linux-kernel - Re: [PATCH 04/10] of: Add Tegra124 EMC bindings

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Date:	Tue, 14 Oct 2014 16:36:45 +0300
From:	Mikko Perttunen <mikko.perttunen@...si.fi>
To:	Mark Rutland <mark.rutland@....com>,
	Tomeu Vizoso <tomeu.vizoso@...labora.com>
CC:	Stephen Warren <swarren@...dia.com>,
	Rhyland Klein <rklein@...dia.com>,
	Thierry Reding <treding@...dia.com>,
	Javier Martinez Canillas <javier.martinez@...labora.co.uk>,
	Mikko Perttunen <mperttunen@...dia.com>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <Pawel.Moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Stephen Warren <swarren@...dotorg.org>,
	Thierry Reding <thierry.reding@...il.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 04/10] of: Add Tegra124 EMC bindings

On 10/10/2014 04:14 PM, Mark Rutland wrote:
> On Fri, Oct 10, 2014 at 01:46:55PM +0100, Tomeu Vizoso wrote:
>> From: Mikko Perttunen <mperttunen@...dia.com>
>>
>> Add binding documentation for the nvidia,tegra124-emc device tree node.
>>
>> Signed-off-by: Mikko Perttunen <mperttunen@...dia.com>
>> Signed-off-by: Tomeu Vizoso <tomeu.vizoso@...labora.com>
>> ---
>>   .../bindings/memory-controllers/tegra-emc.txt      | 41 ++++++++++++++++++++++
>>   1 file changed, 41 insertions(+)
>>   create mode 100644 Documentation/devicetree/bindings/memory-controllers/tegra-emc.txt
>>
>> diff --git a/Documentation/devicetree/bindings/memory-controllers/tegra-emc.txt b/Documentation/devicetree/bindings/memory-controllers/tegra-emc.txt
>> new file mode 100644
>> index 0000000..6282c6b
>> --- /dev/null
>> +++ b/Documentation/devicetree/bindings/memory-controllers/tegra-emc.txt
>> @@ -0,0 +1,41 @@
>> +Tegra124 SoC EMC controller
>> +
>> +Required properties :
>> +- compatible : "nvidia,tegra124-emc".
>> +- reg : Should contain 1 entry:
>> +  - EMC register set
>> +
>> +The node should contain a "timings@i" subnode for each supported RAM type
>> +  (see field RAM_CODE in register PMC_STRAPPING_OPT_A)
>> +Required properties for "timings@i" nodes :
>> +- nvidia,ram-code : Should contain the value of RAM_CODE this timing set
>> +  is used for.
>> +
>> +Each "timings@i" node should contain "timing@j" subnodes. One "timing@j"
>> +  node should exist for each supported EMC clock rate.
>
> What do the i and j correspond to?

Arbitrary integers, preferably starting from 0 and increasing, similar 
as with regulator@i etc. Each set of `j's is independent. (Actually, 
these values are never used anywhere for anything).

>
>> +Required properties for "timing@j" nodes :
>> +- clock-frequency : Should contain the memory clock rate.
>> +- nvidia,parent-clock-frequency : Should contain the rate of the EMC
>> +  clock's parent clock.
>
> Why are both of these properties necessary?
>
> What is the relationship between the two?

There is a certain set of memory timings specified by the hardware 
manufacturer for each board type. Each memory timing contains the memory 
rate, parent clock and parent clock frequency and many other values 
which you mention later.

>
>> +- clocks : Must contain an entry for each entry in clock-names.
>> +  See ../clocks/clock-bindings.txt for details.
>> +- clock-names : Must include the following entries:
>> +  - emc-parent : EMC's parent clock.
>
> Surely the clocks are a property of the EMC, and not the individual
> timings?

The required parent for each timing is specified by the manufacturer.

>
>> +- The following properties contain EMC timing characterization values:
>> +  - nvidia,emc-zcal-cnt-long
>> +  - nvidia,emc-auto-cal-interval
>> +  - nvidia,emc-ctt-term-ctrl
>> +  - nvidia,emc-cfg
>> +  - nvidia,emc-cfg-2
>> +  - nvidia,emc-sel-dpd-ctrl
>> +  - nvidia,emc-cfg-dig-dll
>> +  - nvidia,emc-bgbias-ctl0
>> +  - nvidia,emc-auto-cal-config
>> +  - nvidia,emc-auto-cal-config2
>> +  - nvidia,emc-auto-cal-config3
>> +  - nvidia,emc-mode-reset
>> +  - nvidia,emc-mode-1
>> +  - nvidia,emc-mode-2
>> +  - nvidia,emc-mode-4
>> +- nvidia,emc-configuration : EMC timing characterization data written to
>> +                             EMC registers.
>
> I have no idea what any of these are. Perhaps these make sense, but I
> cannot tell.

Magic values specified by the manufacturer. There is some sparse 
documentation in the Tegra TRM.

TL;DR: The whole EMC sequence is rather magical and complicated so we 
stick to manufacturer-specified values rather than trying guess on our 
own. (Although this patch already calculates some stuff that's safe to 
calculate and that is manually specified by the downstream NVIDIA kernel.)

>
> Mark.
>

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