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

[Laurent Pinchart]

Hi Boris,

On Thursday 10 July 2014 14:56:26 Boris BREZILLON wrote:
> On Thu, 10 Jul 2014 13:16:21 +0200 Laurent Pinchart wrote:
> > On Monday 07 July 2014 18:42:59 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
> > > 
> > > diff --git a/Documentation/devicetree/bindings/drm/atmel-hlcdc-dc.txt
> > > b/Documentation/devicetree/bindings/drm/atmel-hlcdc-dc.txt new file mode
> > > 100644
> > > index 0000000..594bdb2
> > > --- /dev/null
> > > +++ b/Documentation/devicetree/bindings/drm/atmel-hlcdc-dc.txt
> > > @@ -0,0 +1,59 @@
> > > +Device-Tree bindings for Atmel's HLCDC (High LCD Controller) DRM driver
> > > +
> > > +The Atmel HLCDC Display Controller is subdevice of the HLCDC MFD
> > > device.
> > > +See Documentation/devicetree/bindings/mfd/atmel-hlcdc.txt for more
> > > details.
> > > +
> > > +Required properties:
> > > + - compatible: value should be one of the following:
> > > +   "atmel,hlcdc-dc"
> > > + - interrupts: the HLCDC interrupt definition
> > > + - pinctrl-names: the pin control state names. Should contain
> > > "default",
> > > +   "rgb-444", "rgb-565", "rgb-666" and "rgb-888".
> > > + - pinctrl-[0-4]: should contain the pinctrl states described by
> > > pinctrl
> > > +   names.
> > 
> > Do you need to switch between the different pinctrl configurations at
> > runtime, or is the configuration selected from the panel type, which
> > doesn't change ?
>
> At the moment no, but if we ever need to support different devices on
> the same RGB connector (actually Atmel's sama5d3xek boards have an
> RGB to HDMI bridge connected on the same RGB connector) and these
> devices do not support the same RGB mode (say your LCD panel supports
> RGB888 and your RGB to HDMI bridge supports RGB555), then depending on
> the output you select you'll have to change your pinctrl config at
> runtime.

Just to make sure I understand the use case correctly, are you talking about 
two devices (for example an RGB666 panel and an RGB888 RGB to HDMI bridge) 
connected to the same output, with the ability to switch between the two at 
runtime ? That's a valid case (on a side note we shouldn't forget that the 
option of using both devices at the same time should be supported as well), 
but I would probably go for a fixed pinctrl configuration that supports both, 
although switching configurations at runtime would be a micro-optimization 
that might make sense.

> I'd say we could get rid of this runtime pinctrl config as a first step
> if DT ABI stability was not required.
> But it is, and I'd like to have a future proof binding to handle these
> tricky cases when they occurs (if they ever do).

I think we have a shortcoming of the pinctrl API here in the general case. The 
API only allows you to select a single configuration per device. Imagine the 
same display controller, with two DPI outputs, each of them configurable in 
444, 565, 666 or 888 modes. With the current API we would have to create 4*4 = 
16 pinctrl configurations for all combinations. That obviously wouldn't scale, 
so we'll have to fix this eventually. From a DT stability point of view, I 
would thus avoid specifying multiple pinctrl configurations now until we come 
up with a standard way to support this use case.

> Anyway, I'm open to any other alternative that could let me add support
> for this later on.
> 
> BTW, is there any reason for not defining an RGB connector type (I'm
> currently defining HLCDC connector as an LVDS connector) ?

Not that I know of. The DRM API has been developed before display on embedded 
systems became such a hot topic. If we had to redo it today, panels might be 
exposed to userspace as such, with a connector. We have to live with the past, 
so the connector will stay, but adding a new RGB connector type could make 
sense (although we might need a different name, in a way the VGA and LVDS 
connectors also carry RGB signals).

> > > + - 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
> > > +   The third cell encodes specific flags describing LCD signals
> > > configuration
> > > +   (see Atmel's datasheet for a full description of these
> > > fields):
> > > +   * bit 0: HSPOL: Horizontal Synchronization Pulse Polarity
> > > +   * bit 1: VSPOL: Vertical Synchronization Pulse Polarity
> > > +   * bit 2: VSPDLYS: Vertical Synchronization Pulse Start
> > > +   * bit 3: VSPDLYE: Vertical Synchronization Pulse End
> > > +   * bit 4: DISPPOL: Display Signal Polarity
> > > +   * bit 7: DISPDLY: LCD Controller Display Power Signal
> > > Synchronization
> > > +   * bit 12: VSPSU: LCD Controller Vertical synchronization Pulse Setup
> > > Configuration
> > > +   * bit 13: VSPHO: LCD Controller Vertical synchronization Pulse Hold
> > > Configuration
> > > +   * bit 16-20: GUARDTIME: LCD DISPLAY Guard Time
> > 
> > If I'm not mistaken, those are HLCDC configuration values that depend on
> > the panel type and characteristics. Shouldn't they then be queries from
> > the panel through the drm_panel API at runtime instead of being specified
> > in DT ? This would likely require extending the drm_panel API.
> 
> HSPOL and VSPOL can be deduced from DRM_MODE_FLAG_[PN]HSYNC and
> DRM_MODE_FLAG_[PN]VSYNC, I'm not sure for the other flags or the
> GUARDTIME value.
> 
> Another question I had regarding these flags is whether they were LCD
> panel specific or a mix of panel and board implementation.
> Take the VSYNC HSYNC polarity, of course the LCD panel defines what it
> expects in terms of polarity, but nothing prevents the HW designer from
> inverting the VSYNC/HSYNC polarity (expect common sense :-)), right ?
>
> A solution would be to override some drm_display_mode settings with
> informations taken from the DT.

Given that I gave the exact same argument during a V4L2 DT bindings design 
review, I can only agree :-) It thus makes sense to specify polarities in the 
HLCDC DT node. The RGB mode, however, should probably be queried from the 
panel, as I don't expect it to be board-dependent but only panel-dependent.

I'm not sure about the other bits in the third cell, maybe we should discuss 
them in more details. I'm always wary when I see DT bindings referring to a 
datasheet :-) Getting the information from the panel by default, with a 
possible override, is an interesting option. You would thus likely need 
several DT properties associated with each connection to a panel. Would it 
then make sense to use the OF graph DT bindings instead of the atmel,panel 
property to specify connections ? You could store per-connection data in the 
endpoint and/or port nodes.

> Thanks for your review.

You're welcome. Sorry for not having had time to review the driver itself. 
Given my limited bandwidth at the moment I've decided to concentrate on the DT 
bindings first.

-- 
Regards,

Laurent Pinchart