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104 lines
3.2 KiB
Markdown
104 lines
3.2 KiB
Markdown
---
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title: "16-device-drivers"
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aliases:
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tags:
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- lecture
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- cosc204
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sr-due: 2022-09-25
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sr-interval: 3
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sr-ease: 250
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---
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# Device controller and driver
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driver Defn: a special kernel module that contrls the operations of a device with the device specific information
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controller Defn: a hardware unit on a device that can know and control a devices status or behaviour, which communicates with the CPU via a driver
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# Types of IO devices
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- character stream vs block
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- swqeuential vs random access
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- synchronous vs async
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- sharable or dedicated
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OS provides a consistent interface for them in the `/dev` folder (unix systems). e.g., ramdisk device is name `/dev/memdrv`. each device has a file under `/dev` that can be handled with file operations read()/write()
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## block devices
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commands (sent from device driver to device controller)
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- read block
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- write block
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- seek block (for random access block devices)
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the sequence of read/write requests can (should) be optimized
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## character devices
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- commands
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- get char
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- put char
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also support buffering (accepting keyboard input line by line) and editing of buffers (e.g., spcial treatement of backspace character)
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# IO models
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IO system calls use different IO models
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- blocking - when a proccess needs IO input it waits to recieve it before continuing
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- non blocking - waits a fixed (small) amount of time, and returns (possibly with some data still untransferred)
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- async - does not wait, is informed with a signal when io is completed
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# IO scheduling and buffering
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for when there are several requests for IO waiting to be serviced
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- first come firs served is not efficient
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IO buffers are used in main memroy to optimise file systems
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when a data block is used often, it is kept is memory and only written to secondary storage once all its accesses are finished
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# example ramdisk driver
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## open
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``` c
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int temp_open (struct inode *inode, struct file *filp){
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// check if the process has the permission to access;
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// suspend the calling process if the device is busy
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// or used by another process;
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// otherwise record the id of the calling process
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// so that the device is owned by the process;
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// initialise or reset the device.
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}
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```
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## read
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```c
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ssize_t temp_read(struct file *filp, char __user *buf,size_t count,loff_t *f_pos){
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// find the location of data;
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// ask the controller to copy data to kernel kbuf
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// copy data to user space kbuf -> buf
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copy_to_user (buf, kbuf, count);
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*f_pos += count; // update pointer
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return count; // return the number of bytes read
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}
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```
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## write
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```c
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ssize_t temp_write(struct file *filp, char __user *bufsize_t count, loff_t *f_pos){
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// copy data from user space buf -> kbuf
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copy_from_user (kbuf, buf, count);
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// find the location of data;
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// ask the controller to copy data from kernel kbuf
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*f_pos += count; // update pointer
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return count; // return the number of bytes written
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}
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```
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## close
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```c
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int temp_release (struct inode *inode, struct file *filp){
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// release the device by removing the id of the process;
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// check if there is any waiting process to wake up;
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// undo anything done at open(), e.g. free memory.
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}
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```
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# UI for IO devices
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