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From: James Bottomley <James.Bottomley@HansenPartnership.com>
To: Christoph Hellwig <hch@lst.de>,
	iommu@lists.linux-foundation.org,
	Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Vladimir Murzin <vladimir.murzin@arm.com>,
	Takashi Iwai <tiwai@suse.de>, Helge Deller <deller@gmx.de>,
	Robin Murphy <robin.murphy@arm.com>,
	Michal Simek <monstr@monstr.eu>,
	linux-arm-kernel@lists.infradead.org,
	linux-m68k@lists.linux-m68k.org, linux-parisc@vger.kernel.org,
	linux-sh@vger.kernel.org, linux-xtensa@linux-xtensa.org,
	linuxppc-dev@lists.ozlabs.org, x86@kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 7/8] parisc: don't set ARCH_NO_COHERENT_DMA_MMAP
Date: Thu, 15 Aug 2019 10:25:52 +0100
Message-ID: <1565861152.2963.7.camel@HansenPartnership.com> (raw)
In-Reply-To: <20190808160005.10325-8-hch@lst.de>

On Thu, 2019-08-08 at 19:00 +0300, Christoph Hellwig wrote:
> parisc is the only architecture that sets ARCH_NO_COHERENT_DMA_MMAP
> when an MMU is enabled.  AFAIK this is because parisc CPUs use VIVT
> caches,

We're actually VIPT but the same principle applies.

>  which means exporting normally cachable memory to userspace is
> relatively dangrous due to cache aliasing.
> 
> But normally cachable memory is only allocated by dma_alloc_coherent
> on parisc when using the sba_iommu or ccio_iommu drivers, so just
> remove the .mmap implementation for them so that we don't have to set
> ARCH_NO_COHERENT_DMA_MMAP, which I plan to get rid of.

So I don't think this is quite right.  We have three architectural
variants essentially (hidden behind about 12 cpu types):

   1. pa70xx: These can't turn off page caching, so they were the non
      coherent problem case
   2. pa71xx: These can manufacture coherent memory simply by turning off
      the cache on a per page basis
   3. pa8xxx: these have a full cache flush coherence mechanism.

(I might have this slightly wrong: I vaguely remember the pa71xxlc
variants have some weird cache quirks for DMA as well)

So I think pa70xx we can't mmap.  pa71xx we can provided we mark the
page as uncached ... which should already have happened in the allocate
and pa8xxx which can always mmap dma memory without any special tricks.

James


  parent reply index

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-08 15:59 remove default fallbacks in dma_map_ops v3 Christoph Hellwig
2019-08-08 15:59 ` [PATCH 1/8] dma-mapping: provide a better default ->get_required_mask Christoph Hellwig
2019-08-08 15:59 ` [PATCH 2/8] dma-mapping: move the dma_get_sgtable API comments from arm to common code Christoph Hellwig
2019-08-08 16:00 ` [PATCH 3/8] dma-mapping: explicitly wire up ->mmap and ->get_sgtable Christoph Hellwig
2019-08-08 16:00 ` [PATCH 4/8] dma-mapping: add a dma_can_mmap helper Christoph Hellwig
2019-08-08 16:00 ` [PATCH 5/8] ALSA: pcm: use dma_can_mmap() to check if a device supports dma_mmap_* Christoph Hellwig
2019-08-08 16:00 ` [PATCH 6/8] arm-nommu: call dma_mmap_from_dev_coherent directly Christoph Hellwig
2019-08-09  9:35   ` Sergei Shtylyov
2019-08-08 16:00 ` [PATCH 7/8] parisc: don't set ARCH_NO_COHERENT_DMA_MMAP Christoph Hellwig
2019-08-15  7:36   ` Christoph Hellwig
2019-08-15  9:25   ` James Bottomley [this message]
2019-08-15 10:50     ` Christoph Hellwig
2019-08-19 14:25       ` Christoph Hellwig
2019-08-08 16:00 ` [PATCH 8/8] dma-mapping: remove CONFIG_ARCH_NO_COHERENT_DMA_MMAP Christoph Hellwig
2019-08-09  7:21   ` Geert Uytterhoeven

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