Re: [PATCH v2] arm64: do not set dma masks that device connection can't handle

Will Deacon
2017-01-10T11:51:34Z

On Mon, Jan 09, 2017 at 10:30:02AM +0300, Nikita Yushchenko wrote:
> It is possible that device is capable of 64-bit DMA addresses, and
> device driver tries to set wide DMA mask, but bridge or bus used to
> connect device to the system can't handle wide addresses.
>
> With swiotlb, memory above 4G still can be used by drivers for streaming
> DMA, but *dev->mask and dev->dma_coherent_mask must still keep values
> that hardware handles physically.
>
> This patch enforces that. Based on original version by
> Arnd Bergmann <arnd@arndb.de>, extended with coherent mask hadnling.
>
> Signed-off-by: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
> CC: Arnd Bergmann <arnd@arndb.de>
> ---
> Changes since v1:
> - fixed issues noted by Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
> - save mask, not size
> - remove doube empty line
>
> arch/arm64/Kconfig | 3 +++
> arch/arm64/include/asm/device.h | 1 +
> arch/arm64/mm/dma-mapping.c | 51 +++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 55 insertions(+)

I still don't think this patch is general enough. The problem you're seeing
with swiotlb seems to be exactly the same problem reported by Feng Kan over
at:

http://lkml.kernel.org/r/CAL85gmA_SSCwM80TKdkZqEe+S1beWzDEvdki1kpkmUTDRmSP7g@mail.gmail.com

[read on; it was initially thought to be a hardware erratum, but it's
actually the inability to restrict the DMA mask of the endpoint that's
the problem]

The point here is that an IOMMU doesn't solve your issue, and the
IOMMU-backed DMA ops need the same treatment. In light of that, it really
feels to me like the DMA masks should be restricted in of_dma_configure
so that the parent mask is taken into account there, rather than hook
into each set of DMA ops to intercept set_dma_mask. We'd still need to
do something to stop dma_set_mask widening the mask if it was restricted
by of_dma_configure, but I think Robin (cc'd) was playing with that.

Will