[PATCH V7 11/12] Documentation: bridge: Add documentation for ps8622 DT properties

[Thierry Reding]

On Mon, Sep 22, 2014 at 02:01:38PM +0530, Ajay kumar wrote:
> Hi Thierry,
> 
> On Mon, Sep 22, 2014 at 1:40 PM, Thierry Reding
> <thierry.reding at gmail.com> wrote:
> > On Thu, Sep 18, 2014 at 11:20:40AM +0530, Ajay kumar wrote:
> >> Hi Tomi,
> >>
> >> On Wed, Sep 17, 2014 at 9:52 PM, Tomi Valkeinen <tomi.valkeinen at ti.com> wrote:
> >> > On 17/09/14 17:29, Ajay kumar wrote:
> >> >> Hi Tomi,
> >> >>
> >> >> Thanks for your comments.
> >> >>
> >> >> On Wed, Sep 17, 2014 at 5:22 PM, Tomi Valkeinen <tomi.valkeinen at ti.com> wrote:
> >> >>> On 27/08/14 17:39, Ajay Kumar wrote:
> >> >>>> Add documentation for DT properties supported by ps8622/ps8625
> >> >>>> eDP-LVDS converter.
> >> >>>>
> >> >>>> Signed-off-by: Ajay Kumar <ajaykumar.rs at samsung.com>
> >> >>>> ---
> >> >>>>  .../devicetree/bindings/video/bridge/ps8622.txt    |   20 ++++++++++++++++++++
> >> >>>>  1 file changed, 20 insertions(+)
> >> >>>>  create mode 100644 Documentation/devicetree/bindings/video/bridge/ps8622.txt
> >> >>>>
> >> >>>> diff --git a/Documentation/devicetree/bindings/video/bridge/ps8622.txt b/Documentation/devicetree/bindings/video/bridge/ps8622.txt
> >> >>>> new file mode 100644
> >> >>>> index 0000000..0ec8172
> >> >>>> --- /dev/null
> >> >>>> +++ b/Documentation/devicetree/bindings/video/bridge/ps8622.txt
> >> >>>> @@ -0,0 +1,20 @@
> >> >>>> +ps8622-bridge bindings
> >> >>>> +
> >> >>>> +Required properties:
> >> >>>> +     - compatible: "parade,ps8622" or "parade,ps8625"
> >> >>>> +     - reg: first i2c address of the bridge
> >> >>>> +     - sleep-gpios: OF device-tree gpio specification for PD_ pin.
> >> >>>> +     - reset-gpios: OF device-tree gpio specification for RST_ pin.
> >> >>>> +
> >> >>>> +Optional properties:
> >> >>>> +     - lane-count: number of DP lanes to use
> >> >>>> +     - use-external-pwm: backlight will be controlled by an external PWM
> >> >>>
> >> >>> What does this mean? That the backlight support from ps8625 is not used?
> >> >>> If so, maybe "disable-pwm" or something?
> >> >> "use-external-pwm" or "disable-bridge-pwm" would be better.
> >> >
> >> > Well, the properties are about the bridge. "use-external-pwm" means that
> >> > the bridge uses an external PWM, which, if I understood correctly, is
> >> > not what the property is about.
> >> >
> >> > "disable-bridge-pwm" is ok, but the "bridge" there is extra. The
> >> > properties are about the bridge, so it's implicit.
> >> Ok. I will use "disable-pwm".
> >
> > Why is this even necessary? According to the datasheet this device has
> > circuitry for backlight control. If so, I'd expect it to expose either a
> > backlight device or a PWM device. That way unless somebody is using the
> > backlight/PWM exposed by the bridge the bridge can simply disable PWM.
> The driver does expose a backlight device.
> And, the decision(whether to expose a backlight device or not) is made
> based on the DT flag "use-external-pwm".
> This was discussed before, and you suggested to use the boolean
> property, refer to this link:
> http://lists.freedesktop.org/archives/dri-devel/2014-July/065048.html

I think you misunderstood what I said, or maybe I didn't explain clearly
what I meant. If the PS8622 provides a backlight there's nothing wrong
with always exposing it. The bridge itself isn't going to be using the
backlight anyway. Rather the panel itself should be using the backlight
device exposed by PS8622 or some separate backlight device.

To illustrate by an example:

	ps8622: ... {
		compatible = "parade,ps8622";
		...
	};

	panel {
		...
		backlight = <&ps8622>;
		...
	};

Or:

	backlight: ... {
		compatible = "pwm-backlight";
		...
	};

	panel {
		...
		backlight = <&backlight>;
		...
	};

What you did in v6 of this series was look up a backlight device and
then not use it. That seemed unnecessary. Looking at v6 again the reason
for getting a phandle to the backlight was so that the device itself did
not expose its own backlight controlling circuitry if an external one
was being used. But since the bridge has no business controlling the
backlight, having the backlight phandle in the bridge is not correct.

So I think what you could do in the driver instead is always expose the
backlight device. If the panel used a different backlight, the PS8622's
internal on simply wouldn't be accessed. It would still be possible to
control the backlight in sysfs, but that shouldn't be a problem (only
root can access it).

That said, I have no strong objections to the boolean property if you
feel like it's really necessary.

Thierry
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