ARM: tegra: support running in non-secure mode

When the CPU is in non-secure (NS) mode (when running U-Boot under a
secure monitor), certain actions cannot be taken, since they would need
to write to secure-only registers. One example is configuring the ARM
architectural timer's CNTFRQ register.

We could support this in one of two ways:
1) Compile twice, once for secure mode (in which case anything goes) and
   once for non-secure mode (in which case certain actions are disabled).
   This complicates things, since everyone needs to keep track of
   different U-Boot binaries for different situations.
2) Detect NS mode at run-time, and optionally skip any impossible actions.
   This has the advantage of a single U-Boot binary working in all cases.

(2) is not possible on ARM in general, since there's no architectural way
to detect secure-vs-non-secure. However, there is a Tegra-specific way to
detect this.

This patches uses that feature to detect secure vs. NS mode on Tegra, and
uses that to:

* Skip the ARM arch timer initialization.

* Set/clear an environment variable so that boot scripts can take
  different action depending on which mode the CPU is in. This might be
  something like:
  if CPU is secure:
    load secure monitor code into RAM.
    boot secure monitor.
    secure monitor will restart (a new copy of) U-Boot in NS mode.
  else:
    execute normal boot process

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/README b/README
index ba57dc5..8a4c5a7 100644
--- a/README
+++ b/README
@@ -621,6 +621,13 @@ The following options need to be configured:
 		exists, unlike the similar options in the Linux kernel. Do not
 		set these options unless they apply!
 
+- Tegra SoC options:
+		CONFIG_TEGRA_SUPPORT_NON_SECURE
+
+		Support executing U-Boot in non-secure (NS) mode. Certain
+		impossible actions will be skipped if the CPU is in NS mode,
+		such as ARM architectural timer initialization.
+
 - Driver Model
 		Driver model is a new framework for devices in U-Boot
 		introduced in early 2014. U-Boot is being progressively