imx7: Update CONFIG_SPL_STACK defaults in Kconfig

Update the Kconfig entry to have the correct defaults for i.MX7
platforms, and move the existing large comment from imx7_spl.h to
doc/imx/common/imx7.txt so that it's not lost.

Signed-off-by: Tom Rini <trini@konsulko.com>

1 files changed, 23 insertions, 0 deletions

diff --git a/doc/imx/common/imx7.txt b/doc/imx/common/imx7.txt
new file mode 100644
index 0000000000..b9db10341a
--- /dev/null
+++ b/doc/imx/common/imx7.txt
@@ -0,0 +1,23 @@
+U-Boot for Freescale i.MX7
+
+SPL Stack size and location notes
+---------------------------------
+See figure 6-22 in i.MX 7Dual/Solo Reference manuals:
+  - IMX7D/S OCRAM free area RAM (OCRAM) is from 0x00910000 to
+    0x00946C00.
+  - Set the stack at the end of the free area section, at 0x00946BB8.
+  - The BOOT ROM loads what they consider the firmware image
+    which consists of a 4K header in front of us that contains the IVT, DCD
+    and some padding. However, the manual also states that the ROM uses the
+    OCRAM_EPCD and OCRAM_PXP areas for itself. While the SPL is free to use
+    this range for stack and malloc, the SPL itself must fit below 0x920000,
+    or the image will be truncated in at least some boot modes like USB SDP.
+    Thus our max size is really 0x00920000 - 0x00912000. If necessary,
+    CONFIG_SPL_TEXT_BASE could be moved to 0x00911000 to gain 4KB of space
+    for the SPL, but 56KB should be more than enough for the SPL.
+  - Pad SPL to 68KB (4KB header + 56KB max size + 8KB extra padding)
+    The extra padding could be removed, but this value was used historically
+    based on an incorrect CONFIG_SPL_MAX_SIZE definition.
+    This allows to write the SPL/U-Boot combination generated with
+    u-boot-with-spl.imx directly to a boot media (given that boot media specific
+    offset is configured properly).