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

[Boris BREZILLON <boris.brezillon@free-electrons.com>]

Hi Laurent,

On Thu, 10 Jul 2014 13:16:21 +0200
Laurent Pinchart <laurent.pinchart@ideasonboard.com> wrote:

> Hi Boris,
> 
> Thank you for the patch.
> 
> 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@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.

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).

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) ?

> 
> > + - 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.

Thanks for your review.

Best Regards,

Boris

-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com