Boring environment setup is done. Let us start coding mybrd driver. First we start with skeleton code to only help us to check whether the driver is loaded correctly. And this chapter will describe how to add our driver code into kernel so that the driver is built and run with kernel. We will start reading kernel source now. Please keep "Linux device driver" and "Understanding the Linux kernel" books close to you.
We want to make our block driver be part of Linux kernel. So we will add our driver source, mybrd.c file, into Linux kernel directory. Driver sources have to be added in drivers directory. You can find many sources and sub-directory with 'ls drivers' command. Our driver is for block device. We will place mybrd.c into drivers/block/ directory.
$ ls drivers/block/
amiflop.c cpqarray.h loop.o paride swim_asm.S
aoe cryptoloop.c Makefile pktcdvd.c swim.c
ataflop.c DAC960.c mg_disk.c ps3disk.c sx8.c
brd.c DAC960.h modules.builtin ps3vram.c umem.c
brd.o drbd modules.order rbd.c umem.h
built-in.o floppy.c mtip32xx rbd_types.h virtio_blk.c
cciss.c hd.c mybrd.c rsxx virtio_blk.o
cciss_cmd.h ida_cmd.h mybrd.o skd_main.c xen-blkback
cciss.h ida_ioctl.h nbd.c skd_s1120.h xen-blkfront.c
cciss_scsi.c Kconfig null_blk.c smart1,2.h xsysace.c
cciss_scsi.h loop.c null_blk.o sunvdc.c z2ram.c
cpqarray.c loop.h osdblk.c swim3.c zram
First let's make an just empty file, mybrd.c. And let's change Kconfig and Makefile like following to build mybrd.c with kernel image.
$ git diff drivers/block/Kconfig
diff --git a/drivers/block/Kconfig b/drivers/block/Kconfig
index 29819e7..6744d48 100644
--- a/drivers/block/Kconfig
+++ b/drivers/block/Kconfig
@@ -365,6 +365,9 @@ config BLK_DEV_RAM
Most normal users won't need the RAM disk functionality, and can
thus say N here.
+config BLK_DEV_MYRAM
+ tristate "myRAM block device support"
+
config BLK_DEV_RAM_COUNT
int "Default number of RAM disks"
default "16"
$ git diff drivers/block/Makefile
diff --git a/drivers/block/Makefile b/drivers/block/Makefile
index 6713290..ec939f2 100644
--- a/drivers/block/Makefile
+++ b/drivers/block/Makefile
@@ -14,6 +14,9 @@ obj-$(CONFIG_PS3_VRAM) += ps3vram.o
obj-$(CONFIG_ATARI_FLOPPY) += ataflop.o
obj-$(CONFIG_AMIGA_Z2RAM) += z2ram.o
obj-$(CONFIG_BLK_DEV_RAM) += brd.o
+obj-$(CONFIG_BLK_DEV_MYRAM) += mybrd.o
+
+
obj-$(CONFIG_BLK_DEV_LOOP) += loop.o
obj-$(CONFIG_BLK_CPQ_DA) += cpqarray.o
obj-$(CONFIG_BLK_CPQ_CISS_DA) += cciss.o
Kconfig file makes menu when you run 'make menuconfig' to set up kernel options. After fixing like above, run 'make menuconfig'. You can find the new option, "myRAM block device support" in "Device drivers ---> Block devices menu". You can activate the option with pressing spacebar key. There is one thing to notice. You must activate the option with '', not 'M'. 'M' means that the driver is not included in kernel image and saved in separated .ko file. '' is for merging driver into kernel image. We will boot kernel image on busybox filesystem. So if you want to seperate driver, you must copy driver file into the busybox filesystem and build filesystem image. You don't want to build filesystem image whenever you change the driver source. So we merge the driver into kernel image to simplify driver test.
.config - Linux/x86 4.4.0 Kernel Configuration
> Device Drivers > Block devices > ─────────────────────────────────────────────
┌───────────────────────────── Block devices ─────────────────────────────┐
│ Arrow keys navigate the menu. <Enter> selects submenus ---> (or empty │
│ submenus ----). Highlighted letters are hotkeys. Pressing <Y> │
│ includes, <N> excludes, <M> modularizes features. Press <Esc><Esc> to │
│ exit, <?> for Help, </> for Search. Legend: [*] built-in [ ] │
│ ┌─────────────────────────────────────────────────────────────────────┐ │
│ │ --- Block devices │ │
│ │ <*> Null test block driver │ │
│ │ < > Normal floppy disk support │ │
│ │ <*> Loopback device support │ │
│ │ (8) Number of loop devices to pre-create at init time │ │
│ │ < > Cryptoloop Support │ │
│ │ *** DRBD disabled because PROC_FS or INET not selected *** │ │
│ │ <*> RAM block device support │ │
│ │ <*> myRAM block device support │ │
│ │ (2) Default number of RAM disks │ │
│ │ (4096) Default RAM disk size (kbytes) │ │
│ │ < > Packet writing on CD/DVD media │ │
│ │ [ ] Very old hard disk (MFM/RLL/IDE) driver
If you select 'Helo' menu, you can see help message of the driver like following.
.config - Linux/x86 4.4.0 Kernel Configuration
> Device Drivers > Block devices ─────────────────────────────────────────────
┌────────────────────── myRAM block device support ───────────────────────┐
│ There is no help available for this option. │
│ Symbol: BLK_DEV_MYRAM [=y] │
│ Type : tristate │
│ Prompt: myRAM block device support │
│ Location: │
│ -> Device Drivers │
│ -> Block devices (BLK_DEV [=y]) │
│ Defined at drivers/block/Kconfig:368 │
│ Depends on: BLK_DEV [=y]
If you select the option for mybrd driver, CONFIG_BLK_DEV_MYRAM value become y. Then make tool checks if obj-$(CONFIG_BLK_DEV_MYRAM) is obj-y and build mybrd.o file.
Yes, adding a new driver file is so easy. Adding menu and source file is all we should do. Every option we select is stored int .config file. You can confirm that CONFIG_BLK_DEV_MYBRD is stored in .config.
$ grep MYRAM .config
CONFIG_BLK_DEV_MYRAM=y
Now adding driver source is done. Now let's try building.
$ make bzImage -j4
CHK include/config/kernel.release
CHK include/generated/uapi/linux/version.h
CHK include/generated/utsrelease.h
CHK include/generated/timeconst.h
CHK include/generated/bounds.h
CHK include/generated/asm-offsets.h
CALL scripts/checksyscalls.sh
CHK include/generated/compile.h
CC drivers/block/mybrd.o
LD drivers/block/built-in.o
LD drivers/built-in.o
LINK vmlinux
LD vmlinux.o
MODPOST vmlinux.o
GEN .version
CHK include/generated/compile.h
UPD include/generated/compile.h
CC init/version.o
LD init/built-in.o
KSYM .tmp_kallsyms1.o
KSYM .tmp_kallsyms2.o
LD vmlinux
SORTEX vmlinux
SYSMAP System.map
VOFFSET arch/x86/boot/voffset.h
CC arch/x86/boot/version.o
OBJCOPY arch/x86/boot/compressed/vmlinux.bin
GZIP arch/x86/boot/compressed/vmlinux.bin.gz
MKPIGGY arch/x86/boot/compressed/piggy.S
AS arch/x86/boot/compressed/piggy.o
LD arch/x86/boot/compressed/vmlinux
ZOFFSET arch/x86/boot/zoffset.h
OBJCOPY arch/x86/boot/vmlinux.bin
AS arch/x86/boot/header.o
LD arch/x86/boot/setup.elf
OBJCOPY arch/x86/boot/setup.bin
BUILD arch/x86/boot/bzImage
Setup is 15708 bytes (padded to 15872 bytes).
System is 2555 kB
CRC cac8eadd
Kernel: arch/x86/boot/bzImage is ready (#20)
Building drivers/block/mybrd.o is completed. And building bzImage is also completed..
I saved source file in github. You can see full source with following link.
https://github.com/gurugio/mybrd/blob/ch01-skel/mybrd.c
I created branches for each chapter. A branch for this chapter is ch01-skel. If you checkout ch01-skel branch, you can check that mybrd.c has minimal code as desribed in this chapter.
mybrd.c includes 5 header files. "linux/init.h", "linux/module.h" and "linux/moduleparam.h" are common for all drivers. You need to know about compiler, linker and loader to understand those files. So I don't describe what they do in detail.
major.h and blkdev.h are, as shown by names, related to device number and block device. If you want to make a device driver and the device is block device, you must include those headers.
mybrd_init() fundtion is the entry pointe of the driver. It's called when the drier is loaded and starts to execute. Every driver should have starting point that is registered by module_init() function. In mybrd.c file, you can see module_init() is called with mybrd_init function pointer at the end of the file.
There are two ways to load a driver. First is loading a driver dynamically after kernel booting is finished. The loader finds a function that is registered by module_init() and calls it. Second is loading a driver when kernel boots. After kernel finishes booting itself, kernel calls all functions that is registered by module_init(). You can select how the driver is loaded when you selects kernel option.
The module_init() function selects entry points of each driver into a specific area. That area is just like an array of function pointers.
The module_exit() is for registering the exit point of the driver.
Every devices in Unix/Linux system exists as files in /dev directory. Everything is file in Unix/Linux system. So we should introduce our driver to kernel and request to create a file in /dev directory. That is what register_blkdev() function does.
Now let's boot kernel. Our driver is loaded automatically because it is merged into kernel. mybrd_init() function prints some message, so we should be able to find following message in kernel messages as following.
/sys # dmesg | grep mybrd
[ 0.323640] mybrd: mybrd major=253
[ 0.323868] mybrd:
[ 0.323868] mybrd: module loaded
We can confirm the driver get a major number from kernel. And we can check /proc/devices if device is registered correctly. The /proc/devices file shows the major number of each devices that are registered to kernel. Our driver registers the name of device as my-ramdisk via register_blkdev() function. So we can find my-ramdisk in the /proc/device file as following.
# grep my-ramdisk /proc/devices
253 my-ramdisk
But you should note that there is no device file in /dev directory. So far, we registered device and get the number from kernel but did not create device file. We will do that in the next chapter.