[PATCH 1/2 v6] of: add helper to parse display timings

Guennadi Liakhovetski g.liakhovetski at gmx.de
Mon Oct 8 01:25:04 PDT 2012

On Fri, 5 Oct 2012, Stephen Warren wrote:

> On 10/04/2012 03:35 PM, Guennadi Liakhovetski wrote:
> > Hi Steffen
> > 
> > Sorry for chiming in so late in the game, but I've long been wanting to 
> > have a look at this and compare with what we do for V4L2, so, this seems a 
> > great opportunity to me:-)
> > 
> > On Thu, 4 Oct 2012, Steffen Trumtrar wrote:
> >> diff --git a/Documentation/devicetree/bindings/video/display-timings.txt b/Documentation/devicetree/bindings/video/display-timings.txt
> >> +timings-subnode
> >> +---------------
> >> +
> >> +required properties:
> >> + - hactive, vactive: Display resolution
> >> + - hfront-porch, hback-porch, hsync-len: Horizontal Display timing parameters
> >> +   in pixels
> >> +   vfront-porch, vback-porch, vsync-len: Vertical display timing parameters in
> >> +   lines
> >> + - clock: displayclock in Hz
> > 
> > You're going to hate me for this, but eventually we want to actually 
> > reference clock objects in our DT bindings. For now, even if you don't 
> > want to actually add clock phandles and stuff here, I think, using the 
> > standard "clock-frequency" property would be much better!
> In a definition of a display timing, we will never need to use the clock
> binding; the clock binding would be used by the HW module that is
> generating a timing, not by the timing definition itself.

You mean clock consumer bindings will be in the display device DT node? 
And the display-timings node will be its child?

> That said, your comment about renaming the property to avoid any kind of
> conceptual conflict is still quite valid. This is bike-shedding, but
> "pixel-clock" might be more in line with typical video mode terminology,
> although there's certainly preference in DT for using the generic term
> clock-frequency that you proposed. Either is fine by me.
> >> +optional properties:
> >> + - hsync-active-high (bool): Hsync pulse is active high
> >> + - vsync-active-high (bool): Vsync pulse is active high
> > 
> > For the above two we also considered using bool properties but eventually 
> > settled down with integer ones:
> > 
> > - hsync-active = <1>
> > 
> > for active-high and 0 for active low. This has the added advantage of 
> > being able to omit this property in the .dts, which then doesn't mean, 
> > that the polarity is active low, but rather, that the hsync line is not 
> > used on this hardware. So, maybe it would be good to use the same binding 
> > here too?
> I agree. This also covers the case where analog display connectors often
> use polarity to differentiate similar modes, yet digital connectors
> often always use a fixed polarity since the receiving device can
> "measure" the signal in more complete ways.
> If the board HW inverts these lines, the same property names can exist
> in the display controller itself, and the two values XORd together to
> yield the final output polarity.
> >> + - de-active-high (bool): Data-Enable pulse is active high
> >> + - pixelclk-inverted (bool): pixelclock is inverted
> > 
> > We don't (yet) have a de-active property in V4L, don't know whether we'll 
> > ever have to distingsuish between what some datasheets call "HREF" and 
> > HSYNC in DT, but maybe similarly to the above an integer would be 
> > preferred. As for pixclk, we call the property "pclk-sample" and it's also 
> > an integer.
> Thinking about this more: de-active-high is likely to be a
> board-specific property and hence something in the display controller,
> not in the mode definition?
> >> + - interlaced (bool)
> > 
> > Is "interlaced" a property of the hardware, i.e. of the board? Can the 
> > same display controller on one board require interlaced data and on 
> > another board - progressive?
> Interlace is a property of a display mode. It's quite possible for a
> particular display controller to switch between interlace and
> progressive output at run-time. For example, reconfiguring the output
> between 480i, 720p, 1080i, 1080p modes. Admittedly, if you're talking to
> a built-in LCD display, you're probably always going to be driving the
> single mode required by the panel, and that mode will likely always be
> progressive. However, since this binding attempts to describe any
> display timing, I think we still need this property per mode.

But why do you need this in the DT then at all? If it's fixed, as required 
per display controller, then its driver will know it. If it's runtime 
configurable, then it's a purely software parameter and doesn't depend on 
the board?

> > BTW, I'm not very familiar with display 
> > interfaces, but for interlaced you probably sometimes use a field signal, 
> > whose polarity you also want to specify here? We use a "field-even-active" 
> > integer property for it.
> I think that's a property of the display controller itself, rather than
> an individual mode, although I'm not 100% certain. My assertion is that
> the physical interface that the display controller is driving will
> determine whether embedded or separate sync is used, and in the separate
> sync case, how the field signal is defined, and that all interlace modes
> driven over that interface will use the same field signal definition.

In general, I might be misunderstanding something, but don't we have to 
distinguish between 2 types of information about display timings: (1) is 
defined by the display controller requirements, is known to the display 
driver and doesn't need to be present in timings DT. We did have some of 
these parameters in board data previously, because we didn't have proper 
display controller drivers... (2) is board specific configuration, and is 
such it has to be present in DT.

In that way, doesn't "interlaced" belong to type (1) and thus doesn't need 
to be present in DT?

Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer

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