[Arm-netbook] MIPI DSI instead of RGB/TTL - the only way out of the blind alley

Luke Kenneth Casson Leighton lkcl at lkcl.net
Wed Dec 14 19:35:10 GMT 2016

On 12/14/16, Benson Mitchell <benson.mitchell+arm-netbook at gmail.com> wrote:
> Glad we're on the same page.


> On Wed, Dec 14, 2016 at 5:36 AM, Luke Kenneth Casson Leighton
> <lkcl at lkcl.net> wrote:
>>  * type II 5.0mm cards may expect housings to provide "upscaling"
>> support for resolutions beyond 1366x768, a notification of the full
>> list of available resolutions in the I2C EEPROM (in the form of DDC
>> data) and, hmmm.... we'll need some form of auto-detection or
>> standards-compliant means and method of communicating the desired
>> resolution.  hmmm....
> I finally found the documentation for display-timings in devicetree.
> As I suspected, it already supports an arbitrary number of modes, and
> specifying one of them as native/preferred.

 ah.  rrright.  okay.  two things:

 (1) the way it's going to have to work is: you read the EOMA68 I2C
EEPROM @ addr 0x51, get the housing "id", and from there *decide which
dtb fragments to load*.  there's a patch related to beagleboards and
so on which is being worked on that allows numbered pointer-references
in devicetree to be replaced with arbitrary pre-compiled binary dtb

 (2) getting the various settings isn't the problem (load DDC data
over I2C like you would from any VESA-compliant monitor), picking the
preferred one according to the devicetree specification isn't the
problem, it's *TELLING* the hardware which one *HAS* been picked
that's the problem.

 now, if this was HDMI, it would not be a problem.  HDMI:
auto-detection is part of tthe protocol.  VGA?  not a problem (at
least with modern monitors) - auto-detection can be designed in.  but
RGB/TTL?  naah.  there's absolutely no meta-data.

 even in instances where LCDs contain DDC data, it's often completely
and utterly wrong, so you have to use it as a "guide", experiment,
then completely ignore it and hard-code the values that *WORKED* into
the linux kernel source / dts.

 now you have two possible (or potentially even more) resolutions to
pick from... how the hell do you tell the IC (whatever it is) which
one to use?

 no idea - so that needs to be resolved, first, before the EOMA68
standard is to be augmented.

 i'm not going to put something into the standard which hasn't at
least been thoroughly researched.


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