Re: [RESEND2 PATCH 0/3] Fix kernel panic in dma-coherent

[Robin Murphy]

Hi Mark,

On 08/02/16 17:50, Mark Rutland wrote:
> On Mon, Feb 08, 2016 at 05:30:49PM +0000, Brian Starkey wrote:
> > Hi,
> >
> > I'm resending these again to try and garner some interest. Without
> > this series, dma-coherent cannot be used on arm64 platforms.
>
> I think you need to characterize that a bit better. I see plenty of
> instances of 'dma-coherent' in dts files, assuming you mean the DT
> dma-coehrent property.
>
> If not, feel free to stop reading now.

Terminology overload: the dma-coherent DT property is about devices 
having cache-coherent access to system memory within the linear map. The 
dma-coherent thing here is pretty much the precise opposite of that - 
namely creating CPU mappings for memory which belongs to the device 
itself and may be behind an I/O bus (e.g. a framebuffer on a video card).

> Currently dma-coherent isn't well-defined,

True dat.

Robin.

> but it's de-facto semantics
> are that a device makes accesses which are coherent with data accesses
> made by CPUs with Normal, Inner Shareable, Inner Write-Back Cacheable,
> Outer Write-Back Cacheable attributes.
>
> > The decision to add MEMREMAP_WC came out of a previous discussion with
> > Dan Williams and Catalin Marinas about the same problem[1].
>
> As pgprot_writecombine is Normal Non-Cacheable, this is a completely
> different idea of coherency to that described by the DT dma-coherent
> property.
>
> We should not overload that to mean different things.
>
> Thanks,
> Mark.
>
> > These patches implement a MEMREMAP_WC flag for memremap(), which can
> > be used to obtain writecombine mappings. This is then used for setting
> > up dma_coherent_mem regions which use the DMA_MEMORY_MAP flag.
> > They apply cleanly on 4.5-rc3.
> >
> > Patch 3 makes sure that the appropriate memset function is used
> > when zeroing coherent allocations, which fixes an alignment fault on
> > arm64.
> >
> > Best Regards,
> > Brian
> >
> > [1] http://lists.infradead.org/pipermail/linux-arm-kernel/2015-December/390857.html
> >
> > Brian Starkey (3):
> >    memremap: add MEMREMAP_WC flag
> >    drivers: dma-coherent: use MEMREMAP_WC for DMA_MEMORY_MAP
> >    drivers: dma-coherent: use memset_io for DMA_MEMORY_IO
> >
> >   drivers/base/dma-coherent.c |   25 ++++++++++++++++++++-----
> >   include/linux/io.h          |    1 +
> >   kernel/memremap.c           |   15 +++++++++++++--
> >   3 files changed, 34 insertions(+), 7 deletions(-)
> >
> > --
> > 1.7.9.5
> >
> >
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