realtek: mach: specify alignment topology

realtek: mach: specify alignment topology

The kernel has two helper defines that guide about hardware
characteristics.

MIPS_L1_CACHE_SHIFT defines the cache line sizes (1<<x) of the
target. It defaults to 5 - so it is assumed that the device has
a cache line size of 32 bytes. This is not true for MIPS 4KEc
cores that are driving the RTL838x SOCs. These cores have 16
byte cache line sizes. Adapt the CONFIG properties for this
target to match the hardware.

ARCH_DMA_MINALIGN definies the alignment for memory allocations.
Other than its name suggests on MIPS devices that have non
coherent DMA kmalloc() respects this configuration. This ensures
that no normal memory is corrupted by DMA blocks that share the
same cache line.

The default for this is 128 bytes. And kernel states itself
"Total overkill for most systems but need as a safe default. Set
this one if any device in the system might do non-coherent DMA".

Realtek devices use non coherent DMA so they are affected by the
setting of ARCH_DMA_MINALIGN. Set this to cache line size for
all devices to reduce memory waste.

Link: https://github.com/openwrt/openwrt/pull/23492
Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>