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

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Message-ID: <53D75B90.7050501@nvidia.com>
Date:	Tue, 29 Jul 2014 11:30:08 +0300
From:	Mikko Perttunen <mperttunen@...dia.com>
To:	Stephen Warren <swarren@...dotorg.org>,
	Andrew Bresticker <abrestic@...omium.org>
CC:	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

Looks like the TRM doesn't document this either. I'll add an option 
("nvidia,short-ram-code" ?) for the next version.

On 22/07/14 20:34, Stephen Warren wrote:
> On 07/22/2014 11:22 AM, Andrew Bresticker wrote:
>> On Tue, Jul 22, 2014 at 9:45 AM, Stephen Warren <swarren@...dotorg.org> wrote:
>>>
>>> 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?
>>
>> No, the DT contains all possible EMC timings for that board.
>>
>>> 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?)
>>
>> Right, there are normally only 2 SDRAM strapping bits available.
>> ChromeOS gets around this by having 4 identical boot device entries in
>> the BCT, so all possible values of STRAPPING_OPT_A[7:6] map to the
>> same boot device.  This allows us to use all 4 strapping bits in
>> coreboot to pick the SDRAM configuration.
>
> OK, that explains how it works.
>
> But that means that when the kernel reads the strapping options, it will
> have to know if it uses just 2 bits (standard) or all 4 bits
> (non-standard) to index into the DT's array of SDRAM configurations.
> We'll need a DT property to represent that.
> --
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