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From: Christoph Hellwig <hch@lst.de>
To: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Christoph Hellwig <hch@lst.de>,
	iommu@lists.linux-foundation.org,
	Marek Szyprowski <m.szyprowski@samsung.com>,
	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 12:50:02 +0200
Message-ID: <20190815105002.GA30805@lst.de> (raw)
In-Reply-To: <1565861152.2963.7.camel@HansenPartnership.com>

On Thu, Aug 15, 2019 at 10:25:52AM +0100, James Bottomley wrote:
> >  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.

Except for the different naming scheme vs the code this matches my
assumptions.

In the code we have three cases (and a fourth EISA case mention in
comments, but not actually implemented as far as I can tell):

arch/parisc/kernel/pci-dma.c says in the top of file comments:

** AFAIK, all PA7100LC and PA7300LC platforms can use this code.

and the handles two different case.  for cpu_type == pcxl or pcxl2
it maps the memory as uncached for dma_alloc_coherent, and for all
other cpu types it fails the coherent allocations.

In addition to that there are the ccio and sba iommu drivers, of which
according to your above comment one is always present for pa8xxx.

Which brings us back to this patch, which ensures that no cacheable
memory is exported to userspace by removing ->mmap from ccio and sba.
It then enabled dma_mmap_coherent for the pcxl or pcxl2 case that
allocates uncached memory, which dma_mmap_coherent does not work
because dma_alloc_coherent already failed for the !pcxl && !pcxl2
and thus there is no memory to mmap.

So if the description is too confusing please suggest a better
one, I'm a little lost between all these code names and product
names (arch/parisc/include/asm/dma-mapping.h uses yet another set).

  reply index

Thread overview: 16+ 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
2019-08-15 10:50     ` Christoph Hellwig [this message]
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
2019-08-29 15:38 ` remove default fallbacks in dma_map_ops v3 Christoph Hellwig

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