Re: [PATCH 0/4] Get EDID late for VGA switcheroo

From: Ramkumar Ramachandra
Date: Tue Jan 07 2014 - 09:31:12 EST

Seth Forshee wrote:
> 1. On many Macbook models the firmware isn't putting LVDS information
> in opregion. Reading the EDID during device initialization also
> fails since the DDC is muxed to the DGPU, so i915 concludes there's
> no LVDS and doesn't register an LVDS connector at all. I also think
> it was impossible to register a new connector at a later time,
> though I'm not sure about that.

On a different thread, Chris is helping me write a series to fill in
the switcheroo-reprobe callback with corresponding get-edid functions.
Do you think it'll be useful?

> 2. If i915 is forced to register the LVDS connector (say via a quirk)
> it still may not know the panel mode and guesses something rather
> conservative. This mode it assumes is used to allocate framebuffer
> memory, which is likely to end up being too small, and I don't think
> there was any support for resizing it later. This resulted in VTs
> not working because there isn't enough framebuffer memory for the
> mode they're trying to use.

I saw your quirk-patch ( Seems
like a rather inelegant approach to register the LVDS connector
unconditionally during initialization; I'll try to avoid going down
this road.

> 3. So what is really wanted is to be able to mux just the DDC over to
> the IGPU when the device is initialized to read the panel mode. One
> possible problem is that the hw might not support muxing the DDC
> separately, but Apple hw does so we'll ignore that for now. The
> basic code for muxing only the DDC isn't too difficult, and I had
> some patches for this at one point. The problem is that the handler
> must register with vga_switcheroo before this is possible, so
> there's a module initialization ordering issue that wasn't easily
> solved. Maybe now it would be possible to do something with
> EPROBE_DEFER, though I'm not sure how i915 would know whether or not
> it should wait for a switcheroo handler.

I didn't understand much of the above; I suppose it'll start making
sense when I'm deeper into the issue.

> I just didn't have enough time to spend on this issue to work through
> all of this. I also never looked at doing this with eDP, which is what I
> thought all the Macbooks have been using for the last couple of years.
> Are you sure your machine uses LVDS and not eDP?

I just assumed that it was LVDS; turns out it's eDP.

I have time to work on the issue now, so I'd appreciate any help I can get.
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