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

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Date:	Tue, 7 Jan 2014 20:00:23 +0530
From:	Ramkumar Ramachandra <artagnon@...il.com>
To:	Ramkumar Ramachandra <artagnon@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andreas Heider <andreas@...tr.de>,
	Chris Wilson <chris@...is-wilson.co.uk>
Subject: Re: [PATCH 0/4] Get EDID late for VGA switcheroo

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 (https://lkml.org/lkml/2012/8/4/128). 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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