Date:   Thu, 8 Jun 2017 17:25:55 +0100
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Vladimir Murzin <vladimir.murzin@....com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Mark Rutland <mark.rutland@....com>,
        Roger Quadros <rogerq@...com>, Joerg Roedel <jroedel@...e.de>,
        sza@....hu, arnd@...db.de,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
        Michal Nazarewicz <mina86@...a86.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Doug Ledford <dledford@...hat.com>,
        Rich Felker <dalias@...c.org>,
        Alan Stern <stern@...land.harvard.edu>, kbuild-all@...org,
        benjamin.gaignard@...aro.org, Rob Herring <robh+dt@...nel.org>,
        akpm@...ux-foundation.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        robin.murphy@....com, alexandre.torgue@...com
Subject: Re: [PATCH v5 0/7] ARM: Fix dma_alloc_coherent() and friends for
 NOMMU

Well, I've no objection to this, but it does need acks from other
people before I can apply it.

There's two patches that touch drivers/base that need Greg's ack.

I'm not sure what's happening with lib/dma-noop.c, there doesn't
appear to be a maintainer list for it, so I guess that's a
free-for-all.

On Thu, Jun 08, 2017 at 09:28:30AM +0100, Vladimir Murzin wrote:
> 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@...e.de>
> > Cc: Christian Borntraeger <borntraeger@...ibm.com>
> > Cc: Michal Nazarewicz <mina86@...a86.com>
> > Cc: Marek Szyprowski <m.szyprowski@...sung.com>
> > Cc: Alan Stern <stern@...land.harvard.edu>
> > Cc: Yoshinori Sato <ysato@...rs.sourceforge.jp>
> > Cc: Rich Felker <dalias@...c.org>
> > Cc: Roger Quadros <rogerq@...com>
> > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > Cc: Rob Herring <robh+dt@...nel.org>
> > Cc: Mark Rutland <mark.rutland@....com>
> > Cc: Doug Ledford <dledford@...hat.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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