linux-kernel - Re: [PATCH 5/8] arm64: dts: tegra186: Add SDMMC4 DQS trim value

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Date:   Thu, 9 Aug 2018 15:52:16 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Aapo Vienamo <avienamo@...dia.com>
Cc:     Ulf Hansson <ulf.hansson@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Mikko Perttunen <mperttunen@...dia.com>,
        linux-mmc@...r.kernel.org, devicetree@...r.kernel.org,
        linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/8] arm64: dts: tegra186: Add SDMMC4 DQS trim value

On Thu, Aug 09, 2018 at 03:02:26PM +0300, Aapo Vienamo wrote:
> On Thu, 9 Aug 2018 13:49:22 +0200
> Thierry Reding <thierry.reding@...il.com> wrote:
> 
> > On Tue, Aug 07, 2018 at 05:00:01PM +0300, Aapo Vienamo wrote:
> > > Add the HS400 DQS trim value for Tegra186 SDMMC4.
> > > 
> > > Signed-off-by: Aapo Vienamo <avienamo@...dia.com>
> > > ---
> > >  arch/arm64/boot/dts/nvidia/tegra186.dtsi | 1 +
> > >  1 file changed, 1 insertion(+)
> > > 
> > > diff --git a/arch/arm64/boot/dts/nvidia/tegra186.dtsi b/arch/arm64/boot/dts/nvidia/tegra186.dtsi
> > > index 6e9ef26..9e07bc6 100644
> > > --- a/arch/arm64/boot/dts/nvidia/tegra186.dtsi
> > > +++ b/arch/arm64/boot/dts/nvidia/tegra186.dtsi
> > > @@ -313,6 +313,7 @@
> > >  		nvidia,pad-autocal-pull-down-offset-1v8-timeout = <0x0a>;
> > >  		nvidia,default-tap = <0x5>;
> > >  		nvidia,default-trim = <0x9>;
> > > +		nvidia,dqs-trim = <63>;
> > >  		status = "disabled";
> > >  	};
> > >    
> > 
> > Isn't this technically dependent on the board layout and as such would
> > belong in the board DTS file? Or does this value work on all existing
> > Tegra186 platforms?
> 
> This value is specified as part of the controller initialization
> sequence in the TRM. I've understood that this (and other tap and trim)
> value(s) are used for compensating the propagation delay differences
> that are caused by the internal SoC layout.

Hmm... it would seem to me like the routing on a board would have a more
significant impact than the SoC internal routing. But perhaps the board-
specific routing is actually what the automatic calibration is used for?

Anyway, if it ever turns out that we need slightly different values on a
given board, we can always override the value in the board DTS file.

Thierry

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