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

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Date:	Tue, 22 Jul 2014 10:22:27 -0700
From:	Andrew Bresticker <abrestic@...omium.org>
To:	Stephen Warren <swarren@...dotorg.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 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.
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