So, why the Nook HD+? While I have alrealy ported Genode to the Samsung Galaxy Nexus phone for the FOSDEM 2013 demo session, I decided to give another device a try for the following reasons:


Booting custom software on a commercial device is usually connected with some obstacles, like breaking the chain of trust. A typical approach (which I have utilized for many devices, including Acer Iconia A500, Samsung Galaxy S2 and Samsung Galaxy Nexus) is porting the u-boot bootloader to be an intermediate chainloader, flashed instead of the linux kernel.

The B&N Nook HD+ does feature the signed kernel-based chain of trust for the internal EMMC memory, but it allows booting unsigned MLO, xloader and u-boot from the external microSD card. That is to say, there already exists the u-boot port, but to make it boot Genode, numerous changes were needed.

First, I’ve obtained the android cwm recovery image for the sd card from the B&N Nook HD+ forum on xda-developers.com [credits go to the forum members verygreen and fat-tire]. After writing the image to the SD card and fixing partition layout in fdisk, I ended up with a VFAT partition containing the MLO, u-boot.bin and uImage. The MLO is the header file for the omap4 CPU which combines the memory initialization table with the x-loader bootloader. The u-boot.bin is the almost vanilla B&N u-boot which initializes the display and boots the uImage [which in the case of sd booting is also u-boot]. We’ll be replacing the uImage with the customized u-boot.

You can obtain the source code for my u-boot from https://github.com/astarasikov/uboot-bn-nook-hd-fastboot and below is the list of problems I’ve solved:


I had to do some changes to make Genode run. I’ll briefly discuss some notable items.

Fiasco.OC cross-compiling

Recently, Genode crew have updated to the latest revision of the Fiasco.OC microkernel and it seems to contain some hardcoded cross-compiler name for ARM. I was reluctant to either fix it or download the ‘proper’ compiler (especially knowing that the one from the Genode toolchain does work for omap4).

So, here is what I’ve done:

Increasing nitpicker memory quota.

Currently nitpicker [window manager providing virtual framebuffers] is hardcoded for some 1024x768 screens (I believe because no one, even at genode, seriously considers using Genode as a daily driver today), so we need to fix the memory limit constant in the following way:

-- a/os/include/nitpicker_session/connection.h
++ b/os/include/nitpicker_session/connection.h
@@ -43,8 +43,8 @@ namespace Nitpicker {
                                char argbuf[ARGBUF_SIZE];
                                argbuf[0] = 0;

Adding LCD support to omap4 framebuffer

Currently, omap4 framebuffer only supports the TV interface for HDMI. To make it reset and configure the LCD interface (which is used in smartphones and tablets), we need to add some register definitions [and fix the incorrect definition of the base address register while we’re at it] to the code according to the omap4 DSS subsystem manual.

diff --git a/os/src/drivers/framebuffer/omap4/dispc.h b/os/src/drivers/framebuffer/omap4/dispc.h
index 23f80df..ea9f602 100644
-- a/os/src/drivers/framebuffer/omap4/dispc.h
++ b/os/src/drivers/framebuffer/omap4/dispc.h
@@ -18,8 +18,14 @@ struct Dispc : Genode::Mmio
        struct Control1 : Register<0x40, 32>

                struct Tv_enable : Bitfield<1, 1> { };

                struct Go_tv : Bitfield<6, 1>
                        enum { HW_UPDATE_DONE    = 0x0,   /* set by HW after updating */
@@ -46,11 +52,17 @@ struct Dispc : Genode::Mmio
                struct Width  : Bitfield<0, 11>  { };
                struct Height : Bitfield<16, 11> { };

         * Configures base address of the graphics buffer

         * Configures the size of the graphics window
diff --git a/os/src/drivers/framebuffer/omap4/driver.h b/os/src/drivers/framebuffer/omap4/driver.h
index d754a97..53517a3 100644

@@ -203,6 +203,7 @@ bool Framebuffer::Driver::init(Framebuffer::Driver::Mode   mode,

        _dispc.write<Dispc::Gfx_size::Sizex>(width(mode) - 1);

Hacking in Nook HD+ display support.

I wanted to make the display work in a quick and dirty way, so I’ve commented out the HDMI init code and replaced with a simple code that reconfigured the framebuffer address for the LCD (we piggyback on the u-boot to initialize the screen). Remember, kids, never ever think of doing this in production. I am deeply ashamed of havind done that. Either way, I’ll show you the code, and the framebuffer driver badly wants some changes:

diff --git a/os/src/drivers/framebuffer/omap4/driver.h b/os/src/drivers/framebuffer/omap4/driver.h
index 53517a3..9287cdc 100644
-- a/os/src/drivers/framebuffer/omap4/driver.h
++ b/os/src/drivers/framebuffer/omap4/driver.h
@@ -76,6 +76,7 @@ class Framebuffer::Driver
                static size_t width(Mode mode)

                        switch (mode) {
                        case MODE_1024_768: return 1024;
@@ -84,6 +85,7 @@ class Framebuffer::Driver
                static size_t height(Mode mode)

                        switch (mode) {
                        case MODE_1024_768: return 768;
@@ -117,12 +119,17 @@ bool Framebuffer::Driver::init(Framebuffer::Driver::Mode   mode,
                                Framebuffer::addr_t         phys_base)
        /* enable display core clock and set divider to 1 */


        /* set load mode */

        if (!_hdmi.issue_pwr_pll_command(Hdmi::Pwr_ctrl::ALL_OFF, _delayer)) {
@@ -196,6 +203,10 @@ bool Framebuffer::Driver::init(Framebuffer::Driver::Mode   mode,
        _dispc.write<Dispc::Size_tv::Height>(height(mode) - 1);

        Dispc::Gfx_attributes::access_t pixel_format = 0;
        switch (format) {
@@ -212,6 +223,7 @@ bool Framebuffer::Driver::init(Framebuffer::Driver::Mode   mode,

@@ -223,6 +235,9 @@ bool Framebuffer::Driver::init(Framebuffer::Driver::Mode   mode,
                PERR("Go_tv timed out");
                return false;

        return true;

Configuration file

Genode is configured via the XML configuration file. Here are some notes We’re using the dde_kit usb_drv driver to provide stubs for networking and input drivers The nic_bridge proxifies the networking for two l4linux instances nitpicker and nit_fb are used to split the display into virtual framebuffers both nic_bridge and nit_fb are using the Genode concepts of the service interfaces and service routing. We’re configuring the services in such a way that they’re using the specific service if needed, and rely on the parent to provide the default service if we dont’ care. For example, take a look at how nic_bridge is configured. The usb_drv features the “provides” section that declares which interfaces the service is allowed to provide. These may be used in the “route” section of the client services. By default, Genode features a deny-all policy, so if you don’t configure something, you have no access to it. usb_drv has some memory leak (or had back in winter and I was lazy to look into it) so I’ve increased the RAM quota and hoped it would survive. It did.

 <start name="usb_drv">
  <resource name="RAM" quantum="40M"/>
   <service name="Input"/>
   <service name="Nic"/>
   <nic mac="2e:60:90:0c:4e:01" />

 <start name="nic_bridge">
  <resource name="RAM" quantum="2M"/>
  <provides><service name="Nic"/></provides>
   <service name="Nic"> <child name="usb_drv"/> </service>
   <any-service> <parent/> <any-child/> </any-service>

Compiling and running

So, how about trying it out yourself?

Install the Genode toolchain (consult the Genode website and sourceforge project) and create the symlinks as explained above.

Get the u-boot source code

git clone git://github.com/astarasikov/uboot-bn-nook-hd-fastboot.git

Compile U-boot.

I recommend using the codesourcery 2010q1-188 toolchain (because not all toolchains produce working code)

export PATH=/path/to/codesourcery/toolchain/bin:$PATH
export ARCH=arm
export CROSS_COMPILE=arm-none-eabi-
make clean
make distclean
make ${U_BOARD}_config
make -j8 
./tools/mkimage -A arm -O linux -T kernel -C none -a 0x88000000 -e 0x88000000 -n foo -d u-boot.bin uImage

Get the genode framework source code

git clone git://github.com/astarasikov/genode.git
git checkout nook_staging

Now, for each directory in the genode tree (base-foc, and non-base directories), go to them and execute “make prepare” to download the required libraries. Well, libports is heavily broken and many packages (openssl, zlib, any more?) fail to download. You can skip libports and qt4 for now.

Prepare the build directory

./tool/create_builddir foc_panda BUILD_DIR=/my/build/dir

Edit the /my/build/dir/etc/build.conf

Uncomment the “REPOSITORIES += “ entries to allow building l4linux and nitpicker

Add the “MAKE += -j8” to the file to build in parallel utilizing all CPU cores.

Add “SPECS += uboot” to the /my/build/dir/etc/specs.conf to force creating the raw image.bin.gz binary.

Compile the Genode framework

cd /my/build/dir
make run/nookhdp_l4linux

Actually running the image

Now, you may wonder how to run the image. The tablet must be in the fastboot mode. Genode expects itself to be loaded at 0x81000000, and the u-boot does make a stupid assumption that linux kernel must be shifted 0x8000 bytes (i.e., 2 pages) from the base address. It should be fixed eventually, but for now, we’re manually substracting the offset from the boot address

gunzip var/run/nookhdp_l4linux/image.bin.gz
fastboot -b 0x80ff8000 boot var/run/nookhdp_l4linux/image.bin


Here is a picture of the Genode running. You can see the screen split into four parts with the help of the Nitpicker window manager. Each screen chunk is a virtual framebuffer provided by the Nit_fb service. Genode is running the Launchpad server (top right corner), the LOG written to the framebuffer (bottom left) and two instances of L4Linux.

Genode on Nook HD+ Photo

So, now it does not do much useful work, but remember it was an overnight proof of concept hacked together with the sole purpose of demonstrating the capabilities of Genode Framework, and this tutorial is basically a collection of anti-patterns.

Future plans