[RFC PATCH v4 0/5] ARM: Fix dma_alloc_coherent() and friends for NOMMU
benjamin.gaignard at linaro.org
Wed Jan 11 05:17:24 PST 2017
2017-01-10 15:18 GMT+01:00 Vladimir Murzin <vladimir.murzin at arm.com>:
> 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 classes of cpu 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
> 2/5). It allows us to offload all bookkeeping on generic coherent DMA
> framework, and it is seems that it might be reused by other
> architectures like c6x and blackfin.
> 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  to allow mmap for NOMMU, his case is also addressed in this
> series (PATCH 1/5 and PATCH 3/5).
I have tested this v4 on my setup (stm32f4, no cache, no MPU) and unfortunately
it doesn't work with my drm/kms driver.
I haven't any errors but nothing is displayed unlike what I have when
using current dma-mapping
I guess the issue is coming from dma-noop where __get_free_pages() is
used instead of alloc_pages()
Since my hardware doesn't have cache or MPU (and so use dma-noop) I
haven't reserved specific memory region.
Buffer addresses and vma parameters look correct... What could I have
miss here ?
>  http://www.armlinux.org.uk/developer/patches/viewpatch.php?id=8633/1
> Vladimir Murzin (5):
> dma: Add simple dma_noop_mmap
> 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/include/asm/dma-mapping.h | 3 +-
> arch/arm/mm/Kconfig | 2 +-
> arch/arm/mm/Makefile | 5 +-
> arch/arm/mm/dma-mapping-nommu.c | 252 +++++++++++++++++++++
> arch/arm/mm/dma-mapping.c | 26 +--
> drivers/base/dma-coherent.c | 59 ++++-
> lib/dma-noop.c | 21 ++
> 8 files changed, 335 insertions(+), 36 deletions(-)
> create mode 100644 arch/arm/mm/dma-mapping-nommu.c
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