linux-kernel - Re: [PATCH 5/8] of: Add Tegra124 EMC bindings

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Message-ID: <53CE9514.1050903@wwwdotorg.org>
Date:	Tue, 22 Jul 2014 10:45:08 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Andrew Bresticker <abrestic@...omium.org>
CC:	Mikko Perttunen <mperttunen@...dia.com>,
	Peter De Schrijver <pdeschrijver@...dia.com>,
	Prashant Gaikwad <pgaikwad@...dia.com>,
	Mike Turquette <mturquette@...aro.org>,
	Thierry Reding <thierry.reding@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH 5/8] of: Add Tegra124 EMC bindings

On 07/21/2014 04:52 PM, Andrew Bresticker wrote:
> On Mon, Jul 21, 2014 at 2:28 PM, Stephen Warren <swarren@...dotorg.org> wrote:
>> On 07/11/2014 10:43 AM, Andrew Bresticker wrote:
>>> On Fri, Jul 11, 2014 at 7:18 AM, Mikko Perttunen <mperttunen@...dia.com> wrote:
>>>> Add binding documentation for the nvidia,tegra124-emc device tree
>>>> node.
>>>
>>>> diff --git a/Documentation/devicetree/bindings/memory-controllers/tegra-emc.txt b/Documentation/devicetree/bindings/memory-controllers/tegra-emc.txt
>>>
>>>> +Required properties :
>>>> +- compatible : "nvidia,tegra124-emc".
>>>> +- reg : Should contain 1 or 2 entries:
>>>> +  - EMC register set
>>>> +  - MC register set : Required only if no node with
>>>> +    'compatible = "nvidia,tegra124-mc"' exists. The MC register set
>>>> +    is first read from the MC node. If it doesn't exist, it is read
>>>> +    from this property.
>>>> +- timings : Should contain 1 entry for each supported clock rate.
>>>> +  Entries should be named "timing@n" where n is a 0-based increasing
>>>> +  number. The timings must be listed in rate-ascending order.
>>>
>>> There are upcoming boards which support multiple DRAM configurations
>>> and require a separate set of timings for each configuration.  Could
>>> we instead have multiple sets of timings with the proper one selected
>>> at runtime by RAM code, as reported by PMC_STRAPPING_OPT_A_0?
>>> Something like:
>>>
>>> emc {
>>>         emc-table@0 {
>>>                 nvidia,ram-code = <0>;
>>>                 timing@0 {
>>>                         ...
>>>                 };
>>>                 ...
>>>         };
>>
>> Until recently, there was a binding for Tegra20 EMC in mainline. We
>> should make sure the Tegra124 driver (or rather binding...) is at least
>> as feature-ful as that was.
>>
>> Furthermore, I thought that with some boards, there were more RAM
>> options that there were available RAM code strap bits. I assume that in
>> mainline, we'll simply have different base DT files for the different
>> sets of configurations? Or, will we want to add another level to the DT
>> to represent different sets of RAM code values? I'm not sure what data
>> source the bootloader uses to determine which set of RAM configuration
>> to use when there aren't enough entries in the BCT for the boot ROM to
>> do this automatically, and whether we have any way to get that value
>> into the kernel, so it could use it for the extra DT level lookup?
> 
> For the ChromeOS boards at least we are neither limited by the number
> of strapping bits (4) nor the number of BCT entries supported by the
> boot ROM (since coreboot does not rely on the boot ROM for SDRAM
> initialization), so having a single set of SDRAM configurations for
> each board, indexed by APBDEV_PMC_STRAPPING_OPT_A_0[7:4] works just
> fine.

Does the bootloader adjust the DT that's passed to the kernel so that
only the relevant single set of EMC timings is contained in the DT?

On a system where the boot ROM initializes RAM, and where the HW design
might have multiple SDRAM configuration, here's what usually happens:

a) The BCT contains N sets of SDRAM configurations.

b) The boot ROM reads the SDRAM strapping bits, and uses them to pick
the correct SDRAM configuration from the N sets in the BCT.

c) The kernel DT has N sets of SDRAM configurations.

d) The kernel reads the SDRAM strapping bits, and uses them to pick the
correct SDRAM configuration from the N sets in the DT.

On the ChromeOS boards (so (a) and (b) above are irrelevant) where N is
too large to fit into APBDEV_PMC_STRAPPING_OPT_A_0[7:4], (c) and (d)
won't work. I assume the kernel DT therefore must be adjusted to only
contain the single SDRAM configuration that is relevant for the current HW?

(isn't STRAPPING_OPT_A split into 2 2-bit fields; 2 bits for SDRAM index
and 2 bits for boot flash index, so max N is quite small?)
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