34 Device Drivers

View from user space : Have special file in /dev associated with it, together with five systems calls:

• open : make device available
• read : read from device
• write : write to device
• ioctl : Perform operations on device (optional)
• close : make device unavailable

Kernel Side

Each file may have functions associated with it which are called when corresponding system calls are made linux/fs.h lists all available operations on files. Device driver implements at least functions for open, read, write and close.

Categorising devices

Kernel also keeps track of

• Physical dependencies between devices. Example: devices connected to a USB-hub
• Buses: Channels between processor and one or more devices. Can be either physical (e.g., pci, usb), or logical
• Classes: Sets of devices of the same type, e.g., keyboards, mice

Handling Interrupts in Device Drivers

Normal cycle of interrupt handling for devices :

• Device sends interrupt
• CPU selects appropriate interrupt handler
• Interrupt handler process interrupt. Two important tasks to be done :
  • Data to be transferred to/from device
  • Waking up process which wait for data transfer to be finished
• Interrupt handler clears interrupt bit of device. Necessary for next interrupt to arrive
  Interrupt processing time must be as short as possible. Data transfer fast, rest of processing slow.
  $\Rightarrow$ Separate interrupt processing in two halves:
• Top Half is called directly by interrupt handler. Only transfers data between device and appropriate kernel buffer and schedules software interrupt to start Bottom half.
• Bottom half still runs in interrupt context and does the rest of the processing (e.g., working through the protocol stack, and waking up processes)