path: root/README
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authorThierry Reding <>2014-12-09 22:25:22 -0700
committerTom Warren <>2014-12-18 13:19:21 -0700
commit1dfdd9ba4e699562a3b282f4fb5efe4c80e6e8c7 (patch)
tree515aa28f47556e989774e9f4caa6d2a2ccdb61f6 /README
parent6e2fca94ff4f754024e46e7d986221a75be611f3 (diff)
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 <> Signed-off-by: Simon Glass <> Signed-off-by: Tom Warren <>
Diffstat (limited to 'README')
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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
+ 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.
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