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I/O Device Connection in Linux

I/O Device Connection in Linux. Michael J. Feinberg Michael.Feinberg@UConn.Edu Ken Cook K.Cook@UConn.Edu CSE258 Linux Project November 30, 1999. Introduction: What are we talking about??. How I/O devices can interact and are connected to the Linux operating system.

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I/O Device Connection in Linux

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  1. I/O Device Connection in Linux Michael J. Feinberg Michael.Feinberg@UConn.Edu Ken Cook K.Cook@UConn.Edu CSE258 Linux Project November 30, 1999

  2. Introduction: What are we talking about?? • How I/O devices can interact and are connected to the Linux operating system. • How architecture is used by Linux (i.e. Interrupts, DMA, etc.) • The device drivers that kernel depends upon to communicate to the device • How the device is represented within Linux • Attempt to note things were taken into account due to Linux’s ability to run a variety of architectures.

  3. Sorry to Interrupt... • Linux takes advantage of an interrupt architecture • Interrupt assignment varies depending on the device. • Some devices are assigned interrupts due to legacy architecture EXAMPLE:FLOPPY DRIVES ARE ASSIGNED IRQ 6 • PCI devices are assigned IRQ’s dynamically on boot-up • Intel x86 Architecture relies on PC BIOS • ALPHA XPD architecture has no BIOS, so assignment is handled by Linux.

  4. ISA, old chum... • In the case of ISA, the device is not aware inherently of its IRQ Linux Deals • Once an action causes an ISA device to be intterrupted, Linux activates all it’s unassigned IRQ’s • If the ISA is fortunate enough to find an unused IRQ, it shall use it as if it were directly assigned.

  5. Handling Interrupts • Uses a set of pointers to data structures containing the addresses of the routines within the device driver that handle the system's interrupts • It is the responsibility of each device driver to request the interrupt that it wants when the driver is initialized. • Kernel determines what device called the interrupt by checking the status registers of the programmable interrupt controllers. • Kernel generates an offset into the irq_action vector. • irq_action vector contains pointers to subroutines for each device • Subroutines are contained in the Device Driver.

  6. Little DMA... • Linux uses and suffers through the limitations of DMA • Limited to 7 DMA channels • These channels can’t be shared • DMA needs a contiguous block of memory • Circumvents virtual memory due to it’s direct nature. • Needs to take percautions so the “Direct” pages aren’t swapped out. (i.e. locking the pages) • DMA only has 24-bits for addressing • DMA can only access the lower 16MB of primary memory

  7. The Structure of DMA • Tracks the usage of the DMA channels using a vector of dma_chan data structures. • one per DMA channel • The dma_chan data structure contains just two fields • a pointer to a string describing the owner of the DMA channel • flag indicating if the DMA channel is allocated or not. • vector of dma_chan data structures that is printed when you cat/proc/dma.

  8. Driver Chooses DMA • The device driver to determine which DMA channel it should use. • some devices have a fixed DMA channel • Sometimes the DMA channel by jumpers • others the device driver can simply pick a free DMA channel • Adds more complication to the driver creation process

  9. Device Driver • The Kernel code that communicates with the device • Ideally the code should be relatively generic for the kernel to know how to handle each device. • Linux handles this by breaking all device drivers into three categories

  10. Terrific Trio Definitions • Character Devices • Read/Write a single byte at a time • Keyboards • Serial Devices • Handle their own buffering • Block Devices • Transfer data blocks @ a time • Data Storage Devices (Hard Drives) • Block sizes usually range 512 to 1024 bytes • Rely on the Kernel to their buffering • Network Devices • Transfers data @ a packet level

  11. THE BENEFIT OF ABSTRACTION • By creating this separation…. • The kernel can ignore the details of a specific device. • Allows the kernel to create generic functions for interaction.

  12. Where to park the drivers... • Compiled into the kernel • Added in as a module….. • So Dynamic…How will Linux handle a varying system???

  13. Desperately Seeking Devices • Hardware is searched for and Registered at boot-up • Devices are “found” by their descriptions are added to a number of System Tables. • character devices are added to the chrdevs vector • block devices are added to blkdevs vector • Network devices are handled through the network systems

  14. Re-cap • It has been established the main controls and interface with the CPU is interrupt and DMA driven • Device driver contains the routines that interrupts use to work with the device. • How are the devices represented, so that the drivers can communicate with them???

  15. It’s all about the files, baby. • All devices with in the system are represented by a Device Special File • Character and Block device “Device Special Files” are generated using the mknod (make node) command. • Network device “Device Special Files” are created during the intialization of the network subsystem. • Technically these files can be found anywhere in the system. • By convention they are found in the /dev directory of the system.

  16. File Description • Linux’s “Device Special File” convention allows devices to be described as any other file on Linux: • DEVICE CLASSIFICATION: • “C” for Character device, “B” for block device. • DEVICE TYPE: • TTY describes is a Teletype type. • MAJOR NUMBER: • Uniquely assigned per device driver in the system • MINOR NUMBER: • Uniquely assigned per partition • DEVICE ID: • Unique name of the “Device Special File”

  17. Benefits of The File Convention • Can take advantage of Linux Permission structures • Can take advantage of Linux Permission structures • CD-ROM is treated as a read-only device • I/O devices can be treated as read/write devices

  18. Finally……. Conclusion • Presentation a very high level view how devices interact with Linux • Started with tools the kernel and architecture have to deal with devices. • Described the means of connecting to the device…The Device Driver. • Covered the how Device can be interfaced, (i.e. what conventions are used by the OS to connect to a device. • Represents the power of keeping things general • The kernel separation of the devices into the 3 categories, provided the flexibily of more devices • File convention allows for an “easier” for programs to deal with the devices. • The extensibility provided by these features is one of the main features to why Linux/Unix has lasted so long

  19. For more information... • Http://www.informit.com ((Free library on a variety of topics including Linux and other OS’s)) Linux SA Survival Guide and Red Hat Linux Unleashed. • http://sunsite.unc.edu/LDP/ldp.html ((The Linux Documentation Project)) David A Rusling’s The Linux Kernel Thank You

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