The LEDs are hooked up using i2c to a driver chip – a is31fl319x which has a corresponding userspace driver leds_is31fl319x
The is31fl319x is located on i2c bus 1 at 0x64, and on ArchLinux, the driver module leds_is31fl319x loads at boot. If you run i2cdetect -y 1 you see that 0x64 has the value UU, indicating that the driver is controlling that address. lsmod confirms this.
If you want to control the LEDs you can write code to talk to the driver.
Alternately you can remove the driver (rmmod leds_is31fl319x), which will allow access to 0x64, and you can use i2c (i2ctransfer) to send instructions to the is31fl319x directly.
For example:
i2ctransfer -y 1 w76@0x64 0x00 0x01 0x01 0x77 0x21 0x07 0x03 0x00 0x04 0x00 0x07 0xff 0x08 0xff 0x09 0xff 0x0a 0xff 0x0b 0xff 0x0c 0xff 0x0d 0xff 0x0e 0xff 0x0f 0xff 0x10 0x00 0x11 0x00 0x12 0x01 0x13 0x02 0x14 0x05 0x15 0x04 0x16 0x03 0x17 0x06 0x18 0x07 0x19 0x09 0x1a 0xa0 0x1b 0xa0 0x1c 0xa0 0x1d 0x07 0x1e 0x07 0x1f 0x07 0x20 0x07 0x21 0x07 0x22 0x07 0x23 0x07 0x24 0x07 0x25 0x07 0x26 0x00 0x03 0x70
