A Quick debugfs Example
This posts shows how to create, build and run a quick debugfs tree on QEMU.
Prerequisites
If you want to build and run the code go through Build the Linux kernel and Busybox and run them on QEMU
The Code
#include <linux/module.h>
#include <linux/debugfs.h>
#include <linux/kernel.h>
#define DEVS 30
#define REGS 32
u16 regs[DEVS][REGS];
static int find_addr_and_reg(void *data, int *addr, int *reg)
{
int i, j;
for (i = 0; i < DEVS; i++) {
for (j = 0; j < REGS; j++) {
if ((u16 *) data == ®s[i][j]) {
*addr = i;
*reg = j;
return 0;
}
}
}
/* something went wrong, should never get here */
return -1;
}
static int debugfs_u16_set(void *data, u64 val)
{
int ret, addr, reg;
ret = find_addr_and_reg((u16 *)data, &addr, ®);
if (ret == 0)
printk(KERN_INFO "addr = %d, reg = %d\n", addr, reg);
else
printk(KERN_INFO "0x%p not found\n", (u16 *)data);
*(u16 *)data = val;
return 0;
}
static int debugfs_u16_get(void *data, u64 *val)
{
int ret, addr, reg;
ret = find_addr_and_reg((u16 *)data, &addr, ®);
if (ret == 0)
printk(KERN_INFO "addr = %d, reg = %d\n", addr, reg);
else
printk(KERN_INFO "0x%p not found\n", (u16 *)data);
*val = *(u16 *)data;
return 0;
}
DEFINE_SIMPLE_ATTRIBUTE(fops_u16, debugfs_u16_get, debugfs_u16_set, "%llu\n");
int init(void)
{
struct dentry *d, *s, *n;
char dev_addr[8];
char reg_addr[8];
u8 i, j;
d = debugfs_create_dir("ds", NULL);
if (!d)
return -ENOMEM;
for (i = 0; i < DEVS; i++) {
snprintf(dev_addr, sizeof(dev_addr), "%i", i);
s = debugfs_create_dir(dev_addr, d);
if (!s)
return -ENOMEM;
for (j = 0; j < REGS; j++) {
snprintf(reg_addr, sizeof(reg_addr), "%i", j);
n = debugfs_create_file(reg_addr, 0644, s, ®s[i][j],
&fops_u16);
if (!n)
return -ENOMEM;
}
}
return 0;
}
module_init(init);
Build It
1. Set up a few variables
STAGE=$HOME/tl
TOP=$STAGE/teeny-linux
K=$STAGE/linux-4.10.6
2. Save the code to a file called
$K/drivers/dump-regs.c
3. Edit this file
vi $K/drivers/Makefle
4. Add this line at the top
obj-y += dump-regs.o
5. Build the kernel
cd $K
make O=$TOP/obj/linux-x86-allnoconfig
If you see something like:
LD arch/x86/boot/setup.elf
OBJCOPY arch/x86/boot/setup.bin
OBJCOPY arch/x86/boot/vmlinux.bin
BUILD arch/x86/boot/bzImage
Setup is 15772 bytes (padded to 15872 bytes).
System is 929 kB
CRC 6f8420cf
Kernel: arch/x86/boot/bzImage is ready (#29)
make[1]: Leaving directory '/home/pfefferz/tl/teeny-linux/obj/linux-x86-allnoconfig'
Things are likely working.
Run It
1. Open a new terminal
STAGE=$HOME/tl
TOP=$STAGE/teeny-linux
qemu-system-x86_64 \
-kernel $TOP/obj/linux-x86-allnoconfig/arch/x86/boot/bzImage \
-initrd $TOP/obj/initramfs.igz \
-nographic -append "earlyprintk=serial,ttyS0 console=ttyS0"
2. Run
If you see something like (you may need to hit Enter to get the prompt):
Freeing unused kernel memory: 452K
Write protecting the kernel read-only data: 4096k
Freeing unused kernel memory: 776K
Freeing unused kernel memory: 1764K
Boot took 0.80 seconds
/bin/sh: can't access tty; job control turned off
/ # input: ImExPS/2 Generic Explorer Mouse as /devices/platform/i8042/serio1/input/input2
random: fast init done
/ #
Things are likely working.
Test It
1. On the terminal in QEMU run
for i in `seq 0 29`;
do
for j in `seq 0 31`;
do
echo 0x1234 > /sys/kernel/debug/ds/$i/$j
done
done
for i in `seq 0 29`;
do
for j in `seq 0 31`;
do
cat /sys/kernel/debug/ds/$i/$j
done
done
If you see lots of output like:
...
ddr = 29, reg = 22
4660
addr = 29, reg = 23
4660
addr = 29, reg = 24
4660
addr = 29, reg = 25
4660
addr = 29, reg = 26
4660
addr = 29, reg = 27
4660
addr = 29, reg = 28
4660
addr = 29, reg = 29
4660
addr = 29, reg = 30
4660
addr = 29, reg = 31
4660
Things are likely working.
Reference