OMAP3: SDRC: Place SDRC AC timing and MR changes in CORE DVFS SRAM code behind Kconfig

The code that reprograms the SDRC memory controller during CORE DVFS,
mach-omap2/sram34xx.S:omap3_sram_configure_core_dpll(), does not
ensure that all L3 initiators are prevented from accessing the SDRAM
before modifying the SDRC AC timing and MR registers.  This can cause
memory to be corrupted or cause the SDRC to enter an unpredictable
state.  This patch places that code behind a Kconfig option,
CONFIG_OMAP3_SDRC_AC_TIMING for now, and adds a note explaining what
is going on.  Ideally the code can be added back in once supporting
code is present to ensure that other initiators aren't touching the
SDRAM.  At the very least, these registers should be reprogrammable
during kernel init to deal with buggy bootloaders.  Users who know
that all other system initiators will not be touching the SDRAM can
also re-enable this Kconfig option.

This is a modification of a patch originally written by Rajendra Nayak
<rnayak@ti.com> (the original is at http://patchwork.kernel.org/patch/51927/).
Rather than removing the code completely, this patch just comments it out.

Thanks to Benoît Cousson <b-cousson@ti.com> and Christophe Sucur
<c-sucur@ti.com> for explaining the technical basis for this and for
explaining what can be done to make this path work in future code.
Thanks to Richard Woodruff <r-woodruff2@ti.com>, Nishanth Menon
<nm@ti.com>, and Olof Johansson <olof@lixom.net> for their comments.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Rajendra Nayak <rnayak@ti.com>
Cc: Christophe Sucur <c-sucur@ti.com>
Cc: Benoît Cousson <b-cousson@ti.com>
Cc: Richard Woodruff <r-woodruff2@ti.com>
Cc: Nishanth Menon <nm@ti.com>
Cc: Olof Johansson <olof@lixom.net>

1 files changed, 12 insertions, 0 deletions

diff --git a/arch/arm/mach-omap2/Kconfig b/arch/arm/mach-omap2/Kconfig
index 7309aab305a9..0cd25ceadb43 100644
--- a/arch/arm/mach-omap2/Kconfig
+++ b/arch/arm/mach-omap2/Kconfig
@@ -128,3 +128,15 @@ config OMAP3_EMU
 	help
 	  Say Y here to enable debugging hardware of omap3
 
+config OMAP3_SDRC_AC_TIMING
+	bool "Enable SDRC AC timing register changes"
+	depends on ARCH_OMAP3 && ARCH_OMAP34XX
+	default n
+	help
+	  If you know that none of your system initiators will attempt to
+	  access SDRAM during CORE DVFS, select Y here.  This should boost
+	  SDRAM performance at lower CORE OPPs.  There are relatively few
+	  users who will wish to say yes at this point - almost everyone will
+	  wish to say no.  Selecting yes without understanding what is
+	  going on could result in system crashes;
+