GPU

The SGX 530 runs Harmattan's own GPU stack as it ships: X with Nokia's driver, GLES2 apps and the compositor, through Harmattan's closed PowerVR libraries. The kernel side is a forward-ported driver with the same interface as Nokia's, kept in its own repository, pvr_sgx_rm696. Speed is on par with stock.

How it fits together

Layer	What	Where
Harmattan's userspace	the SGX DDK 1.4.14.2514 libraries (*_r125.so, libsrv_um, libpvr2d), Nokia's X driver (DRI2 over pvr2d), pvrsrvinit_r125	the phone's root filesystem, unchanged
The driver	pvrsrvkm.ko (services and the SGX, /dev/pvrsrvkm) and omaplfb.ko (the display class on /dev/fb0)	pvr_sgx_rm696, built against the kernel by kernel/build.sh (PVR=)
The kernel	omapfb rather than omapdrm (Harmattan's X driver and omaplfb use omapfb), the registered_fb export for omaplfb (b5ec71112), Nokia's DSS notifier the driver's flips complete on (d75b49787), the GPU node from omap36xx's device tree	linux_mainline_rm696

At boot Harmattan's own /etc/init/sgx.conf loads the modules as on stock (modprobe omaplfb brings pvrsrvkm with it) and runs pvrsrvinit_r125, which sets the SGX up through the driver.

The driver is Nokia's N900 driver as Maemo Leste forward-ported it to 5.0, brought to 6.18: it probes from the device tree, powers the SGX through runtime PM, and its flip events, event timestamps and mappings of framebuffer memory (EGL images for the camera viewfinder and video) are fixed. The driver's README lists each change.

Keeping Nokia's interface

The libraries are closed, so the driver must answer their ioctls exactly as Nokia's module did. check-abi.py (in the driver's repository) compares the two modules' ioctl dispatch without a phone: the same ioctl index range, the same 11 unhandled indices, the same 24 indices sharing a handler, and the same first kernel call in each comparable handler (59 of 59). It checks the dispatch, not the layout of each ioctl's arguments; those come from the same DDK headers, and a mismatch would show at SGX initialisation or on the first GL call. The driver also refuses to build with any option set but Harmattan's (release, EDM trace, command trace), because the libraries check it at start-up.

Power and clocks

• The SGX runs at 195.2 MHz (CORE/2, the N9's maximum), powers down 1 ms after it idles and is polled every 10 ms while powered, as Nokia's driver.
• While the SGX is clocked it holds L3 at full speed (the sgx holder in omap3_l3, see Power), and CORE reaches OFF only with the SGX off.
• CPU floor: while the SGX is busy the CPU stays at or above 800 MHz (/sys/module/pvrsrvkm/parameters/cpu_min_khz, 0 turns it off). Stock had none.

Speed

On Harmattan's own GLES2 benchmark runs (tests/glesbench.c, the same binary on both kernels) 6.18 matches or beats stock in fill, draw and swap rates and is about 2% slower on texture uploads; the table is in Performance.

Checking it

On the phone (as root, or through the rescue SSH):

cat /proc/pvr/version                                   # the driver's DDK version, once loaded
grep -E 'pvrsrvkm|omaplfb' /proc/modules
cat /sys/module/pvrsrvkm/parameters/cpu_min_khz

The build kit's tests/ has three GPU checks, run in Harmattan's session with the display on: pvr2d-test.c (a GPU fill and copy through libpvr2d, every pixel checked), gles2-test.c (a GLES2 triangle through EGL on X, pixels read back) and glesbench.c (frame rates and CPU time per mode).

Limits

• Only Harmattan's DDK 1.4.14.2514 userspace: the driver keeps that one interface, so other GPU stacks (Mesa, newer DDKs) don't run on it.
• The interface check covers the ioctl dispatch, not the argument structures (see above).