Re: [PATCH V2 1/3] scsi: mptxsas: try 64 bit DMA when 32 bit DMA fails

From: Arnd Bergmann
Date: Tue Nov 10 2015 - 14:14:28 EST

On Tuesday 10 November 2015 11:00:59 Timur Tabi wrote:
> On 11/10/2015 10:47 AM, Arnd Bergmann wrote:
> > What BenH was worried about here is that the driver sets different masks
> > for streaming and coherent mappings, which is indeed a worry that
> > could hit us on ARM as well, but I suppose we'll have to deal with
> > that in platform code.
> >
> > Setting both masks to 32-bit is something that a lot of drivers do,
> > and without IOMMU enabled, you'd hit the same bug on all of them.
> Also note that I think that on PowerPC, the mask is set to 32 by default
> for all devices. I don't think we do that on ARM64. So on PowerPC,
> some drivers get away with not explicitly setting the mask.

If the mask is 64-bit by default on ARM64, that is a bug that we need
to fix urgently. Can you verify this?

A lot of PCI devices can only do 32-bit DMA, and we have plenty
of drivers that don't bother setting a mask at all because the 32-bit
mask is the default on all other architectures.

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