tvservice -n does not detect USB powered display name but dumps EDID correctly

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#1,105 6 comments 0 reactions 0 assignees View on GitHub

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
25/100
Issue type
Bug
Clarity
Needs clarification
Activity status
Stale
Tech stack
raspberry-pi
Domain
embedded-iot

Research direction

Start with config.txt, the bootcode_delay=10 workaround, and the tvservice -n versus tvservice -d behavior described in the report. Reproduce the USB-powered display setup and trace when USB power and EDID detection occur; done means the display name is detected and the [EDID=...] filter works without a separate PSU.

Written by the indexing model from the issue text.

Description

Hello,

I am working with a Raspberry Pi 3B+ and a 1024x600 touchscreen interfaced through HDMI and USB through a TI TFP401 decoder that needs a special config.txt line for it to work properly, as the TFP401 doesn't support scaling. I wanted to add an [EDID=...] filter in config.txt but to no avail as tvservice -n returns

[E] No device present

all the while EDID is read and parsed correctly through tvservice -d then decoded through edid-decode to validate correctness.

The display is powered by the Raspberry Pi itself through USB and this apparently is the same issue as reported in #401 but with the workaround of the bootcode_delay=10 entry in config.txt is not working as the USB hub is not powered (enough?) in bootcode apparently and it is therefore not reading the EDID.

Powering the display separately through a dedicated PSU makes everything work ok on the EDID side but since the display has a touchscreen and it is powered via the same USB cable as the touchscreen transmits data on, powering the display separately is not a very desirable option as it would entail not using the touchscreen at all.

Is there any chance that there could be a config.txt entry to early-power the USB in bootcode too, or at least power the USB before the delay is taken into consideration?
Alternately, would it be possible to move the EDID detect phase later in the bootcode, after the USB powering event?
Do you have any other suggestion to help with this?

Thank you!

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