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

Frieder Paape frieder at paape.io
Mon Jan 25 17:47:58 GMT 2021

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.

On 1/25/21 7:57 PM, Jacob Lifshay wrote:
> On Mon, Jan 25, 2021, 10:23 Luke Kenneth Casson Leighton <lkcl at lkcl.net>
> wrote:
>> On Monday, January 25, 2021, Jacob Lifshay <programmerjake at gmail.com>
>> wrote:
>>> We should seriously consider using DisplayPort instead of HDMI, it
>> supports
>>> nearly all the same features, can be implemented without requiring
>>> closed-source firmware since HDCP is optional, and supports higher
>>> resolutions and other more modern stuff than DVI.
>> this is some good research.
> well, it's mostly just a brain dump, I didn't research anything today.
>> DVI is directly pin-compatible with HDMI, it is the broken/braindead HDCP,
>> transferred over the DVI wires, that "makes" it HDMI.
>> i have absolutely no problem with leaving out HDCP, or at least providing a
>> way for software implementations of it.
>> the problem with leaving out HDMI:
>> * you can't use TVs as monitors
> * some TVs esp. at higher resolutions flatly refuse to communicate DVI-only
> DVI support is a requirement of the HDMI spec, so they should support it.
> DVI itself is limited though.
>> the problem with leaving out both HDMI and DVI:
>> * eDP is nowhere near the adoption rate
> 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.
>> the problem with eDP:
>> * it'a nowhere near as ubiquitous as say LVDS, RGBTTL or even MIPI.
> 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.
>> much as i despise MIPI it has low-power benefits for this SoC.
>>> DVI:
>>> * simple/cheap
>>> * patent licensing is free
>>> * max resolution is 1920x1200 at 60Hz, 1080p60 is supported
>>> * no audio
>>> * no variable refresh rate (though we might be able to get it to work by
>>> breaking the spec).
>> i have not confirmed it but i believe this may not be quite correct: the
>> I2C channel transfers EDID data just like with VGA. add that I2C channel
>> and i would be very surprised if it didn't just "work".
> well, DVI and HDMI and DisplayPort all have/require the I2C channel, I was
> assuming we'd implement it. I've never heard of variable refresh rate
> working in DVI mode, though. The display would probably just send different
> compatibility EDID data when connecting using DVI.
>>> * no high bit-depth
>>> * forward compatible with HDMI (can run over same HDMI physical
>> connector,
>>> you can get wires-only cheap (~$5) adaptors to physical DVI-D connector
>> for
>>> old monitors).
>>> DisplayPort:
>>> * HDCP optional
>>> * patent licensing is free
>>> * supports >4K, high refresh rate, variable refresh rate (important for
>>> power-saving and games), high bit-depth (HDR and better color gamut than
>>> sRGB).
>>> * supports audio output
>>> * works over USB type-C
>>> * the modern standard for computer monitors
>>> So, I think we should have 1 DisplayPort output (perhaps shared with USB
>>> type-C) if we're licensing someone elses design, as well as 1 DVI-D over
>> a
>>> HDMI connector for physical compatibility with TVs that only support HDMI
>>> and older monitors that only support DVI.
>> i agree this is sane / sensible / workable. i have no idea how to cost it
>> up though.
>> bear in mind though that SoCs are RADICALLY different from "The Average
>> Intel quotes mobile quotes processor" where they think 8 Amps @ 0.6 v is
>> "low".
> DisplayPort is a supported output in many modern cell phones via usb-c, so
> the power draw can't be that awful...
>> power draw and raw data bandwidth for 4k monitors is MENTAL.
> yup, hence why we might recommend only using 1080p or 1440p for the 1x32
> DDR3/4.
>> i found a datasheet online for DDR4 that estimated power consumption for
>> "high performance" (lots of FIFOs, 8x AXI4 Buses) DDR4 and it was ONE WATT.
>> that's insane: it's 30% of the entire power budget for the entire SoC.
>> we likewise have to be extremely careful about the interface selection
>> itself, to ensure that driving eDP (which unlike MIPI was never designed
>> for low-power scenarios) is not overloading the power budget.
> yup, hence why we can use a different interface for internal display for
> very-low-power scenarios, if needed. Variable refresh rate also helps.
> Jacob
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