Android/SGX on the Gumstix Overo

Daniel Beer

9 Aug 2015

This document explains how I, with some assistance and quite a bit of background research by Simon Guest at TracMap, got the PowerVR SGX drivers (3_01_00_03) working with Android 4.3 on the Gumstix Overo. What's described here isn't a finished product, but it's enough to get SurfaceFlinger up and running with hardware acceleration.

Throughout this document, I'm going to assume familiarity with the basics of AOSP development (the use of repo and so forth). Some of this is written from memory, and the state of the respositories described below might have changed slightly in the meantime. Be prepared to fill in some blanks and/or alter these instructions to suit your situation.

Obtaining the necessary files

Our starting point was the Gumdroid repositories, maintained by Ash Charles at Gumstix. The manifest repository contains branches called overo and dev/overo. We mixed and matched a little between the two. In particular:

Get the repositories described above, swapping out the kernel and u-boot components as described. You'll also need to install the SGX Android SDK. Obtain the repository which contains the installer:

git clone git://gitorious.org/rowboat/ti_android_sgx_sdk.git

Inside this repository, you'll find an installer named OMAP35x_Android_Graphics_SDK_setuplinux_3_01_00_03.bin. Run that to unpack the SDK. Within the unpacked SDK, the binaries we're interested in are located in the gfx_rel_es3.x_android subdirectory. There is also source for the kernel bridge driver, but it's for a fairly old kernel version.

NOTE: Since gitorious.org now appears to be non-functional, I've uploaded a copy of the installer to Dropbox. The SHA256 hash of the file is 566fe0e82af4431e1135fc655115faff8196bbbf05f8134a5f46ee82e082aec9:

Modifications

We need to add the PVR bridge driver to the Linux kernel, and we need to add a few userspace components to the system. We also need to modify the Android frameworks to work around some quirks in the SGX userspace drivers.

Kernel bridge driver

The first thing you need to do is integrate the bridge drivers into the kernel with this patch. There are two separate drivers: one manages general communication between userspace and the SGX hardware (pvrsrvkm), and the other manages display flip-chains (omaplfb).

To enable these drivers, make sure your kernel configuration has these options set:

CONFIG_SGX_PVRSRV=y
CONFIG_SGX_OMAPLFB=y

This patch registers drivers only. You will also need to declare instances of the hardware to get them working. In your board-specific setup file (probably arch/arm/mach-omap2/board-overo.c) you'll need something like this:

static struct platform_device pvrsrvkm_device = {
        .name = "pvrsrvkm",
        .id = -1
};

static struct platform_device omaplfb_device = {
        .name = "omaplfb",
        .id = -1
};

static inline void __init overo_init_sgx(void)
{
        platform_device_register(&pvrsrvkm_device);
        platform_device_register(&omaplfb_device);
}

Remember to call overo_init_sgx somewhere in your init_machine function (perhaps overo_init).

Userspace drivers

As stated above, the files we need from the SDK are in the gfx_rel_es3.x_android subdirectory of the SDK package. We copy these into place in the device/gumstix/common/sgx/proprietary/3.x subdirectory in the Android build tree.

But that's not all! There is a bug in glDiscardFramebufferEXT in libGLESv2_POWERVR_SGX530_121.so which causes crashes. Since this function is only an optimization hint, we work around this problem by patching the first instruction to a return, making the call a no-op:

0000c160 <glDiscardFramebufferEXT>:
    c160:       e12fff1e        bx      lr

You can use this program to make this modification to your version:

gcc -O1 -Wall -o sgx_bp sgx_bp.c
./sgx_bp libGLESv2_POWERVR_SGX530_121.so

Ash Charles has already included an Android.mk for the proprietary subdirectory in the development branch, but we need to modify it a bit, as the names of the binaries to be installed are slightly different. Our modified version is here.

There are two more things that need to be tidied up in device/gumstix/overo. First, we need to set OMAPES := 3.x in BoardConfig.mk. The other thing we need to do is make sure the SGX userspace init program gets run on boot. Add this stanza to init.overo.rc:

service pvrsrvkm /system/bin/pvrsrvinit
    class main
    oneshot

Hardware composer

Gumdroid doesn't provide an hwcomposer HAL, but there is a skeleton implementation in hardware/libhardware/modules/hwcomposer. Starting from this, we produced a basic module which provides a VSYNC signal, but supports only GLES composition. The necessary files are here:

This should produce a HAL module called hwcomposer.overo.so, which is installed in the usual HAL location in the filesystem /system/lib/hw.

Frameworks

Finally, some of the framework components need modification to work around SGX quirks. Patches for this purpose are listed below:

Booting

Make sure U-Boot is passing a command-line parameter configuring the kernel's framebuffer driver to use 32-bit colour. For example:

omapfb.mode=dvi:640x480MR-32@60

Also make sure that you have sufficient video RAM to allocate at least a couple of flip-chain buffers.

The first boot takes quite a long time, as Android will compile packages ahead of time. After a few minutes, you should see the leaf-fall animation appear on screen.

If you're having trouble, run through the following checklist:

Limitations

This hasn't been thoroughly tested. It's quite possible that some kinds of acceleration (video, perhaps) have been broken as a result of the use of GL_TEXTURE_2D in place of GL_TEXTURE_EXTERNAL_OES.