On Mon, 27 Mar 2023 at 14:18, Arnd Bergmann [off-list ref] wrote:

From: Arnd Bergmann <arnd@arndb.de>

The cache management operations for noncoherent DMA on ARMv6 work
in two different ways:

 * When CONFIG_DMA_CACHE_RWFO is set, speculative prefetches on in-flight
   DMA buffers lead to data corruption when the prefetched data is written
   back on top of data from the device.

 * When CONFIG_DMA_CACHE_RWFO is disabled, a cache flush on one CPU
   is not seen by the other core(s), leading to inconsistent contents
   accross the system.

As a consequence, neither configuration is actually safe to use in a
general-purpose kernel that is used on both MPCore systems and ARM1176
with prefetching enabled.

We could add further workarounds to make the behavior more dynamic based
on the system, but realistically, there are close to zero remaining
users on any ARM11MPCore anyway, and nobody seems too interested in it,
compared to the more popular ARM1176 used in BMC2835 and AST2500.

The Oxnas platform has some minimal support in OpenWRT, but most of the
drivers and dts files never made it into the mainline kernel, while the
Arm Versatile/Realview platform mainly serves as a reference system but
is not necessary to be kept working once all other ARM11MPCore are gone.

Take the easy way out here and drop support for multiprocessing on
ARMv6, along with the CONFIG_DMA_CACHE_RWFO option and the cache
management implementation for it. This also helps with other ARMv6
issues, but for the moment leaves the ability to build a kernel that
can run on both ARMv7 SMP and single-processor ARMv6, which we probably
want to stop supporting as well, but not as part of this series.

Cc: Neil Armstrong <neil.armstrong@linaro.org>
Cc: Daniel Golle <daniel@makrotopia.org>
Cc: Linus Walleij <redacted>
Cc: linux-oxnas@groups.io
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Acked-by: Ard Biesheuvel <ardb@kernel.org>