[PATCH 1/3] video: add support for getting video mode from device tree

[Grant Likely]

On Wed, Apr 28, 2010 at 7:43 AM, Anatolij Gustschin <agust at denx.de> wrote:
> On Mon, 01 Mar 2010 14:45:20 +1100
> Benjamin Herrenschmidt <benh at kernel.crashing.org> wrote:
>
>> On Sat, 2010-02-27 at 22:44 -1000, Mitch Bradley wrote:
>>
>> > As it turns out, I'm doing exactly that - exporting verbatim EDID
>> > data
>> > as the value of the "edid" property - for the display node on the Via
>> > version of the OLPC machine.  The kernel driver uses it instead of
>> > trying to obtain the EDID data from the monitor, because the builtin
>> > OLPC display cannot supply EDID data through the usual hardware
>> > interfaces.
>>
>> This is actually a common practice (though EDID is most often in
>> uppercase) on Apple hardware too. It has issues though in the sense that
>> it doesn't carry proper connector information and falls over in many
>> multi-head cases.
>>
>> I think passing the EDID data, when available, is thus the right thing
>> to do indeed, however that doesn't solve two problems:
>>
>>  - Where to put that property ? This is a complicated problem and we
>> might argue on a binding for weeks because video cards typically support
>> multiple outputs, etc. etc... I think the best for now is to stick as
>> closely as possible to the existing OF fb binding, and have "display"
>> nodes for each output, which can eventually contain an EDID. We might
>> also want to add a string that indicate the connector type. Specific
>> drivers might want to define additional properties here where it makes
>> sense such as binding of those outputs to CRTCs or such, up to you.
>
> Putting EDID to display node would be really sufficient for LCDs in
> our case. Other systems might define this additional connector type
> property.
>
>>
>>  - We -also- want a way to specify the "default" mode as set by the
>> firmware, at least on some devices. The EDID gives capabilities, and
>> often for LCDs also the "preferred" mode which is almost always the
>> "default" mode ... but could be different. In order to avoid "flicking",
>> the driver might wants to know what is the currently programmed mode.
>> For that, having split out properties makes sense, though I would like
>> to either prefix them all with "mode," or stick them in a sub-node of
>> the display at .
>
> I would propose defining following properties in the case the
> programmed mode is different from "default" mode:
>
> display at ...{
>        compatible = "<vendor>,<model>"
>        EDID = [edid-data];
>
>        current-mode {
>                pixel_clock = <value>;
>                horizontal_active = <value>;
>                horizontal_blank = <value>;
>                vertical_active = <value>;
>                vertical_blank = <value>;
>                horizontal_active = <value>;
>                hsync_offset = <value>;
>                hsync_pulse_width = <value>;
>                vsync_offset = <value>;
>                vsync_pulse_width = <value>;
>                hsync_positive;
>                vsync_positive;
>        }
> };
>
> The firmware can set the "default" mode using the EDID's preferred
> Detailed Timing Descriptor data. If on some devices it sets another
> mode than the preferred mode, then the firmware can insert a
> "current-mode" sub-node with currently programmed mode. The driver
> can check for this sub-node and use it's data and if it isn't present,
> it can use the preferred timing data from EDID. The names of the
> properties here are actually what Detailed Timing Descriptor in EDID
> specifies. What do you think?

If you really want to do that, then I think it is okay.  I really
don't know enough about the problem space to say whether or not that
particular description is a good binding or not, but regardless it
should be a video-controller-specific binding.  The name of the node
should probably be prefixed with "<manufacturer>," and it should be
documented along with the display controller's device tree binding.
If another controller wants/needs a different binding format (for the
current mode), then that is fine too (unless you can make a really
good argument that this current-mode binding is perfect and no other
layout should ever be required).  :-)

Cheers,
g.

>
> Thanks,
> Anatolij
>



-- 
Grant Likely, B.Sc., P.Eng.
Secret Lab Technologies Ltd.