und
puts the CPU into I/O disabled state by preserving the current PSW flags.

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 an non deterministicly 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 asynchronosly 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 resume_IO() function returns:

        0  - suspended channel program is resumed
-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.

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 :

      0 - successful completion or request successfully initiated
-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.


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.