[RFC v2 PATCH 0/3] Fix dma_alloc_coherent() and friends for NOMMU

[Alexandre Torgue]

Hi Vladimir,

On 12/13/2016 02:45 PM, Vladimir Murzin wrote:
> Hi,
>
> 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. Since we have MPU support in
> Linux for R-class only and M-class setting MPU in bootloader, proposed
> interface to advertise such memory is via "memdma=size at start" command
> line option, to avoid clashing with normal memory (which usually comes
> from dts) it'd be safer to use it together with "mem=" command line
> option. Meanwhile, I'm open to suggestions for the better way telling
> Linux of such memory.
>
> For configuration without cache support (like Cortex-M3/M4) dma
> operations are forced to be coherent and wired with dma-noop. Such
> decision is made based on cacheid global variable. In case cpu
> supports caches and no coherent memory region is given - dma is
> disallowed. Probably, some other important checks are missing, so I'll
> all my ears :)
>
> To make life easier NOMMU dma operations are kept in separate
> compilation unit.
>
> Thanks!
>
> Changelog:
>
>     RFC v1 -> RFC v2
>            - s/dmac_unmap_area/dmac_map_area in __dma_page_cpu_to_dev()
> 	   - removed unrelated changes in nommu.c
>
> Vladimir Murzin (3):
>   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
>
>  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    |  262 ++++++++++++++++++++++++++++++++++++
>  arch/arm/mm/dma-mapping.c          |   26 +---
>  arch/arm/mm/mm.h                   |    3 +
>  arch/arm/mm/nommu.c                |    6 +
>  7 files changed, 278 insertions(+), 29 deletions(-)
>  create mode 100644 arch/arm/mm/dma-mapping-nommu.c
>

First, thanks for this series.

I tested it on stm32f746 platform. Main issues related to cache and DMA 
are fixed but I still have an issue using dma_zalloc_alloc API. 
Allocated memory is not set to zero.
Can you have a look on it please?

Thanks
Alex