Re: Using LVDS in a iMX6Q/D from Barebox

[Sascha Hauer <s.hauer@pengutronix.de>]

On Tue, Feb 14, 2017 at 11:32:44AM +0100, gianluca wrote:
> On 02/10/2017 08:35 AM, Sascha Hauer wrote:
> > Hi Gianluca,
> > 
> > On Thu, Feb 09, 2017 at 03:37:41PM +0100, gianluca wrote:
> > > Hello,
> > > I would like to know if there is a clear way on using the lvds pins to drive
> > > a LVDS display in a custom made board, based on iMX6Q (in the near future
> > > the iMX6Dual).
> > 
> > I think what you are looking for is of_device_enable_and_register() or
> > of_device_enable_and_register_by_name(). You can call it on either the
> > lvds device node or the hdmi device node, depending on whether you found
> > an EEPROM or not.
> > 
> 
> I think this is not necessary as during boot I can see clearly:
> 
> > imx-ipuv3 2400000.ipu: IPUv3H probed
> > imx-ipuv3 2800000.ipu: IPUv3H probed
> > imx-ldb ldb.10: probe failed: Invalid argument
> > imx-hdmi 120000.hdmi: Detected HDMI controller 0x13:0xa:0xa0:0xc1
> 
> So barebox is trying to bring-up the imx-ldb but fails.
> 
> So I suppose something is wrong in my device tree structure.

Nope, there's everything right.

I can reproduce this here on a GuF Santaro board. LVDS works fine on
current barebox as long as I leave hdmi disabled in the device tree.
When I enable hdmi I also get a "failed to get modes".

Normally when different outputs are available then one would expect
that each one of them is routed to some framebuffer, maybe with some
sane default and configurable during runtime. The current code falls
short here.
What happens is each framebuffer looks around for outputs, all finding
hdmi and lvds. Now the hdmi output may not have a monitor connected
and returns no valid mode. In this case the lvds is not even asked for
modes.
Also in the current code it can happen that the lvds is asked for modes
which are then applied to hdmi.
Where we currently are is that it should work when exactly one output
is enabled. Does lvds work properly when hdmi is disabled? In this
case I would suggest that you manipulate your devicetree so that exactly
one output is enabled.

Sascha

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