Pegatron BYT-T1 eDP/LVDS resolution problem (VBT vs EDID)

Pegatron BYT-T1 eDP/LVDS resolution problem (VBT vs EDID)

[Edgardo Gho]

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Hi,
I have an AIO computer with a Pegatron BYT-T1 motherboard. Looks like a 
cheap motherboard (using J1900).
It has an innolux 1366x768 panel (LVDS). The Motherboard also has LVDS 
written on the connector.
I'm running kernel 4.0.5 on it (tinycore custom kernel) and xrandr 
displays the port as eDP1 (not LVDS).
 From what I can tell the panel is LVDS so I'm a little confused why the 
kernel picks it up as eDP. The CPU is valleyview.

The problem I have is this:
When drm/i915 reads the EDID on eDP1, it gets a bunch of stuff 
(including 1920x1080) and sets the resolution to that.
But the panel supports 1366x768 as max. That seems to be saved on VBT 
since if I use vga=0x37f I get 1366x768 on fb.
But when drm reads 1920x1080 from EDID it sets that and the panel goes 
into a weird mode (displaying colors) which I believe is a testing mode.

I could try to fix the EDID, and also could generate one and use the 
drm_kms_helper to pass a binary EDID.
What I don't understand is where is this EDID coming from. LVDS supports 
EDID read? I don't see any i2c cable on the LVDS cable so I don't 
understand how it can be reading the EDID.
Is the EDID saved on BIOS somehow?

I changed the code as suggested by this thread:
VLV: eDP: panel timings / resolution data from VBT,    not via i2c from 
eDP EDID
http://www.spinics.net/lists/intel-gfx/msg64312.html

and I'm not letting edp read the EDID, so it fallbacks to VBT and 
everything works great.
But I'm still not convinced that the EDID data comes from the LVDS Panel 
(since it should have different info)

Hope anyone can share some insights on what is going on.
Thanks

-- 
Edgardo Gho
edgardo.g@telikin.com


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Re: Pegatron BYT-T1 eDP/LVDS resolution problem (VBT vs EDID)

[Jani Nikula]

On Thu, 18 Jun 2015, Edgardo Gho <edgardo.g@telikin.com> wrote:
> Hi,
> I have an AIO computer with a Pegatron BYT-T1 motherboard. Looks like a 
> cheap motherboard (using J1900).
> It has an innolux 1366x768 panel (LVDS). The Motherboard also has LVDS 
> written on the connector.
> I'm running kernel 4.0.5 on it (tinycore custom kernel) and xrandr 
> displays the port as eDP1 (not LVDS).
>  From what I can tell the panel is LVDS so I'm a little confused why the 
> kernel picks it up as eDP. The CPU is valleyview.

This would be because BYT does not have LVDS, and apparently your
motherboard has an eDP->LVDS adapter.

> The problem I have is this:
> When drm/i915 reads the EDID on eDP1, it gets a bunch of stuff 
> (including 1920x1080) and sets the resolution to that.
> But the panel supports 1366x768 as max. That seems to be saved on VBT 
> since if I use vga=0x37f I get 1366x768 on fb.
> But when drm reads 1920x1080 from EDID it sets that and the panel goes 
> into a weird mode (displaying colors) which I believe is a testing mode.
>
> I could try to fix the EDID, and also could generate one and use the 
> drm_kms_helper to pass a binary EDID.
> What I don't understand is where is this EDID coming from. LVDS supports 
> EDID read? I don't see any i2c cable on the LVDS cable so I don't 
> understand how it can be reading the EDID.
> Is the EDID saved on BIOS somehow?

I assume the eDP->LVDS adapter responds to the EDID reads. The problem
is, apparently it reports a preferred mode that the LVDS does not
support, and our driver insists on using only the preferred mode for
eDP.

> I changed the code as suggested by this thread:
> VLV: eDP: panel timings / resolution data from VBT,    not via i2c from 
> eDP EDID
> http://www.spinics.net/lists/intel-gfx/msg64312.html
>
> and I'm not letting edp read the EDID, so it fallbacks to VBT and 
> everything works great.
> But I'm still not convinced that the EDID data comes from the LVDS Panel 
> (since it should have different info)
>
> Hope anyone can share some insights on what is going on.
> Thanks

BR,
Jani.


-- 
Jani Nikula, Intel Open Source Technology Center
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