[Libre-soc-dev] HDMI, DVI, DisplayPort on SBC (like Raspberry Pi) and on BMC

Luke Kenneth Casson Leighton lkcl at lkcl.net
Mon Jan 25 22:48:54 GMT 2021

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

On Mon, Jan 25, 2021 at 6:57 PM Jacob Lifshay <programmerjake at gmail.com> wrote:

> in desktop computers DisplayPort is near 100% adoption in new devices, been
> that way for a few years. In fact a few of the new video cards don't even
> support anything but DisplayPort iirc.


> Yeah, I was talking mostly about external video outputs (full DP, not eDP),
> having LVDS for board-internal display output is reasonable. We should try
> to pick an internal video protocol that supports variable refresh rate,
> since that is a major power saver in low-power devices. In fact, that's
> what variable refresh rate was originally designed for iirc.

ok so richard herveille's vga_lcd RTL does exactly this.  it's
*vastly* programmable, and the linux kernel source code i have for it
provides full access to all capabilities.

i've used the re-programmability to e.g. drive a 1366x768 LCD panel at
only 30fps in order to reduce noice.  that was via a SN75LVDS83b, and
on the IC3128 (where i got the vga_lcd kernel source from) i actually
put it into monochrome mode, and got it to do 1280x720, because the
designers of the IC3128 forgot to put enough Wishbone Bus bandwidth on
:)  it was only 400mhz.

that was all RGB/TTL.  and i would expect there to be a converter
(internally) between RGB/TTL and the outgoing PHYs.

> DisplayPort is a supported output in many modern cell phones via usb-c, so
> the power draw can't be that awful...

ahh so that's how they did it.

On Mon, Jan 25, 2021 at 7:12 PM Frieder Paape <frieder at paape.io> wrote:
> Is it not possible to just use a DP to HDMI adapter if one wanted to use
> HDMI? Of course that still does not solve the power concerns.

Chrontel have a boat-load of converters here.
TI TFP410a converts RGBTTL to DVI/HDMI
Solomon SSD2828 converts RGBTTL to MIPI
SN75LVDS83b converts RGBTTL to LVDS

there's a bunch of solutions, it's just that customers get irritated
by the extra cost of these converter ICs

On Mon, Jan 25, 2021 at 7:26 PM Frieder Paape <frieder at paape.io> wrote:
> You can find lots of laptops these days which don't ship with hdmi
> anymore, so I think it should definitely be considered to ditch this
> closed firmware port in a libre project.

that's probably because they have USB-C.  which we have to be very
careful there not to introduce *another* batch of closed firmware.


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