Add a simple malloc() implementation for pre-relocation

If we are to have driver model before relocation we need to support some
way of calling memory allocation routines.

The standard malloc() is pretty complicated:

1. It uses some BSS memory for its state, and BSS is not available before
relocation

2. It supports algorithms for reducing memory fragmentation and improving
performace of free(). Before relocation we could happily just not support
free().

3. It includes about 4KB of code (Thumb 2) and 1KB of data. However since
this has been loaded anyway this is not really a problem.

The simplest way to support pre-relocation malloc() is to reserve an area
of memory and allocate it in increasing blocks as needed. This
implementation does this.

To enable it, you need to define the size of the malloc() pool as described
in the README. It will be located above the pre-relocation stack on
supported architectures.

Note that this implementation is only useful on machines which have some
memory available before dram_init() is called - this includes those that
do no DRAM init (like tegra) and those that do it in SPL (quite a few
boards). Enabling driver model preior to relocation for the rest of the
boards is left for a later exercise.

Signed-off-by: Simon Glass <sjg@chromium.org>

1 files changed, 13 insertions, 0 deletions

diff --git a/README b/README
index 4ac7399698..11e9d31285 100644
--- a/README
+++ b/README
@@ -3736,6 +3736,19 @@ Configuration Settings:
 - CONFIG_SYS_MALLOC_LEN:
 		Size of DRAM reserved for malloc() use.
 
+- CONFIG_SYS_MALLOC_F_LEN
+		Size of the malloc() pool for use before relocation. If
+		this is defined, then a very simple malloc() implementation
+		will become available before relocation. The address is just
+		below the global data, and the stack is moved down to make
+		space.
+
+		This feature allocates regions with increasing addresses
+		within the region. calloc() is supported, but realloc()
+		is not available. free() is supported but does nothing.
+		The memory will be freed (or in fact just forgotton) when
+		U-Boot relocates itself.
+
 - CONFIG_SYS_BOOTM_LEN:
 		Normally compressed uImages are limited to an
 		uncompressed size of 8 MBytes. If this is not enough,