On Thu, Sep 03, 2015 at 04:55:59PM -0500, Rob Herring wrote: > On Thu, Sep 3, 2015 at 3:47 AM, Thierry Reding <treding@xxxxxxxxxx> wrote: > > On Wed, Sep 02, 2015 at 03:17:57PM -0500, Rob Herring wrote: > > [...] > >> Are there any eDP panels which don't have EDID and need panel details in DT? > > > > Most panels need information other than EDID. They typically have some > > requirements regarding the power up sequence that aren't to be found > > anywhere in EDID or detectable by some other mechanism. A decision was > > therefore made a long time ago to require panels to be listed in DT with > > a specific compatible string. That way all of these details can be > > stashed away in drivers that know how to deal with these kinds of > > details. > > I guess I was being hopeful that eDP was improving that situation. Unfortunately not. eDP helps with a number of things (DPCD and link training take care of a lot of the issues), but it doesn't provide a solution for everything. To my knowledge in most cases the power up sequence isn't strictly necessary to get the panel to operate. But there have been instances where this is absolutely required if you want to avoid visual artifacts as you set a mode. A lot of panels that I've come across require receiving a couple of frames before they guarantee that something will actually be displayed on the screen. So if your code is too quickly enabling backlight, and your backlight doesn't happen to have just the right amount of ramp-up time you might end up seeing garbage on the screen for a couple of frames. It would be great if somebody came up with, say, an EDID extension to describe these requirements, but I'm not sure if even that would give us panels that could be driven with a generic driver. Often the power supplies or reset/enable signals are very different across panels. So describing it all generically would become messy rather quickly. Thierry
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