CM-X270: Linux: Kernel

Overview

Linux kernel for CM-X270 modules provides support for on-board peripherals. Current support coverage is specified at O/S support coverage map page. CompuLab provides ready-to-run binary kernel images, and source code of the modifications and additions made to the Linux kernel to work properly with CM-X270 modules.

Building Linux kernel for CM-X270

Instructions for building Linux kernel for CM-X270 can be found here.

CM-X270 specific additions and changes to Linux kernel 2.6.24

The CM-X270 platform support was merged into the mainline kernel starting from version 2.6.24. There are still some features that are not present in the mainline kernel:

• Fixes to suspend/resume functionality
• Change in serial port mappings to ensure consistency between CM-X270L and CM-X270W
• Support for UART device integrated into PCI bridge
• Support for Wi2Wi W2SW0001 wireless LAN (based on Marvell 8686 chipset)

The Linux kernel 2.6.24 for CM-X270 by default uses MTD drivers to access the on-board NAND flash. The CompuLab proprietary NAND flash driver is included in the Linux kernel package for CM-X270 but is not compiled into the binary image. If you would like to use it you should disable the MTD driver and enable the CompuLab NAND flash driver in the kernel configuration.

CM-X270 specific additions and changes to Linux kernel 2.6.16

• Basic platform support
• PCI support with ITE8152 companion chip
• Touchscreen support
• NAND flash support

Using mainline Linux kernel on CM-X270

It is possible to use mainline Linux kernel starting from version 2.6.24 on CM-X270 "as is". Just keep in mind that features and fixes listed in CM-X270 specific additions and changes to Linux kernel 2.6.24 section are not present in the mainline kernel and the serial ports are mapped to different ttys on L and W variants as summarized in the below table:

Serial ports mapping on CM-X270L

UART	SB-X270 port name	Linux tty in CompuLab kernel release	Linux tty in mainline kernel
BTUART	COM-A	ttySA0	ttyS1
FFUART	COM-B	ttySA1	ttyS0
STUART	COM-D	ttySA2	ttyS2
ITE8152 UART	COM-B	ttyS0	not present

Serial ports mapping on CM-X270W

UART	SB-X270 port name	Linux tty in CompuLab kernel release	Linux tty in mainline kernel
FFUART	COM-A	ttySA0	ttyS0
BTUART	COM-D	ttySA1	ttyS1
STUART	COM-C	ttySA2	ttyS2
ITE8152 UART	COM-B	ttyS0	not present

Device support details

Basic Platform Support

The CM-X270 platform basic support is implemented in the linux/arch/arm/mach-pxa/cm-x270.c. This file includes registration of the devices integrated in the PXA270 SoC, definitions of platform-specific configuration of these devices, platfrom-specific power management code and static mapping for memory mapped IO. This file also includes several routines needed for proper operation of the PCI bridge.

PCI support

The board support for CM-X270 introduces driver for ITE 8152 companion chip and platform specific drivers for proper initialization of ITE 8152 on the CM-X270. The bridge driver implementation resides in linux/arch/arm/mach-pxa/it8152.[ch] in Linux kernel 2.6.16 and in linux/arch/arm/common/it8152.[ch] in Linux kernel 2.6.24,; parts of the CM-X270 specific initialization code are in linux/arch/arm/mach-pxa/cm-x270-pci.c. There are also changes in linux/arch/arm/kernel/bios32.c that are needed to use the ITE 8152 PCI bridge.

PCI Ethernet Cards

The CompuLab stock kernel does not include built-in support for PCI Ethernet cards. A driver for popular RealTek 8139 card is available as a loadable kernel module in Debian Linux for CM-X270 package. The module is called 8139too and can be enabled using

modprobe 8139too

Sound and Touchsreen

CM-X270 uses UCB1400 AC97 codec with integrated touchscreen controller.

The touchscreen controller driver for UCB1400 for Linux 2.6.16 is taken from patch 3075/2 in the ARM Linux patch system and slightly modified to suit CM-X270 hardware configuration.

Both audio and touchscreen drivers are modularized if you use default CM-X270 configuration. The following commands enable audio support and touchscreen support respectively:

modprobe snd-pxa2xx-ac97
modprobe ucb1x00-ts

Note, that enabling touchscreen driver without AC97 controller driver is meaningless, since ucb1x00-ts driver uses AC97 bus to access the touchscreen device.

More information about Linux audio subsystem can be found at ALSA project homepage

AC'97 and PCI interaction

The following note is relevant only for cards with 'B' option and without 'A' or 'AT' options.

CM-X270 contains local bus arbitration logic needed to arbiter bus accesses between the CPU and PCI bridge. This arbitration logic and AC97 controller share the same input clock. Linux AC97 driver tests whether there's an audio codec attached to the AC97 bus. If the driver does not detect a codec, it disables PXA270 AC97 controller, and its clock altogether. Disabling this clock, in turn, disables the local bus arbitration, and makes PCI bridge useless.

To avoid such unpleasant situation we'd recommend not to enable sound drivers, unless you need them.

I2C

Standard I2C and SPI drivers can be used to access these buses on CM-X270. The I2C drivers can be easily enabled using Linux kernel configuration. In the Device drivers menu enable the I2C support end enter the I2C support -> I2C Hardware Bus support submenu. Then enable Intel PXA2XX I2C adapter:

.config - Linux Kernel v2.6.24 Configuration
──────────────────────────────────────────────────────────────────────────────
 ┌─────────────────────── I2C Hardware Bus support ────────────────────────┐
 │ Arrow keys navigate the menu. <Enter> selects 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 [ ] excluded <M> module < >            │
 │ ┌─────────────────────────────────────────────────────────────────────┐ │
 │ │ < > ALI 1535                                                        │ │
 │ │ < > ALI 1563                                                        │ │
 │ │ < > ALI 15x3                                                        │ │
 │ │ < > AMD 756/766/768/8111 and nVidia nForce                          │ │
 │ │ < > AMD 8111                                                        │ │
 │ │ < > GPIO-based bitbanging I2C                                       │ │
 │ │ < > Intel 82801 (ICH)                                               │ │
 │ │ < > Intel 810/815                                                   │ │
 │ │ <*> Intel PXA2XX I2C adapter (EXPERIMENTAL)                         │ │
 │ │ [ ] Intel PXA2XX I2C Slave comms support                            │ │
 │ └────v(+)─────────────────────────────────────────────────────────────┘ │
 ├─────────────────────────────────────────────────────────────────────────┤
 │                     <Select> < Exit > < Help >                          │
 └─────────────────────────────────────────────────────────────────────────┘

This will enable the I2C controller driver. To access a device attached to I2C bus you should have a driver for that device, or you can use "I2C device interface" driver for raw read and write access.

SPI

Please refer to SPI support for CM-X270 article.

USB device

USB device controller drivers for PXA270 are not available in mainline kernels, and this is the main reason we do not support USB device functionality on CM-X270. Yet, there are several patches available at openzaurus project, Handhelds.org, Raphael Assenat site dedicated to Linux on CompuLab boards, and probably others. We never tried these patches, and we don't know which of them will work on CM-X270.

See also

• Getting started with Linux on CM-X270
• Linux Development for XScale modules
• Debian Linux for CM-X270
• Linux MTD drivers on CM-X270