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>

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;
+

arch/arm/mach-omap2/sram34xx.S

@@ -91,8 +91,19 @@
  *  new SDRC_ACTIM_CTRL_B_1 register contents
  *  new SDRC_MR_1 register value
  *
- * If the param SDRC_RFR_CTRL_1 is 0, the parameters
- *  are not programmed into the SDRC CS1 registers
+ * If the param SDRC_RFR_CTRL_1 is 0, the parameters are not programmed into
+ * the SDRC CS1 registers
+ *
+ * NOTE: This code no longer attempts to program the SDRC AC timing and MR
+ * registers.  This is because the code currently cannot ensure that all
+ * L3 initiators (e.g., sDMA, IVA, DSS DISPC, etc.) are not accessing the
+ * SDRAM when the registers are written.  If the registers are changed while
+ * an initiator is accessing SDRAM, memory can be corrupted and/or the SDRC
+ * may enter an unpredictable state.  In the future, the intent is to
+ * re-enable this code in cases where we can ensure that no initiators are
+ * touching the SDRAM.  Until that time, users who know that their use case
+ * can satisfy the above requirement can enable the CONFIG_OMAP3_SDRC_AC_TIMING
+ * option.
  */
 ENTRY(omap3_sram_configure_core_dpll)
 	stmfd	sp!, {r1-r12, lr}	@ store regs to stack
@@ -219,6 +230,7 @@ configure_sdrc:
 	ldr	r12, omap_sdrc_rfr_ctrl_0_val	@ fetch value from SRAM
 	ldr	r11, omap3_sdrc_rfr_ctrl_0	@ fetch addr from SRAM
 	str	r12, [r11]			@ store
+#ifdef CONFIG_OMAP3_SDRC_AC_TIMING
 	ldr	r12, omap_sdrc_actim_ctrl_a_0_val
 	ldr	r11, omap3_sdrc_actim_ctrl_a_0
 	str	r12, [r11]
@@ -228,11 +240,13 @@ configure_sdrc:
 	ldr	r12, omap_sdrc_mr_0_val
 	ldr	r11, omap3_sdrc_mr_0
 	str	r12, [r11]
+#endif
 	ldr	r12, omap_sdrc_rfr_ctrl_1_val
 	cmp	r12, #0			@ if SDRC_RFR_CTRL_1 is 0,
 	beq	skip_cs1_prog		@  do not program cs1 params
 	ldr	r11, omap3_sdrc_rfr_ctrl_1
 	str	r12, [r11]
+#ifdef CONFIG_OMAP3_SDRC_AC_TIMING
 	ldr	r12, omap_sdrc_actim_ctrl_a_1_val
 	ldr	r11, omap3_sdrc_actim_ctrl_a_1
 	str	r12, [r11]
@@ -242,6 +256,7 @@ configure_sdrc:
 	ldr	r12, omap_sdrc_mr_1_val
 	ldr	r11, omap3_sdrc_mr_1
 	str	r12, [r11]
+#endif
 skip_cs1_prog:
 	ldr	r12, [r11]		@ posted-write barrier for SDRC
 	bx	lr