ARM: Implement non-cached memory support

Implement an API that can be used by drivers to allocate memory from a
pool that is mapped uncached. This is useful if drivers would otherwise
need to do extensive cache maintenance (or explicitly maintaining the
cache isn't safe).

The API is protected using the new CONFIG_SYS_NONCACHED_MEMORY setting.
Boards can set this to the size to be used for the non-cached area. The
area will typically be right below the malloc() area, but architectures
should take care of aligning the beginning and end of the area to honor
any mapping restrictions. Architectures must also ensure that mappings
established for this area do not overlap with the malloc() area (which
should remain cached for improved performance).

While the API is currently only implemented for ARM v7, it should be
generic enough to allow other architectures to implement it as well.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>

1 files changed, 19 insertions, 0 deletions

diff --git a/README b/README
index 4ca04d0489..5af345b086 100644
--- a/README
+++ b/README
@@ -4007,6 +4007,25 @@ Configuration Settings:
                 boards which do not use the full malloc in SPL (which is
                 enabled with CONFIG_SYS_SPL_MALLOC_START).
 
+- CONFIG_SYS_NONCACHED_MEMORY:
+                Size of non-cached memory area. This area of memory will be
+                typically located right below the malloc() area and mapped
+                uncached in the MMU. This is useful for drivers that would
+                otherwise require a lot of explicit cache maintenance. For
+                some drivers it's also impossible to properly maintain the
+                cache. For example if the regions that need to be flushed
+                are not a multiple of the cache-line size, *and* padding
+                cannot be allocated between the regions to align them (i.e.
+                if the HW requires a contiguous array of regions, and the
+                size of each region is not cache-aligned), then a flush of
+                one region may result in overwriting data that hardware has
+                written to another region in the same cache-line. This can
+                happen for example in network drivers where descriptors for
+                buffers are typically smaller than the CPU cache-line (e.g.
+                16 bytes vs. 32 or 64 bytes).
+
+                Non-cached memory is only supported on 32-bit ARM at present.
+
 - CONFIG_SYS_BOOTM_LEN:
                 Normally compressed uImages are limited to an
                 uncompressed size of 8 MBytes. If this is not enough,