On Mon, 27 Mar 2023 at 14:18, Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
From: Arnd Bergmann <arnd@xxxxxxxx> 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@xxxxxxxxxx> Cc: Daniel Golle <daniel@xxxxxxxxxxxxxx> Cc: Linus Walleij <linus.walleij@xxxxxxxxxx> Cc: linux-oxnas@xxxxxxxxx Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
Acked-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
