+# RISC-V 3D GPU
+
+at FOSDEM 2018 when Yunsup and the team announced the U540 there was
+some discussion about this: it was one of the questions asked. one of
+the possibilities raised there was that maddog was heading something:
+i've looked for that effort, and have not been able to find it [jon is
+getting quite old, now, bless him. he had to have an operation last
+year. he's recovered well].
+
+ also at the Barcelona Conference i mentioned in the
+very-very-very-rapid talk on the Libre RISC-V chip that i have been
+tasked with, that if there is absolutely absolutely no other option,
+it will use Vivante GC800 (and, obviously, use etnaviv). what *that*
+means is that there's a definite budget of USD $250,000 available
+which the (anonymous) sponsor is definitely willing to spend... so if
+anyone can come up with an alternative that is entirely libre and
+open, i can put that initiative to the sponsor for evaluation.
+
+ basically i've been looking at this for several months, so have been
+talking to various people (jeff bush from nyuzi [1] and chiselgpu [2],
+frank from gplgpu [3], VRG for MIAOW [4]) to get a feel for what would
+be involved.
+
+ * miaow is just an OpenCL engine that is compatible with a subset of
+AMD/ATI's OpenCL assembly code. it is NOT a GPU. they have
+preliminary plans to *make* one... however the development process is
+not open. we'll hear about it if and when it succeeds, probably as
+part of a published research paper.
+
+ * nyuzi is a *modern* "software shader / renderer" and is a
+replication of the intel larrabee architecture. it explored the
+concept of doing recursive software-driven rasterisation (as did
+larrabee) where hardware rasterisation uses brute force and often
+wastes time and power. jeff went to a lot of trouble to find out
+*why* intel's researchers were um "not permitted" to actually put
+performance numbers into their published papers. he found out why :)
+one of the main facts that jeff's research reveals (and there are a
+lot of them) is that most of the energy of a GPU is spent getting data
+each way past the L2/L1 cache barrier, and secondly much of the time
+(if doing software-only rendering) you have several instruction cycles
+where in a hardware design you issue one and a separate pipeline takes
+over (see videocore-iv below)
+
+ * chiselgpu was an additional effort by jeff to create the absolute
+minimum required tile-based "triangle renderer" in hardware, for
+comparative purposes in the nyuzi raster engine research. synthesis
+of such a block he pointed out to me would actually be *enormous*,
+despite appearances from how little code there is in the chiselgpu
+repository. in his paper he mentions that the majority of the time
+when such hardware-renderers are deployed, the rest of the GPU is
+really struggling to keep up feeding the hardware-rasteriser, so you
+have to put in multiple threads, and that brings its own problems.
+it's all in the paper, it's fascinating stuff.
+
+ * gplgpu was done by one of the original developers of the "Number
+Nine" GPU, and is based around a "fixed function" design and as such
+is no longer considered suitable for use in the modern 3D developer
+community (they hate having to code for it), and its performance would
+be *really* hard to optimise and extend. however in speaking to jeff,
+who analysed it quite comprehensively, he said that there were a large
+number of features (4-tuple floating-point colour to 16/32-bit ARGB
+fixed functions) that have retained a presence in modern designs, so
+it's still useful for inspiration and analysis purposes. you can see
+jeff's analysis here [7]
+
+ * an extremely useful resource has been the videocore-iv project [8]
+which has collected documentation and part-implemented compiler tools.
+the architecture is quite interesting, it's a hybrid of a
+Software-driven Vector architecture similar to Nyuzi plus
+fixed-functions on separate pipelines such as that "take 4-tuple FP,
+turn it into fixed-point ARGB and overlay it into the tile"
+instruction. that's done as a *single* instruction to cover i think 4
+pixels, where Nyuzi requires an average of 4 cycles per pixel. the
+other thing about videocore-iv is that there is a separate internal
+"scratch" memory area of size 4x4 (x32-bit) which is the "tile" area,
+and focussing on filling just that is one of the things that saves
+power. jeff did a walkthrough, you can read it here [10] [11]
+
+ so on this basis i have been investigating a couple of proposals for
+RISC-V extensions: one is Simple-V [9] and the other is a *small*
+general-purpose memory-scratch area extension, which would be
+accessible only on the *other* side of the L1/L2 cache area and *ONLY*
+accessible by an individual core [or its hyperthreads]. small would
+be essential because if a context-switch occurs it would be necessary
+to swap the scratch-area out to main memory (and back).
+general-purpose so that it's useful and useable in other contexts and
+situations.
+
+ whilst there are many additional reasons - justifications that make
+it attractive for *general-purpose* usage (such as accidentally
+providing LD.MULTI and ST.MULTI for context-switching and efficient
+function call parameter stack storing, and an accidental
+single-instruction "memcpy" and "memzero") - the primary driver behind
+Simple-V has been as the basis for turning RISC-V into an
+embedded-style (low-power) GPU (and also a VPU).
+
+ one of the things that's lacking from RVV is parallelisation of
+Bit-Manipulation. RVV has been primarily designed based on input from
+the Supercomputer community, and as such it's *incredible*.
+absolutely amazing... but only desirable to implementt if you need to
+build a Supercomputer.
+
+ Simple-V i therefore designed to parallelise *everything*. custom
+extensions, future extensions, current extensions, current
+instructions, *everything*. RVV, once it's been implemented in gcc
+for example, would require heavy-customisation to support e.g.
+Bit-Manipulation, would require special Bit-Manipulation Vector
+instructions to be added *to RVV*... all of which would need to AGAIN
+go through the Extension Proposal process... you can imagine how that
+would go, and the subsequent cost of maintenance of gcc, binutils and
+so on as a long-term preliminary (or if the extension to RVV is not
+accepted, after all the hard work) even a permanent hard-fork.
+
+ in other words once you've been through the "Extension Proposal
+Process" with Simple-V, it need never be done again, not for one
+single parallel / vector / SIMD instruction, ever again.
+
+ that would include for example creating a fixed-function 3D "FP to
+ARGB" custom instruction. a custom extension with special 3D
+pipelines would, with Simple-V, not need to also have to worry about
+how those operations would be parallelised.
+
+ this is not a new concept: it's borrowed directly from videocore-iv
+(which in turn probably borrowed it from somewhere else).
+videocore-iv call it "virtual parallelism". the Vector Unit
+*actually* has a 4-wide FPU for certain heavily-used operations such
+as ADD, and a ***ONE*** wide FPU for less-used operations such as
+RECIPSQRT.
+
+ however at the *instruction* level each of those operations,
+regardless of whether they're heavily-used or less-used they *appear*
+to be 16 parallel operations all at once, as far as the compiler and
+assembly writers are concerned. Simple-V just borrows this exact same
+concept and lets implementors decide where to deploy it, to best
+advantage.
+
+
+> 2. If it’s a good idea to implement, are there any projects currently
+> working on it?
+
+ i haven't been able to find any: if you do please do let me know, i
+would like to speak to them and find out how much time and money they
+would need to complete the work.
+
+> If the answer is yes, would you mind mention the project’s name and
+> website?
+>
+> If the answer is no, are there any special reasons that nobody not
+> implement it yet?
+
+ it's damn hard, it requires a *lot* of resources, and if the idea is
+to make it entirely libre-licensed and royalty-free there is an extra
+step required which a proprietary GPU company would not normally do,
+and that is to follow the example of the BBC when they created their
+own Video CODEC called Dirac [5].
+
+ what the BBC did there was create the algorithm *exclusively* from
+prior art and expired patents... they applied for their own patents...
+and then *DELIBERATELY* let them lapse. the way that the patent
+system works, the patents will *still be published*, there will be an
+official priority filing date in the patent records with the full text
+and details of the patents.
+
+this strategy, where you MUST actually pay for the first filing
+otherwise the records are REMOVED and never published, acts as a way
+of preventing and prohibiting unscrupulous people from grabbing the
+whitepapers and source code, and trying to patent details of the
+algorithm themselves just like Google did very recently [6]
+
+* [0] https://www.youtube.com/watch?v=7z6xjIRXcp4
+* [1] https://github.com/jbush001/NyuziProcessor/wiki
+* [2] https://github.com/asicguy/gplgpu
+* [3] https://github.com/jbush001/ChiselGPU/
+* [4] http://miaowgpu.org/
+* [5] https://en.wikipedia.org/wiki/Dirac_(video_compression_format)
+* [6] https://yro.slashdot.org/story/18/06/11/2159218/inventor-says-google-is-patenting-his-public-domain-work
+* [7] https://jbush001.github.io/2016/07/24/gplgpu-walkthrough.html
+* [8] https://github.com/hermanhermitage/videocoreiv/wiki/VideoCore-IV-Programmers-Manual
+* [9] libre-riscv.org/simple_v_extension/
+* [10] https://jbush001.github.io/2016/03/02/videocore-qpu-pipeline.html
+* [11] https://jbush001.github.io/2016/02/27/life-of-triangle.html
+
+
projects / libreriscv.git / commitdiff