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    SubjectRe: [PATCH v5 0/7] ARM: Fix dma_alloc_coherent() and friends for NOMMU
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    Ping!

    On 24/05/17 11:24, Vladimir Murzin wrote:
    > Short story:
    >
    > Without these patches coherent DMA is broken for András and Alexandre,
    > so they cannot safely enable DMA on their platforms.
    >
    > Patches have been circulated on a list since last year without much
    > attention to changes in dma-coherent.c and dma-noop.c. Meanwhile, ARM
    > bits have been reviewed and there is no strict objection to get them
    > merged. Unfortunately, applying only ARM bits doesn't help much and
    > the original issue would still exist.
    >
    > Please, let me know how to move with this fix forward?
    >
    > Long story:
    >
    > It seems 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 versions 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 any 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 at 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@suse.de>
    > Cc: Christian Borntraeger <borntraeger@de.ibm.com>
    > Cc: Michal Nazarewicz <mina86@mina86.com>
    > Cc: Marek Szyprowski <m.szyprowski@samsung.com>
    > Cc: Alan Stern <stern@rowland.harvard.edu>
    > Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
    > Cc: Rich Felker <dalias@libc.org>
    > Cc: Roger Quadros <rogerq@ti.com>
    > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    > Cc: Rob Herring <robh+dt@kernel.org>
    > Cc: Mark Rutland <mark.rutland@arm.com>
    > Cc: Doug Ledford <dledford@redhat.com>
    >
    > Changelog:
    > v4 -> v5
    > - rebased on v4.12-rc2
    > - updated description for CONFIG_ARM_DMA_MEM_BUFFERABLE
    >
    > 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 | 8 +-
    > 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, 359 insertions(+), 45 deletions(-)
    > create mode 100644 arch/arm/mm/dma-mapping-nommu.c
    >

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