[RESEND PATCH v3 06/11] drm: add DT bindings documentation for atmel-hlcdc-dc driver

[Laurent Pinchart]

Hi Thierry,

On Tuesday 15 July 2014 12:52:54 Thierry Reding wrote:
> On Tue, Jul 15, 2014 at 12:43:02PM +0200, Laurent Pinchart wrote:
> > On Tuesday 15 July 2014 12:37:19 Thierry Reding wrote:
> >> On Tue, Jul 15, 2014 at 12:20:02PM +0200, Laurent Pinchart wrote:
> >>> On Tuesday 15 July 2014 12:06:19 Boris BREZILLON wrote:
> >>>> On Mon, 14 Jul 2014 12:05:43 +0200 Thierry Reding wrote:
> >>>>> On Mon, Jul 07, 2014 at 06:42:59PM +0200, Boris BREZILLON wrote:
> >>>>>> The Atmel HLCDC (HLCD Controller) IP available on some Atmel SoCs
> >>>>>> (i.e. at91sam9n12, at91sam9x5 family or sama5d3 family) provides a
> >>>>>> display controller device.
> >>>>>> 
> >>>>>> The HLCDC block provides a single RGB output port, and only
> >>>>>> supports LCD panels connection to LCD panels for now.
> >>>>>> 
> >>>>>> The atmel,panel property link the HLCDC RGB output with the LCD
> >>>>>> panel connected on this port (note that the HLCDC RGB connector
> >>>>>> implementation makes use of the DRM panel framework).
> >>>>>> 
> >>>>>> Connection to other external devices (DRM bridges) might be added
> >>>>>> later by mean of a new atmel,xxx (atmel,bridge) property.
> >>>>>> 
> >>>>>> Signed-off-by: Boris BREZILLON <boris.brezillon at free-electrons.com>
> >>>>>> ---
> >>>>>> 
> >>>>>>  .../devicetree/bindings/drm/atmel-hlcdc-dc.txt     | 59 ++++++++++
> >>>>>>  1 file changed, 59 insertions(+)
> >>>>>>  create mode 100644
> >>>>>>  Documentation/devicetree/bindings/drm/atmel-hlcdc-dc.txt
> >>> 
> >>> [snip]
> >>> 
> >>>>>> + - atmel,panel: Should contain a phandle with 2 parameters.
> >>>>>> +   The first cell is a phandle to a DRM panel device
> >>>>>> +   The second cell encodes the RGB mode, which can take the
> >>>>>> following values:
> >>>>>> +   * 0: RGB444
> >>>>>> +   * 1: RGB565
> >>>>>> +   * 2: RGB666
> >>>>>> +   * 3: RGB888
> >>>>> 
> >>>>> These are properties of the panel and should be obtained from the
> >>>> panel directly rather than an additional cell in this specifier.
> >>>> 
> >>>> Okay.
> >>>> What's the preferred way of doing this ?
> >>>> What about defining an rgb-mode property in the panel node.
> >>> 
> >>> You could do that, but it won't help you much, as the HLCDC driver
> >>> must not parse properties from the panel node. You should instead
> >>> extend the drm_panel API with a function to retrieve panel properties.
> >>> The HLCDC driver will then query the panel driver at runtime for the
> >>> interface type. The panel driver will get the information from
> >>> hardcoded data in the driver, from DT or possibly in some cases by
> >>> querying the panel hardware directly.
> >> 
> >> My preference for this would be that we either add this to some existing
> >> structure (struct drm_display_info seems like a good candidate) or if
> >> the number of parameters grows out of hands, then maybe even introduce a
> >> new type of device that's specific for the interface. DRM panels are an
> >> abstraction for panels, that is, interface-agnostic, and if we start
> >> exposing interface specific parameters things will start to become very
> >> unwieldy.
> > 
> > I agree with the goal of keeping drm_panel interface-agnostic. However,
> > one way or another, interface parameters will need to be communicated
> > between the panel driver and the controller driver. My preference, if we
> > need to extend the number and/or scope of parameters beyond what
> > drm_display_info could reasonably contain, would be to implement a new
> > drm_panel operation to query/configure interface parameters, using a
> > structure that contains the interface type and a union of type-specific
> > structures. This would keep the API generic in the sense of not requiring
> > explicit knowledge of all interfaces in the drivers, while offering the
> > flexibility we need with a way to easily detect the interface type at
> > runtime and react on unknown/unsupported types.
>
> That's exactly what I was hoping could be avoided. If instead we modeled
> the interface type as a bus, we could for example have an lvds_bus along
> with an lvds_device and then use that as the natural place to store
> these properties. Much like we do for DSI.

And I believe that's what we should avoid ;-) First of all, let's not forget 
that Linux models control busses, not data busses. DSI is a special case as it 
combines the control and data busses, but in the general case the same 
implementation isn't possible. An LVDS panel controlled through I2C needs to 
be an I2C device sitting on an I2C bus.

Then, I believe it would make all drivers simpler if we had a single object 
type to deal with, with proper abstractions for bus types. A drm_panel that 
can model panels regardless of the data bus type, with one operation that 
conveys bus-specific information, makes storing the objects and communicating 
with them simpler than having to deal with different kind of devices.

-- 
Regards,

Laurent Pinchart
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