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README on the SDRAM Controller for the LH7a40X
The standard configuration for the SDRAM controller generates a sparse
memory array. The precise layout is determined by the SDRAM chips. A
default kernel configuration assembles the discontiguous memory
regions into separate memory nodes via the NUMA (Non-Uniform Memory
Architecture) facilities. In this default configuration, the kernel
is forgiving about the precise layout. As long as it is given an
accurate picture of available memory by the bootloader the kernel will
execute correctly.
The SDRC supports a mode where some of the chip select lines are
swapped in order to make SDRAM look like a synchronous ROM. Setting
this bit means that the RAM will present as a contiguous array. Some
programmers prefer this to the discontiguous layout. Be aware that
may be a penalty for this feature where some some configurations of
memory are significantly reduced; i.e. 64MiB of RAM appears as only 32
There are a couple of configuration options to override the default
behavior. When the SROMLL bit is set and memory appears as a
contiguous array, there is no reason to support NUMA.
CONFIG_LH7A40X_CONTIGMEM disables NUMA support. When physical memory
is discontiguous, the memory tables are organized such that there are
two banks per nodes with a small gap between them. This layout wastes
some kernel memory for page tables representing non-existent memory.
CONFIG_LH7A40X_ONE_BANK_PER_NODE optimizes the node tables such that
there are no gaps. These options control the low level organization
of the memory management tables in ways that may prevent the kernel
from booting or may cause the kernel to allocated excessively large
page tables. Be warned. Only change these options if you know what
you are doing. The default behavior is a reasonable compromise that
will suit all users.
A typical 32MiB system with the default configuration options will
find physical memory managed as follows.
node 0: 0xc0000000 4MiB
0xc1000000 4MiB
node 1: 0xc4000000 4MiB
0xc5000000 4MiB
node 2: 0xc8000000 4MiB
0xc9000000 4MiB
node 3: 0xcc000000 4MiB
0xcd000000 4MiB
Setting CONFIG_LH7A40X_ONE_BANK_PER_NODE will put each bank into a
separate node.