Broadcom VideoCore IV architecture overview
Broadcom has decided to open-source their graphics driver for one of their VideoCore IV powered Android chipsets. This is an awesome and welcome step. They also released an architecture manual giving details for many things. I will try and summarize some of the information known about VideoCore IV so far.
VideoCore IV refers to a family of closely-related GPUs. Implementations have shown up in various chipsets. For example, BCM2835 used in Raspberry Pi, BCM2763 used in several Nokia Symbian Belle handsets (eg: Nokia Pureview 808, 701,700 etc), BCM21553 in Android handsets such as Samsung Galaxy Y and and BCM28155 in Android handsets such as Samsung Galaxy SII Plus.
Overview: Various chipsets have their own peculiarities. In the Raspberry Pi and Nokia flavors, the VideoCore IV consists of two distinct processors. The first processor is the actual programmable graphics core, which I will refer to as PGC. The second processor is a coprocessor. This embedded processor, not to be confused with the main CPU, runs its own operating system and handles almost all the actual work of the OpenGL driver. For example, shader compilation is done on this embedded processor and not on the main CPU in the Raspberry Pi and Nokia flavors. The OpenGL driver on these devices just is a shim that passes calls to the embedded coprocessor via RPC-like mechanism. My speculation (low-confidence) is that the BCM21553, for which Broadcom released the source code, does not have the embedded coprocessor and the driver runs on the main CPU. The Nokia variants have an additional detail that these feature an 128MB LPDDR2 on-package memory dedicated for GPU, separate from the 512MB RAM in these devices, to provide a high-bandwidth (at the time) graphics RAM for the GPU. Raspberry Pi does not have this buffer and the GPU reads/writes from the main memory.
GPU core: VideoCore’s PGC is a tile-based renderer (TBR). Apart from fixed function parts, the programmable portion of the chip is organized into “slices”, which are similar to say “compute units” in GCN. Each slice consists of upto 4 SIMD units called QPUs, one special function unit (SFU),one or two texture and memory units (TMUs) as well as some caches. The architectural diagram shows upto 4 slices, but I guess the actual number may vary between chipsets (not confirmed).
QPU (SIMD ALUs): QPU consists of two SIMD ALUs. The ALUs are not symmetric. Each of these ALUs is physically 4-wide (i.e. 128-bit), but one of them is an “add” unit and the other is a “mul” unit, and handle add and multiply floating-point operations respectively along with some other ops such as integer and logical ops. The QPU is a dual-issue processor, capable of feeding one add and one mul instruction per cycle to each of the units. Logically, each ALU in the QPU is actually a 16-way machine that executes a 16-way instruction in 4 cycles. Thus, overall, each QPU can perform 8 flops/cycle. Thus, each slice can do upto 32 flops/cycle. Each QPU has access to a 4kB of registers, as well as a few accumulators. Registers are organized as two register files of 2kB each. Each register file is organized as 32 vector registers, where each vector register is 64 bytes (16 x 4bytes) which makes sense given the 16-way logical view of the QPU. Each QPU can run two threads.
Memory (TMUs and VPM): TMUs have their own L1 cache, and there is also a separate L2 cache that is shared across slices. Cache sizes are unknown. QPUs read/write vertex data through a separate path called the Vertex Pipe Manager (VPM). VPM is a system-wide shared unit and appears to have a buffer of either 8kB or 16kB. VPM performs DMA from main memory to read/write vertex data into the buffer. VPM is optimized essentially for reading/writing vectors of data from/to main memory and from/to the QPUs vector register files. Vertex fetch is general enough to implement memory gather operations, but it is not clear if scatter is also supported.
RPi and Conclusions: Consider the Raspberry Pi. We already know that the published frequency is 250MHz and that the QPUs can do 24 gflops and the TMUs can do 1.5 GTexel/s. Thus, per-clock, the GPU performs 96 flops/cycle and 6 texels/cycle. Likely, this is achieved through 3 slices each with 4 QPUs and 2 TMUs. Overall, VideoCore IV is an interesting architecture. Performance-wise, the implementation in the Raspberry Pi does not compare to modern mobile GPUs such as Adreno 330 or Mali T600 series but then again the Raspberry Pi is using an old SoC that was meant to be cost-conscious even at that time. For a low-cost GPU, VideoCore IV looks to be quite competent. It will be interesting to see what Broadcom is cooking up for VideoCore V.