linux-kernel - Re: [PATCH 2/2] ARM: dts: da850: add a node for the LCD controller

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Message-ID: <1615142.iysWriV8vb@avalon>
Date:   Mon, 17 Oct 2016 15:44:48 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Tomi Valkeinen <tomi.valkeinen@...com>
Cc:     Sekhar Nori <nsekhar@...com>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Kevin Hilman <khilman@...libre.com>,
        Michael Turquette <mturquette@...libre.com>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        Peter Ujfalusi <peter.ujfalusi@...com>,
        Russell King <linux@...linux.org.uk>,
        Karl Beldan <karl.beldan@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        arm-soc <linux-arm-kernel@...ts.infradead.org>,
        linux-drm <dri-devel@...ts.freedesktop.org>,
        linux-devicetree <devicetree@...r.kernel.org>,
        Jyri Sarha <jsarha@...com>, David Airlie <airlied@...ux.ie>,
        Maxime Ripard <maxime.ripard@...e-electrons.com>,
        Karl Beldan <kbeldan@...libre.com>
Subject: Re: [PATCH 2/2] ARM: dts: da850: add a node for the LCD controller

Hi Tomi,

On Monday 17 Oct 2016 15:29:23 Tomi Valkeinen wrote:
> On 17/10/16 14:40, Laurent Pinchart wrote:
> > On Monday 17 Oct 2016 10:33:58 Tomi Valkeinen wrote:
> >> On 17/10/16 10:12, Sekhar Nori wrote:
> >>> On Monday 17 October 2016 11:26 AM, Tomi Valkeinen wrote:
> >>>> On 15/10/16 20:42, Sekhar Nori wrote:
> >>>>>> diff --git a/arch/arm/boot/dts/da850.dtsi
> >>>>>> b/arch/arm/boot/dts/da850.dtsi
> >>>>>> index f79e1b9..32908ae 100644
> >>>>>> --- a/arch/arm/boot/dts/da850.dtsi
> >>>>>> +++ b/arch/arm/boot/dts/da850.dtsi
> >>>>>> @@ -399,6 +420,14 @@
> >>>>>>  				<&edma0 0 1>;
> >>>>>>  			dma-names = "tx", "rx";
> >>>>>>  		};
> >>>>>> +
> >>>>>> +		display: display@...000 {
> >>>>>> +			compatible = "ti,am33xx-tilcdc", "ti,da850-
tilcdc";
> >>>>> 
> >>>>> This should instead be:
> >>>>> 
> >>>>> compatible = "ti,da850-tilcdc", "ti,am33xx-tilcdc";
> >>>>> 
> >>>>> as the closest match should appear first in the list.
> >>>> 
> >>>> Actually I don't think that's correct. The LCDC on da850 is not
> >>>> compatible with the LCDC on AM335x. I think it should be just
> >>>> "ti,da850-tilcdc".
> >>> 
> >>> So if "ti,am33xx-tilcdc" is used, the display wont work at all? If thats
> >>> the case, I wonder how the patch passed testing. Bartosz?
> >> 
> >> AM3 has "version 2" of LCDC, whereas DA850 is v1. They are quite
> >> similar, but different.
> >> 
> >> The driver gets the version number from LCDC's register, and acts based
> >> on that, so afaik the compatible string doesn't really affect the
> >> functionality (as long as it matches).
> >> 
> >> But even if it works with the current driver, I don't think
> >> "ti,am33xx-tilcdc" and "ti,da850-tilcdc" are compatible in the HW level.
> > 
> > If the hardware provides IP revision information, how about just "ti,lcdc"
> > ?
>
> Maybe, and I agree that's the "correct" way, but looking at the history,
> it's not just once or twice when we've suddenly found out some
> difference or bug or such in an IP revision, or the integration to a
> SoC, that can't be found based on the IP revision.
> 
> That's why I feel it's usually safer to have the SoC revision there in
> the compatible string.
> 
> That said, we have only a few different old SoCs with LCDC (compared to,
> say, OMAP DSS) so in this case perhaps just "ti,lcdc" would be fine.

You obviously know more than I do on this topic so I'll trust your opinion. If 
the version register isn't enough I'm fine with multiple compatible strings.

-- 
Regards,

Laurent Pinchart