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

[Vladimir Murzin <vladimir.murzin@arm.com>]

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
>