linux-kernel - Re: [RESEND PATCH 1/2] devicetree: mxsfb: add reset-active property

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Date:	Tue, 12 Jan 2016 14:44:12 +0200
From:	Tomi Valkeinen <tomi.valkeinen@...com>
To:	Mans Rullgard <mans@...sr.com>, Rob Herring <robh+dt@...nel.org>
CC:	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	<devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-fbdev@...r.kernel.org>
Subject: Re: [RESEND PATCH 1/2] devicetree: mxsfb: add reset-active property


On 12/01/16 12:22, Mans Rullgard wrote:
> Some boards connect the LCD_RESET pin to a reset input on the
> display panel.  On these boards, this pin must be set to the
> proper level for the display to function.
> 
> This adds an optional "reset-active" property to the "display"
> subnode such that devicetrees can specify the desired polarity
> of the LCD_RESET pin.
> 
> Signed-off-by: Mans Rullgard <mans@...sr.com>
> ---
>  Documentation/devicetree/bindings/display/mxsfb.txt | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/display/mxsfb.txt b/Documentation/devicetree/bindings/display/mxsfb.txt
> index 96ec5179c8a0..cb7212a6bdf2 100644
> --- a/Documentation/devicetree/bindings/display/mxsfb.txt
> +++ b/Documentation/devicetree/bindings/display/mxsfb.txt
> @@ -13,6 +13,11 @@ Required properties:
>  - bits-per-pixel : <16> for RGB565, <32> for RGB888/666.
>  - bus-width : number of data lines.  Could be <8>, <16>, <18> or <24>.
>  
> +Optional properties:
> +- reset-active : <0>: reset pin is active low
> +                 <1>: reset pin is active high
> +                 omitted: reset pin not used
> +
>  Required sub-node:
>  - display-timings : Refer to binding doc display-timing.txt for details.

So maybe this is fine for the mxsfb if it doesn't support any kind of
panel drivers, and there's no plan to extend it. Otherwise the LCD_RESET
pin could perhaps be exposed as a GPIO for the panel drivers.

But even so, I think the definition of "reset" is a bit vague.

I know panels for which "reset" is a pulse, you assert it for a short
period. Other panels take reset more like a on/off switch. If I'm not
mistaken, this one is the latter kind.

I don't see any sleeps related to reset in the code. If reset is
asserted when the display is blanked, is it guaranteed that the reset
stays asserted long enough until the display is enabled again?

 Tomi


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