Re: [PATCH v4 0/7] ARM: Fix dma_alloc_coherent() and friends for NOMMU

From: Vladimir Murzin
Date: Tue May 02 2017 - 04:32:50 EST


Gentle ping!

On 24/04/17 11:16, Vladimir Murzin wrote:
> It seem that addition of cache support for M-class CPUs uncovered
> latent bug in DMA usage. NOMMU memory model has been treated as being
> always consistent; however, for R/M CPU classes memory can be covered
> by MPU which in turn might configure RAM as Normal i.e. bufferable and
> cacheable. It breaks dma_alloc_coherent() and friends, since data can
> stuck in caches now or be buffered.
>
> This patch set is trying to address the issue by providing region of
> memory suitable for consistent DMA operations. It is supposed that
> such region is marked by MPU as non-cacheable. Robin suggested to
> advertise such memory as reserved shared-dma-pool, rather then using
> homebrew command line option, and extend dma-coherent to provide
> default DMA area in the similar way as it is done for CMA (PATCH
> 4/7). It allows us to offload all bookkeeping on generic coherent DMA
> framework, and it seems that it might be reused by other architectures
> like c6x and blackfin.
>
> While reviewing/testing previous vesrions of the patch set it turned
> out that dma-coherent does not take into account "dma-ranges" device
> tree property, so it is addressed in PATCH 3/7.
>
> For ARM, dedicated DMA region is required for cases other than:
> - MMU/MPU is off
> - cpu is v7m w/o cache support
> - device is coherent
>
> In case one of the above conditions is true dma operations are forced
> to be coherent and wired with dma_noop_ops.
>
> To make life easier NOMMU dma operations are kept in separate
> compilation unit.
>
> Since the issue was reported in the same time as Benjamin sent his
> patch [1] to allow mmap for NOMMU, his case is also addressed in this
> series (PATCH 1/7 and PATCH 2/7).
>
> Thanks!
>
> [1] http://www.armlinux.org.uk/developer/patches/viewpatch.php?id=8633/1
>
> Cc: Joerg Roedel <jroedel@xxxxxxx>
> Cc: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>
> Cc: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> Cc: Yoshinori Sato <ysato@xxxxxxxxxxxxxxxxxxxx>
> Cc: Rich Felker <dalias@xxxxxxxx>
> Cc: Roger Quadros <rogerq@xxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: Rob Herring <robh+dt@xxxxxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Doug Ledford <dledford@xxxxxxxxxx>
>
> Changelog:
> v3 -> v4
> - rebased on v4.11-rc7
> - made CONFIG_ARM_DMA_MEM_BUFFERABLE optional for CPU_V7M
> - added Arnd's Acked-by
>
> v2 -> v3
> - fixed warnings reported by Alexandre and kbuild robot
>
> v1 -> v2
> - rebased on v4.11-rc1
> - added Robin's Reviewed-by
> - dedicated flag is introduced to use dev->dma_pfn_offset
> rather than mem->device_base in case memory region is
> configured via device tree (so Tested-by discarded there)
>
> RFC v6 -> v1
> - dropped RFC tag
> - added Alexandre's Tested-by
>
>
> Vladimir Murzin (7):
> dma: Take into account dma_pfn_offset
> dma: Add simple dma_noop_mmap
> drivers: dma-coherent: Account dma_pfn_offset when used with device
> tree
> drivers: dma-coherent: Introduce default DMA pool
> ARM: NOMMU: Introduce dma operations for noMMU
> ARM: NOMMU: Set ARM_DMA_MEM_BUFFERABLE for M-class cpus
> ARM: dma-mapping: Remove traces of NOMMU code
>
> .../bindings/reserved-memory/reserved-memory.txt | 3 +
> arch/arm/Kconfig | 1 +
> arch/arm/include/asm/dma-mapping.h | 2 +-
> arch/arm/mm/Kconfig | 4 +-
> arch/arm/mm/Makefile | 5 +-
> arch/arm/mm/dma-mapping-nommu.c | 253 +++++++++++++++++++++
> arch/arm/mm/dma-mapping.c | 29 +--
> drivers/base/dma-coherent.c | 74 +++++-
> lib/dma-noop.c | 29 ++-
> 9 files changed, 355 insertions(+), 45 deletions(-)
> create mode 100644 arch/arm/mm/dma-mapping-nommu.c
>