Re: [PATCH 00/21] dma-mapping: unify support for cache flushes

Lad, Prabhakar
2023-05-25T07:47:21Z

Hi Arnd,

On Mon, Mar 27, 2023 at 1:14 PM Arnd Bergmann <arnd@kernel.org> wrote:
>
> From: Arnd Bergmann <arnd@arndb.de>
>
> After a long discussion about adding SoC specific semantics for when
> to flush caches in drivers/soc/ drivers that we determined to be
> fundamentally flawed[1], I volunteered to try to move that logic into
> architecture-independent code and make all existing architectures do
> the same thing.
>
> As we had determined earlier, the behavior is wildly different across
> architectures, but most of the differences come down to either bugs
> (when required flushes are missing) or extra flushes that are harmless
> but might hurt performance.
>
> I finally found the time to come up with an implementation of this, which
> starts by replacing every outlier with one of the three common options:
>
> 1. architectures without speculative prefetching (hegagon, m68k,
> openrisc, sh, sparc, and certain armv4 and xtensa implementations)
> only flush their caches before a DMA, by cleaning write-back caches
> (if any) before a DMA to the device, and by invalidating the caches
> before a DMA from a device
>
> 2. arc, microblaze, mips, nios2, sh and later xtensa now follow the
> normal 32-bit arm model and invalidate their writeback caches
> again after a DMA from the device, to remove stale cache lines
> that got prefetched during the DMA. arc, csky and mips used to
> invalidate buffers also before the bidirectional DMA, but this
> is now skipped whenever we know it gets invalidated again
> after the DMA.
>
> 3. parisc, powerpc and riscv already flushed buffers before
> a DMA_FROM_DEVICE, and these get moved to the arm64 behavior
> that does the writeback before and invalidate after both
> DMA_FROM_DEVICE and DMA_BIDIRECTIONAL in order to avoid the
> problem of accidentally leaking stale data if the DMA does
> not actually happen[2].
>
> The last patch in the series replaces the architecture specific code
> with a shared version that implements all three based on architecture
> specific parameters that are almost always determined at compile time.
>
> The difference between cases 1. and 2. is hardware specific, while between
> 2. and 3. we need to decide which semantics we want, but I explicitly
> avoid this question in my series and leave it to be decided later.
>
> Another difference that I do not address here is what cache invalidation
> does for partical cache lines. On arm32, arm64 and powerpc, a partial
> cache line always gets written back before invalidation in order to
> ensure that data before or after the buffer is not discarded. On all
> other architectures, the assumption is cache lines are never shared
> between DMA buffer and data that is accessed by the CPU. If we end up
> always writing back dirty cache lines before a DMA (option 3 above),
> then this point becomes moot, otherwise we should probably address this
> in a follow-up series to document one behavior or the other and implement
> it consistently.
>
> Please review!
>
> Arnd
>
> [1] https://lore.kernel.org/all/20221212115505.36770-1-prabhakar.mahadev-lad.rj@bp.renesas.com/
> [2] https://lore.kernel.org/all/20220606152150.GA31568@willie-the-truck/
>
> Arnd Bergmann (21):
> openrisc: dma-mapping: flush bidirectional mappings
> xtensa: dma-mapping: use normal cache invalidation rules
> sparc32: flush caches in dma_sync_*for_device
> microblaze: dma-mapping: skip extra DMA flushes
> powerpc: dma-mapping: split out cache operation logic
> powerpc: dma-mapping: minimize for_cpu flushing
> powerpc: dma-mapping: always clean cache in _for_device() op
> riscv: dma-mapping: only invalidate after DMA, not flush
> riscv: dma-mapping: skip invalidation before bidirectional DMA
> csky: dma-mapping: skip invalidating before DMA from device
> mips: dma-mapping: skip invalidating before bidirectional DMA
> mips: dma-mapping: split out cache operation logic
> arc: dma-mapping: skip invalidating before bidirectional DMA
> parisc: dma-mapping: use regular flush/invalidate ops
> ARM: dma-mapping: always invalidate WT caches before DMA
> ARM: dma-mapping: bring back dmac_{clean,inv}_range
> ARM: dma-mapping: use arch_sync_dma_for_{device,cpu}() internally
> ARM: drop SMP support for ARM11MPCore
> ARM: dma-mapping: use generic form of arch_sync_dma_* helpers
> ARM: dma-mapping: split out arch_dma_mark_clean() helper
> dma-mapping: replace custom code with generic implementation
>
Do you plan to send v2 for this series?

Cheers,
Prabhakar

> arch/arc/mm/dma.c | 66 ++------
> arch/arm/Kconfig | 4 +
> arch/arm/include/asm/cacheflush.h | 21 +++
> arch/arm/include/asm/glue-cache.h | 4 +
> arch/arm/mach-oxnas/Kconfig | 4 -
> arch/arm/mach-oxnas/Makefile | 1 -
> arch/arm/mach-oxnas/headsmp.S | 23 ---
> arch/arm/mach-oxnas/platsmp.c | 96 -----------
> arch/arm/mach-versatile/platsmp-realview.c | 4 -
> arch/arm/mm/Kconfig | 19 ---
> arch/arm/mm/cache-fa.S | 4 +-
> arch/arm/mm/cache-nop.S | 6 +
> arch/arm/mm/cache-v4.S | 13 +-
> arch/arm/mm/cache-v4wb.S | 4 +-
> arch/arm/mm/cache-v4wt.S | 22 ++-
> arch/arm/mm/cache-v6.S | 35 +---
> arch/arm/mm/cache-v7.S | 6 +-
> arch/arm/mm/cache-v7m.S | 4 +-
> arch/arm/mm/dma-mapping-nommu.c | 36 ++--
> arch/arm/mm/dma-mapping.c | 181 ++++++++++-----------
> arch/arm/mm/proc-arm1020.S | 4 +-
> arch/arm/mm/proc-arm1020e.S | 4 +-
> arch/arm/mm/proc-arm1022.S | 4 +-
> arch/arm/mm/proc-arm1026.S | 4 +-
> arch/arm/mm/proc-arm920.S | 4 +-
> arch/arm/mm/proc-arm922.S | 4 +-
> arch/arm/mm/proc-arm925.S | 4 +-
> arch/arm/mm/proc-arm926.S | 4 +-
> arch/arm/mm/proc-arm940.S | 4 +-
> arch/arm/mm/proc-arm946.S | 4 +-
> arch/arm/mm/proc-feroceon.S | 8 +-
> arch/arm/mm/proc-macros.S | 2 +
> arch/arm/mm/proc-mohawk.S | 4 +-
> arch/arm/mm/proc-xsc3.S | 4 +-
> arch/arm/mm/proc-xscale.S | 6 +-
> arch/arm64/mm/dma-mapping.c | 28 ++--
> arch/csky/mm/dma-mapping.c | 46 +++---
> arch/hexagon/kernel/dma.c | 44 ++---
> arch/m68k/kernel/dma.c | 43 +++--
> arch/microblaze/kernel/dma.c | 38 ++---
> arch/mips/mm/dma-noncoherent.c | 75 +++------
> arch/nios2/mm/dma-mapping.c | 57 +++----
> arch/openrisc/kernel/dma.c | 62 ++++---
> arch/parisc/include/asm/cacheflush.h | 6 +-
> arch/parisc/kernel/pci-dma.c | 33 +++-
> arch/powerpc/mm/dma-noncoherent.c | 76 +++++----
> arch/riscv/mm/dma-noncoherent.c | 51 +++---
> arch/sh/kernel/dma-coherent.c | 43 +++--
> arch/sparc/Kconfig | 2 +-
> arch/sparc/kernel/ioport.c | 38 +++--
> arch/xtensa/Kconfig | 1 -
> arch/xtensa/include/asm/cacheflush.h | 6 +-
> arch/xtensa/kernel/pci-dma.c | 47 +++---
> include/linux/dma-sync.h | 107 ++++++++++++
> 54 files changed, 721 insertions(+), 699 deletions(-)
> delete mode 100644 arch/arm/mach-oxnas/headsmp.S
> delete mode 100644 arch/arm/mach-oxnas/platsmp.c
> create mode 100644 include/linux/dma-sync.h
>
> --
> 2.39.2
>
> Cc: Vineet Gupta <vgupta@kernel.org>
> Cc: Russell King <linux@armlinux.org.uk>
> Cc: Neil Armstrong <neil.armstrong@linaro.org>
> Cc: Linus Walleij <linus.walleij@linaro.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Guo Ren <guoren@kernel.org>
> Cc: Brian Cain <bcain@quicinc.com>
> Cc: Geert Uytterhoeven <geert@linux-m68k.org>
> Cc: Michal Simek <monstr@monstr.eu>
> Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
> Cc: Dinh Nguyen <dinguyen@kernel.org>
> Cc: Stafford Horne <shorne@gmail.com>
> Cc: Helge Deller <deller@gmx.de>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
> Cc: Paul Walmsley <paul.walmsley@sifive.com>
> Cc: Palmer Dabbelt <palmer@dabbelt.com>
> Cc: Rich Felker <dalias@libc.org>
> Cc: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: Max Filippov <jcmvbkbc@gmail.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Robin Murphy <robin.murphy@arm.com>
> Cc: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
> Cc: Conor Dooley <conor.dooley@microchip.com>
> Cc: linux-snps-arc@lists.infradead.org
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-oxnas@groups.io
> Cc: linux-csky@vger.kernel.org
> Cc: linux-hexagon@vger.kernel.org
> Cc: linux-m68k@lists.linux-m68k.org
> Cc: linux-mips@vger.kernel.org
> Cc: linux-openrisc@vger.kernel.org
> Cc: linux-parisc@vger.kernel.org
> Cc: linuxppc-dev@lists.ozlabs.org
> Cc: linux-riscv@lists.infradead.org
> Cc: linux-sh@vger.kernel.org
> Cc: sparclinux@vger.kernel.org
> Cc: linux-xtensa@linux-xtensa.org
>
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