udl vs udlfb and EDID trouble

[Alex Deucher]

On Thu, Aug 30, 2012 at 8:24 AM, Tvrtko Ursulin
<tvrtko.ursulin at onelan.co.uk> wrote:
>
> Hi all,
>
> I've been playing with some DisplayLink adapters, six of them to be precise
> connected to six identical HDMI monitors.
>
> Under kernel 3.5 things did not really work for me apparently because of the
> udl driver's inability to fetch EDID. Interestingly, logged EDID blocks were
> different for each of the six monitors which suggests a serious bug somewhere.
>
> I've then went to see if I can load my own EDID, because I remember doing it
> via /sys/class/graphics/fb?/edid before, but that file was not there.
> (Is it possible to override EDID with the udl driver?)
>
> To cut the long story short, I figured out there are two drivers for this
> hardware. Fedora based 3.5 kernel builds and uses udl, while the older 3.3.2
> I played with before builds and uses udlfb. (Why two drivers?)

udlfb was the original driver.  It's a basic kernel fb driver.  udl is
a drm driver that Dave wrote based on udlfb.  The drm variant will
eventually allow you to hotplug displaylink adapters in X and use
acceleration provided by another GPU.

>
> Now going back to 3.3.2 and udlfb, EDID got correctly fetched from all six
> monitors.

Sounds like a bug in the udl i2c code.  Probably worth comparing the
relevant udl and uldfb code; it may be a simple fix.

Alex