Unix Device Taxonomy

1
Unix Device Taxonomy

• clock does not transfer data
• Block addressable storage devices
• Data moved in substantial size units
• Blocks are cached
• Character all else
• Originally referred to true single character at a
  time devices (e.g., keyboards and serial lines)
• Now, includes networks and raw (unstructured)
  interfaces

2
Unix Device Drivers

• Device-dependent software
• Handles device-specific operations plus generic
  functions like management of request queue and
  buffers
• Handles both layers 3 4
• Device-dependent commands
• Interrupt handling
• Single driver controls any number of copies of
  device

3
Unix Device Drivers

• Driver split into 2 parts
• Top half (device driver layer)
• Called by API, so runs in context of caller
• Interfaces with rest of Unix
• Includes synchronous routines invoked by syscall
• Maintains queue of I/O operations underway or to
  be performed
• Block devices queue buf structure (buffer)
  character devices queues clist structure
• Syscall runs through top half, then blocks the
  calling process (waiting for I/O to complete)

4
Unix Device Drivers

• Driver split into 2 parts
• Bottom half (interrupt handler layer)
• Caused to run by interrupt system, so runs in
  kernel context
• Directly interfaces with device
• Includes asynchronous routines invoked by
  interrupt handler
• Drains queue and unblocks top half when I/O
  operation is done
• Note that top bottom halves both access the
  queue, so need synchronization method

5
Unix I/O

• Devices mounted as special files
• Block and character devices appear in file system
  name space
• Advantages
• File I/O and device I/O look same to applications
• Devices can be protected in the same way as files
• Only superuser can create devices
• Inode stores class (block or character) and major
  and minor device numbers
• Device driver located by class and major device
  number
• Major indexes into array for that class (bdevsw
  for block device array cdevsw for character
  array) and identifies entry points for device
  driver
• Minor passed to driver to identify specific
  device

6
Block Special Files

• Goal is to minimize number of data transfers
• Block device is buffered by block cache (or
  buffer)
• Cache hit eliminates need for I/O operation
• Cache flushed on command and periodically
  (usually every 30 secs.)
• Sync flush whole cache
• Fsync flush blocks of single file
• Block devices often have a second, character
  interface for unbuffered I/O

7
Character Special Files

• BSD and classic Unix use c-lists
• Block of up to 64 characters
• Characters from character devices (mouses,
  terminals) buffered into chain of c-lists
• Line discipline
• Process data in c-lists before transfer to
  process
• 3 models
• Raw no processing
• Cbreak no line editting
• Cooked character stream
• Carriage returns mapped to line feeds
• Tabs mapped to spaces
• Remove erased characters

8
Character Special Files Streams

• Motivations
• Only one line discipline may be in use at a time
• Network protocols naturally implemented as
  pipelines
• Properties
• Stream head module that connects to process and
  implements syscall (read, write, etc.)
• Stream bottom device driver
• Other stream modules may be dynamically
  inserted/removed in between head and bottom
• Standard inter-module interface allows different
  programmers/vendors to write modules that work
  together
• Analogous to pipes in user-space, but more
  powerful because data flows in 2 directions and
  numerous data flow controls

9
Character Special Files Streams

• Data flow control capabilities
• Call neighboring modules put routine to enqueue
  a message (containing data and/or control
  information)
• Put is non-blocking if put-onto queue is full,
  caller puts message on its own outgoing queue
• Asynchronous except at stream head where user
  process will block waiting for I/O to complete
• At stream bottom (device driver), device may drop
  messages if its buffer is full
• Each modules service routine is scheduled by OS
  - Service routine drains incoming from neighbors
  and tries to push along outgoing messages to
  neighbors
• Each module maintains read/write queue to ensure
  correct order of processing
• Messages may specify priorities and service may
  reorder message processing
• Ability to multiplex modules can split or merge
  streams

10
Character Special Files Streams

• Advantages
• Provides modular and incremental framework for
  writing device drivers and network protocols
• Modules may be used by different streams and by
  different devices
• Example network modules used by different
  network protocols (ethernet vs. token ring)
• Supports message boundaries through flow control
  unstructured byte stream can be converted into
  structured messages
• System V, streams used for
• Named pipes
• Interprocess communication
• Network protocols
• Many character devices (e.g., terminals)