[libre-riscv-dev] Fall 2022 Interfaces

Luke Kenneth Casson Leighton lkcl at lkcl.net
Fri Jun 12 12:03:19 BST 2020

On Friday, June 12, 2020, Jacob Lifshay <programmerjake at gmail.com> wrote:

> On Thu, Jun 11, 2020, 19:59 Yehowshua <yimmanuel3 at gatech.edu> wrote:
> > Was talking with Pine64 and Raptor about Fall2022 interfaces which I’m
> > trying to finalize.
> > We can do everything including HDMI, ethernet, DDR4(via OpenCAPI OMI),
> and
> > PCIE with SERDES lanes.
> >
> hdmi has issues with libre licensing and DRM content,

indeed.  DVI however does not (and is pin compatible).  HDMI is just a
protocol over DVI.

>  if we're going that
> fast, we should just go with display port or similar, since it's an open
> standard, I think.

i added Richard Herveille's RGBTTL to the list.  a TI TFP410a will convert
to at least 1600 x 1200 DVI no problem.

the TFP410a is a well known solution.

if we make it DDR RGBTTL then 1920x1080 is also no problem, via some
Chrontel ASICs.

> > We need 10rx and 14tx SERDES.

that is one hell of a lot.

> > 4rx and 4tx for the OMI will need to run at 25GHz for DDR4 over OMI.

that may not be enough to handle framebuffers as well as run an OS.
framebuffers are scarily demanding of memory bandwidth and they absolutely
MUST have total absolute top priority.

you cannot have a video scan line go blank.  evverrr.

to give some idea: 1920 x 1080 is 2 million pixels.  each pixel is 4
bytes.   that is 64 million bytes per frame.  60 frames? that is 360
MEGABYTES per second.

DDR3 800 Mhz is only 6400 MB per second so we are looking at 20 percent of
memory bandwidth taken by feeding the framebuffer!!!!

if you want 4k that is now 1200 MEGABYTES per second just to feed the
frickin framebuffer!

doesn't even matter if it contains blank lines, that's 1.2 gigabytes per
second full of zeros!

> >
> 25GHz seems *really* fast, probably impractically so unless going to *lots*
> of effort, did you mean 2.5GHz?

no it really is 25GHz.

we will need Rudi from ASICS.ws for this.  he can handle that kind of
work.  when i went to visit him he was doing some SERDES PHY related work
at these speeds.

i do not know if 45nm can handle it, though.


crowd-funded eco-conscious hardware: https://www.crowdsupply.com/eoma68

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