[RFC PATCH v4 0/5] ARM: Fix dma_alloc_coherent() and friends for NOMMU
vladimir.murzin at arm.com
Wed Jan 11 06:34:58 PST 2017
On 11/01/17 13:17, Benjamin Gaignard wrote:
> 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 guess the same is for fbmem, but would be better to have confirmation since
amba-clcd I use has not been ported to drm/kms (yet), so I can't test.
> 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()
> in dma-mapping.
Unless I've missed something bellow is a call stack for both
So the difference is that nommu case in dma-mapping.c memzeros memory, handles
DMA_ATTR_NO_KERNEL_MAPPING and does optimisation of memory usage.
Is something from above critical for your driver?
> 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 ?
No ideas, sorry...
>>  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
More information about the linux-arm-kernel