Re: [PATCHv15 3/7] video: add of helper for display timings/videomode

[Steffen Trumtrar <s.trumtrar@pengutronix.de>]

Hi,

On Fri, Dec 07, 2012 at 03:12:48PM +0100, Philipp Zabel wrote:
> Hi,
> 
> Am Montag, den 26.11.2012, 18:56 +0200 schrieb Tomi Valkeinen:
> > On 2012-11-26 18:10, Steffen Trumtrar wrote:
> > > Hi,
> > > 
> > > On Mon, Nov 26, 2012 at 04:38:36PM +0200, Tomi Valkeinen wrote:
> > 
> > >>> +optional properties:
> > >>> + - hsync-active: hsync pulse is active low/high/ignored
> > >>> + - vsync-active: vsync pulse is active low/high/ignored
> > >>> + - de-active: data-enable pulse is active low/high/ignored
> > >>> + - pixelclk-inverted: pixelclock is inverted (active on falling edge)/
> > >>> +				non-inverted (active on rising edge)/
> > >>> +				     ignored (ignore property)
> > >>
> > >> I think hsync-active and vsync-active are clear, and commonly used, and
> > >> they are used for both drm and fb mode conversions in later patches.
> > >>
> > >> de-active is not used in drm and fb mode conversions, but I think it's
> > >> also clear.
> > >>
> > >> pixelclk-inverted is not used in the mode conversions. It's also a bit
> > >> unclear to me. What does it mean that pix clock is "active on rising
> > >> edge"? The pixel data is driven on rising edge? How about the sync
> > >> signals and DE, when are they driven? Does your HW have any settings
> > >> related to those?
> > >>
> > > 
> > > Those are properties commonly found in display specs. That is why they are here.
> > > If the GPU does not support the property it can be omitted.
> > 
> > So what does the pixelclk-inverted mean? Normally the SoC drives pixel
> > data on rising edge, and the panel samples it at falling edge? And
> > vice-versa for inverted? Or the other way around?
> >
> > When is hsync/vsync set? On rising or falling edge of pclk?
> >
> > My point here is that the pixelclk-inverted is not crystal clear thing,
> > like the hsync/vsync/de-active values are.
> >
> > And while thinking about this, I realized that the meaning of
> > pixelclk-inverted depends on what component is it applied to. Presuming
> > normal pixclk means "pixel data on rising edge", the meaning of that
> > depends on do we consider the SoC or the panel. The panel needs to
> > sample the data on the other edge from the one the SoC uses to drive the
> > data.
> > 
> > Does the videomode describe the panel, or does it describe the settings
> > programmed to the SoC?
> 
> How about calling this property pixelclk-active, active high meaning
> driving pixel data on rising edges and sampling on falling edges (the
> pixel clock is high between driving and sampling the data), and active
> low meaning driving on falling edges and sampling on rising edges?
> It is the same from the SoC perspective and from the panel perspective,
> and it mirrors the usage of the other *-active properties.
> 

I think, this would not be a bad idea. I would include Philipps description in the
display-timing.txt, as it makes the meaning pretty clear; at least to me.

What do the others think about this?

Regards,
Steffen

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