Embedded systems 2/7 J.-M Friedt Introduction Virtual memory access Embedded systems 2/7 Kernel module basics Using the kernel: timers J.-M Friedt Conclusion & lab session FEMTO-ST/d´epartement temps-fr´equence [email protected] slides at jmfriedt.free.fr September 9, 2020 1 / 24 Embedded systems 2/7 J.-M Friedt Operating system: the need for Introduction drivers Virtual memory access Kernel module basics Using the kernel: timers • Hardware abstraction: hide low level functions so that the developer Conclusion & lab session can focus on the functionalities provided by the peripheral ! a single entry point providing system calls (open, read, write, close) hiding access to hardware • Homogeneous interface to all peripherals (\Everything is a file") • Only the kernel can access hardware resources (DMA, interrupts) • Share resources and make sure only one process can access a given hardware function • Add functionalities to the Linux kernel: modules 2 / 24 Embedded systems 2/7 J.-M Friedt Virtual memory/hardware memory Introduction Hardware memory addressing Virtual memory • hardware memory: a value on the address bus identifies which access peripheral is active Kernel module basics • each peripheral decodes the address bus to detect whether it is the Using the kernel: target of a message timers • Conclusion & lab only one peripheral must match a given physical address (otherwise, session conflict) Virtual memory addressing • each process has its own address space • memory organization independent of physical constraints • dynamic loading binaries and associated libraries • MMU: translates between hardware and virtual memory addresses Virtual organization (process) Physical organization (CPU) 4096 Page 1 4096 Page 1 Page 2 Page 2 page & 0xff..f000 page & 0xff..f000 Page 3 Page 3 page & 0x00..0fff page & 0x00..0fff Page N Page N 3 / 24 Embedded systems 2/7 J.-M Friedt Hardware access from userspace Introduction Through /dev/mem Virtual memory access • advantage: not going through kernel abstraction layers (fast) Kernel module • drawback: not going through kernel abstraction layers (no handling basics Using the kernel: of concurrent memory access) timers #include < f c n t l . h> Conclusion & lab #include <s y s /mman . h> session #define MAP SIZE 4096UL // MMU page size #define MAP MASK ( MAP SIZE−1) // mask i n t main(int argc, char ∗∗ a r g v ) f i n t fd; void ∗ map base , ∗ v i r t a d d r ; u n s i g n e d long read result , writeval; o f f t target=0x12345678; // physical@ f d = open ( "/dev/mem" ,O RDWR j O SYNC) ; // MMU access map base=mmap(0 , MAP SIZE , PROT READ j PROT WRITE , n MAP SHARED, fd , t a r g e t & ~MAP MASK) ; v i r t a d d r=map base+(target & MAP MASK) ; // virt.@ r e a d r e s u l t =∗((unsigned long ∗) v i r t a d d r ) ; // read mem p r i n t f ( "0x%X(%p):0x%X\n" ,target ,virt a d d r , r e a d r e s u l t ) ; // ∗ ((unsigned long ∗) virt a d d r)= writeval;// write munmap( map base , MAP SIZE); close(fd); return 0; g 4 / 24 Embedded systems 2/7 J.-M Friedt The devmem tool Introduction Virtual memory access Kernel module basics Using the kernel: timers • Fast prototyping when accessing processor registers Conclusion & lab 1 session • Available in busybox • Use: Read/write from physical address ADDRESS Address to act upon WIDTH Width (8/16/...) VALUE Data to be written 1$BUILDROOT/output/build/busybox-1.30.1/miscutils/devmem.c 5 / 24 Embedded systems 2/7 J.-M Friedt Hardware access from the kernel Introduction Virtual memory • access Userspace: mmap on the /dev/mem pseudo-file Kernel module • Kernel space: ioremap function after requesting the address range basics Using the kernel: used by the peripheral timers #include <l i n u x / i o . h> // ioremap Conclusion & lab #define IO BASE 0xe000a000 // UG585p.1347 session and in the initialization function i f (request m e m r e g i o n ( IO BASE,0x2e4 , "GPIO test" )==NULL) printk (KERN ALERT "mem request failed" ); j m f gpio=(u32)ioremap(IO BASE, 0x2e4); // UG585p.1349 w r i t e l (1<<9, j m f gpio+0x204) ; // dir. of MIO9 To free the resource: r e l e a s e m e m r e g i o n ( IO BASE, 0x2e4); Check if the requested memory range is available (otherwise, kernel panic) 6 / 24 Embedded systems 2/7 J.-M Friedt Basic kernel module structure Introduction Kernel module = \plugin" requiring strarting and ending functions Virtual memory access #include <linux/module.h> /∗ Needed by all modules ∗/ Kernel module #include <linux/kernel.h> /∗ Needed for KERN INFO ∗/ basics #include <linux/init .h> /∗ Needed for the macros ∗/ Using the kernel: timers i n t hello s t a r t (void); Conclusion & lab session v o i d hello e n d (void); i n t hello s t a r t ( ) // init m o d u l e(void) f printk (KERN INFO "Hello\n" ); r e t u r n 0; g v o i d hello e n d ( ) // cleanup m o d u l e(void) f printk (KERN INFO "Goodbye\n" ); g m o d u l e i n i t ( h e l l o s t a r t ) ; m o d u l e e x i t ( h e l l o e n d ) ; No printf (write to /var/log/messages with printk accessible with dmesg), no floating point operation, no file operations. 7 / 24 Embedded systems 2/7 J.-M Friedt Kernel module compilation Introduction Virtual memory A kernel module must link to the running kernel: access • Kernel module requires kernel sources ($BR/output/build/linux*) or at least basics configured headers (linux-headers-X.Y.Z-amd64 on Using the kernel: timers Debian/GNU Linux) Conclusion & lab • dedicated Makefile calling the Linux kernel Makefile modules session method on PC: obj−m += mymod . o a l l : make −C /lib/modules/$ (shell uname −r ) / b u i l d n M=$ (PWD) modules or for cross-compiling, linking to the Buildroot $BR obj−m +=mymod . o a l l : make ARCH=arm CROSS COMPILE=arm−l i n u x − n −C $ (BR)/output/build/linux −XXX n M=$ (PWD) modules 8 / 24 Embedded systems 2/7 J.-M Friedt Communicating from userspace Introduction with the kernel Virtual memory access Kernel module Through /dev basics Using the kernel: • write, read, or control (ioctl) timers Conclusion & lab • each kernel module implements its own answer to the system calls session • each peripheral is identified by its class (major number) and its index (minor number) brw-rw---- 1 root disk 8, 0 Feb 28 06:21 sda brw-rw---- 1 root disk 8, 1 Feb 28 06:21 sda1 brw-rw---- 1 root disk 8, 2 Feb 28 06:21 sda2 brw-rw---- 1 root disk 8, 3 Feb 28 06:21 sda3 [...] crw-rw---- 1 root dialout 4, 64 Feb 28 07:21 ttyS0 crw-rw---- 1 root dialout 4, 65 Feb 28 07:21 ttyS1 crw-rw---- 1 root dialout 4, 66 Feb 28 07:21 ttyS2 crw-rw---- 1 root dialout 4, 67 Feb 28 07:21 ttyS3 In case an entry is missing (e.g. created by udev) in /dev : mknod 9 / 24 Embedded systems 2/7 2 J.-M Friedt Architecture of a POSIX Introduction compliant OS Virtual memory access Kernel module basics Using the kernel: Userspace timers Conclusion & lab applications (libc) session /dev/ttyUSB /dev/partport0 Module 1 Module 2 Module 3 (USB) (GPIO) (fs) Monolithic kernel (scheduler, interrupts, timers ...) system fonctions, networking, process scheduling and memory sharing, vfs 2/dev directory: pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap10.html 10 / 24 Embedded systems 2/7 J.-M Friedt The /dev directory Introduction • includes pseudo-files linking the kernel with the user space Virtual memory access • each device is defined with a major number and, within each class, Kernel module basics a minor number Using the kernel: • timers on the kernel side, a driver links to the same major number and Conclusion & lab provides implementations of the system calls session s t a t i c struct file operations fops= • f . r e a d : qadc read , open() . open : qadc open , • close() . u n l o c k e d i o c t l : q a d c i o c t l , • write() .release: qadc r e l e a s e , • read() // no.write on an ADC! • ioctl() g ; s t a t i c int i n i t q a d c i n i t (void) f ... r e g i s t e r chrdev (qadc major , "ppp" ! ,! , &f o p s ) ; ... g m o d u l e i n i t ( q a d c i n i t ) ; m o d u l e e x i t ( q a d c e x i t ) ; 11 / 24 Embedded systems 2/7 J.-M Friedt Input/Output Control (ioctl) Introduction • mechanism breaking the \Everything is a file” philosophy Virtual memory • access configures a peripheral by providing \what" index and as an Kernel module argument the \value" basics • no standardized command: the header file common to the driver Using the kernel: timers and the userspace program provides the list of IOCTL calls 3 Conclusion & lab • example: OSS (Open Sound System ) in session include/uapi/linux/soundcard.h of the kernel source: /∗ Use ioctl(fd, OSS GETVERSION,&int) to get the v e r s i o n number of the currently active driver.