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SubjectRe: HARDWARE: Open-Source-Friendly Graphics Cards -- Viable?


Jeff Garzik wrote:
> I've thought about this a bit already, and had some chip designers
> correct my thoughts on a few things. Here are my comments, in no
> particular order:
>
>
> 1) I agree it probably wouldn't be cost effective without selling to
> OEMs in volume

I have concluded that.

>
> 2) AGP/PCI-Express is practically required, if you want OEM sales, IMO

AGP and PCI are very similar in terms of the state machine, although the
signal drivers are different. I expect we'll come out with PCI and AGP
versions first and then PCIE soon after. Any "early access" developer
boards would be PCI-only.

>
> 3) Your main bottlenecks are video RAM bandwidth to/from the GPU,
> PCI/AGP bus bandwidth, and system RAM bandwidth.

The biggest bottleneck is the PCI/AGP bus.

>
> 4) I am a bit dubious that FPGA will perform at a useful clock speed.

You can either clock fast, like Intel, or parallelize more, like AMD.
Both work.

Of course, for an FPGA, both make it more expensive.

>
> 5) Key question: generic GPU or not?
>
> From what I've read in ARS Technica and other tech sites, ATI and
> NVIDIA chips are moving towards a more generic, programmable CPU model.
> Presumably on current (or future?) chips, you will push bytecoded
> shaders direct to your video card. Essentially some future GPUs will be
> highly specialized, yet generic, CPUs with their own ISA.
>
> On the other hand, if you only support a small number of graphics
> operations, it may be easier for the first rev of your chip to do all
> the 2D operations in silicon.

It sounds like the best thing to do is implement a 3D pipeline and bolt
on a translator to do 2D stuff.

What I don't know is how much 3D I can get away with not implementing.

>
>
> 6) My preference: generic OpenGL programming interface.
>
> I feel my own personal design for video hardware interface is better
> than ATI or NVIDIA: present to the OS driver a generic, open, public
> OpenGL interface, that very rarely (if ever) changes. This must be a
> simple interface: only a few key operations, such as "transfer {display
> list | shader | texture | ...} data to card" or "execute display list"
> should be presented to the OS driver.
>
> The interface should have a standard "fall back to software rendering"
> response message, for minimalist hardware.
>
> If your hardware presents a standard GL interface to OS driver,
>
> a) there is a high potential industry standardization, if it's done right
>
> b) reduce complexity in the OS driver.
>
> c) stop the "driver rat race". After the OS driver is initially
> written, the only maintenance costs are keeping up to date with the
> latest OpenGL standards. You have very low cross-hardware support
> costs, because all the hardware presents the same interface to the OS
> driver.
>
> d) makes it possible to add value to your hardware without changing the
> OS driver
>
> e) makes it possible for multiple video chip vendors to compete, without
> worrying about OS driver issues, or Linux support issues.
>
> f) this isn't a terribly new idea :) But it's a good one, IMHO.
>
> g) even on minimalist 2D-only hardware, you can implement this interface
>

At this moment, I'm taking a cue from the Linux driver ABI and thinking
that standardizing the interface would be more limiting than helpful.
While it might be a pain to have to carry around multiple driver
versions, the fact that it's all open source kinda makes it easy to make
drastic changes without hurting anything.

Plus, I don't expect to get it perfect the first time. The first design
will have to be very good, but I'm still expecting lots of complaints
and suggestions from developers. The second design would probably end
up being a lot different from the first. The third design, I expect,
would change lots of things again, because by then, I'll look like an
idiot if I don't have bump mapping and fully programmable virtex shaders.

>
> 7) two-chip solution
>
> One thing I have pondered, with regards to #6: what about implementing
> a multi-core solution? One core to handle the graphics operations and
> control the video, and one core a much more generic microcontroller that
> runs ucLinux, and handles the GL "slow path" stuff. The advantage of
> this approach is
>
> a) 100% of 2D and 3D GL is "done in hardware". The portions of GL that
> are not handled by the GPU core are handled by the microcontroller core,
> which is running a generic firmware.
>
> b) Since a microcontroller is included, you can upgrade the firmware
> quite easily to support new OpenGL features.
>
> c) you don't necessarily have to follow my design of one purpose built
> GPU core, and one generic microcontroller core. If your FPGA is big
> enough, you can have plenty of execution engines (that's the beauty of
> hardware, it's inherently parallel).
>
> But OTOH, maybe a multi-core solution will add too much latency into the
> picture, I dunno.

I've thought about that too. Two FPGAs are cheaper than one which is
twice the size. I could have one chip for the host interface, VGA, and
perhaps the microcontroller, 2^N chips for rendering pipelines, and
another for the video/memory controllers.

Not cheap, though. But at least in this case, it'll also LOOK
correspondingly expensive. :)

>
>
> 8) I don't find an open source video BIOS terribly exciting. Yes, I do
> think it should be open source, but the interest is largely theoretical.
> A video BIOS only does a couple things... implements VESA/etc. BIOS
> calls, and initializes the hardware based on characteristics specific to
> the video board (i.e. OEM not GPU details, such as video RAM setup and
> clocking, which video inputs are actually implemented on the board,
> ...). I don't see there being a big programmer or developer interest

I agree it's not a big deal. But there's no reason to hide anything
there. Besides, if someone wants to port it to an obscure platform,
they don't have to ask for anything.

>
> Such a video BIOS would probably have to be BSD-licensed, since the
> video BIOS code may wind up being #included into an OEM's system BIOS.

Yes, that is my plan.

I want to release all software under a BSD license (and MIT) so that it
can be used under that license or converted to GPL at the option of the
user.


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