RE: [PATCH 15/21] ARM: dma-mapping: always invalidate WT caches before DMA

[David Laight <David.Laight@ACULAB.COM>]

From: Arnd Bergmann
> Sent: 31 March 2023 11:39
...
> Most architectures that have write-through caches (m68k,
> microblaze) or write-back caches but no speculation (all other
> armv4/armv5, hexagon, openrisc, sh, most mips, later xtensa)
> only invalidate before DMA but not after.
> 
> OTOH, most machines that are actually in use today (armv6+,
> powerpc, later mips, microblaze, riscv, nios2) also have to
> deal with speculative accesses, so they end up having to
> invalidate or flush both before and after a DMA_FROM_DEVICE
> and DMA_BIDIRECTIONAL.

nios2 is a simple in-order cpu with a short pipeline
(it is a soft-cpu made from normal fpga logic elements).
Definitely doesn't do speculative accesses.
OTOH any one trying to run Linux on it needs their head examined.

	David

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