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Studies on User Level Device Driver in Embedded Environment （ Progress Report ）

Studies on User Level Device Driver in Embedded Environment （ Progress Report ）. Katsuya Matsubara IGEL Co.,Ltd. Stories so far. Investigated behaviors of threads RT Task vs Non-RT Task. Switching Time(WRITE Task->Vertual DD->Read Task). Wakeup Vertual DD->Read Task).

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Studies on User Level Device Driver in Embedded Environment （ Progress Report ）

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  1. Studies on User Level Device Driverin Embedded Environment（Progress Report） Katsuya Matsubara IGEL Co.,Ltd. IGEL Co.,Ltd. / Renesas Solution Corp.

  2. Stories so far • Investigated behaviors of threads • RT Task vs Non-RT Task Switching Time(WRITE Task->Vertual DD->Read Task) Wakeup Vertual DD->Read Task) Response Time(microsec) Response Time(microsec) Number of Load Tasks Number of Load Tasks IGEL Co.,Ltd. / Renesas Solution Corp.

  3. Background and Objectives • Want to implement device drivers in user level • Easy to devleop • Reduce system down due to driver bugs • （Avoid GPL） • Some issues on implementing user level device drivers • Access to I/O memory, physical memory • Forwarding of Interupttion Requests（IRQ） • Response performance to interuption • ‥‥ • New features in Kernel 2.6 • NPTL(Native POSIX Thread Library) • Sceduler improvement（O(1) scheduler etc.） • Kernel preemption • …. IGEL Co.,Ltd. / Renesas Solution Corp.

  4. Facts in Development of Embedded Software • More development of drivers for new devices • Applications directly control devices • To minimize latency • More intimate relation between devices and applications, thant desktop PC applications • Hiding hardware specification IGEL Co.,Ltd. / Renesas Solution Corp.

  5. User Level Device Drivers Implementing Device Drivers in User Space • Expected effects • Use of abundant tools/libraries (e.g. files) • Ease of development and debugging • Less system hang ups • Less distance to applications • …. • Related research • Peter Chubb, “Get more device drivers out of kernel,” OLS2004. IGEL Co.,Ltd. / Renesas Solution Corp.

  6. Features Requiredfor Device Drivers • Access to I/O Memory • Device control through register I/O, data I/O • Interrupt Receipt and Handling • Physical Memory Allocation and Access • Necessary for devices which access directly to memory using DMA, etc. • In case of DMA, allocation of consecutive memory may be required • Lock of CPU，Realtime • Interface to Applications IGEL Co.,Ltd. / Renesas Solution Corp.

  7. Interrupt Receipt and Handlingin User Level Device Drivers • Interrupt Reciept by File I/O • Prepare device files for each interrupt number • read() the device file and block until interrupt occurs IGEL Co.,Ltd. / Renesas Solution Corp.

  8. Memory Accessin User Level Device Drivers • Direct Access to I/O Memory with Memory Map(mmap) • open(), mmap() the /dev/mem or specific device file • Once mmap’ed, then accessible as usual • For Physical Memory, Allocate, then mmap IGEL Co.,Ltd. / Renesas Solution Corp.

  9. Interface to Applicationsin User Level Device Drivers • As User Level Device Driver can Exist in the Same User Space as Applications, Such Interfaces as Interprocess Communications, Shared Memory, etc. can Implementable • Easy to Integrate with Applications IGEL Co.,Ltd. / Renesas Solution Corp.

  10. CPU Lock, Realtimein User Level Device Drivers • With RT Threads, to Some Extent… • To be discussed IGEL Co.,Ltd. / Renesas Solution Corp.

  11. Implementation（Serial Driver） • Implementation Environment • RTS7751R2D • Renesas SH4 • SM501 companion chip embedding UART • 8250 compatible • 2 modes: 1 byte I/O and FIFO buffer I/O • Linux 2.6.13.4 • CONFIG_PREEMPT=y • Glibc 2.3.3 IGEL Co.,Ltd. / Renesas Solution Corp.

  12. Processing • Interrupt Handling • Receiving Data • Emptying Send Buffer[?] • I/O Memory Access • SM501 Register I/O IGEL Co.,Ltd. / Renesas Solution Corp.

  13. I/O Memory Map Driver Interrupt Hook Driver SM501 UART Software Architecture Buffer Data I/O User Level UART D.Driver Terminal App Linux Kernel IGEL Co.,Ltd. / Renesas Solution Corp.

  14. Interrupt Hook Driver static int irqhook_proc_open(struct inode *inop, struct file *filp) { ... request_irq(ipp->irq, irqhook_proc_irq_handler, SA_INTERRUPT, ipp->devname, ipp); ... } static ssize_t irqhook_proc_read(struct file *filp, char __user *bufp, size_t len, loff_t *ppos) { .... prepare_to_wait(&ipp->q, &wait, TASK_INTERRUPTIBLE); pending = atomic_read(&ipp->count); if (pending == 0) schedule(); ..... } IGEL Co.,Ltd. / Renesas Solution Corp.

  15. I/O Memory Map Driver int iommap_mmap(struct file *filp, struct vm_area_struct *vma) { size_t size = vma->vm_end - vma->vm_start; unsigned long offset = vma->vm_pgoff << PAGE_SHIFT; vma->vm_flags |= VM_RESERVED; vma->vm_flags |= VM_IO; vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot); /* Map each page to users' virtual memory space */ if (io_remap_page_range(vma, vma->vm_start, offset, size, vma->vm_page_prot)) return -EAGAIN; return 0; } IGEL Co.,Ltd. / Renesas Solution Corp.

  16. UART Driver while (1) { .... if (read(fd, &n_pending, sizeof(int) > 0) { if ((status = SM501_UART0_LINESTAT(&iomem)) & 0x01) { do { buffer[count]= SM501_UART0_RXBUFF(&iomem); status = SM501_UART0_LINESTAT(&iomem); count++; if(count >= BUFFER_SIZE){ loop++; count = 0; if(loop >= LOOP_COUNT) goto end; } while((status & 0x01)&&(max_count-- > 0)); } .... } IGEL Co.,Ltd. / Renesas Solution Corp.

  17. Experiment Just started experimenting… • Serial I/O • Measured throughput with 1MB READ/WRITE for serial device • Used SM501 byte I/O mode • Kernel drivers are those included in 2.6.13.4 • User level device driver was executed as RT task • User level device driver was integrated with serial I/O process • Connected R2D board and note PC(TeraTerm) with serial cross cable IGEL Co.,Ltd. / Renesas Solution Corp.

  18. Experiment Results • Data Size：1MB • Buffer Size： 1KB • Baud Rate： 300 or 38400 bps • Mesured values：Average of the 10 trial results of the same experiment IGEL Co.,Ltd. / Renesas Solution Corp.

  19. Impressions • Negligible Overhead At least in Low Load Environment (where no one intervenes)? • ULDD is rather faster for 38.4kbps • Deserves analysis – My action item • System Stability, Ease of Coding and Debugging, and Abundance of User Land Features are Attractive IGEL Co.,Ltd. / Renesas Solution Corp.

  20. Issues • Behavior in Heavy Load Environment • RT tasks / Non-RT tasks • Comparison with PC Environment • Evaluation with Other Devices • Summary of Characteristics of Kernel Level Device Drivers and User Level Device Drivers • Seeking for Novelty… IGEL Co.,Ltd. / Renesas Solution Corp.

  21. Discussion IGEL Co.,Ltd. / Renesas Solution Corp.

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