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

[Sinan Kaya]

On 11/10/2015 11:47 AM, Arnd Bergmann wrote:
> On Tuesday 10 November 2015 11:06:40 Sinan Kaya wrote:
> > On 11/10/2015 3:38 AM, Arnd Bergmann wrote:
> >   > No, as Timur found, the driver is correct and it intentionally
> > > sets the 32-bit mask, and that is guaranteed to work on all sane
> > > hardware. Don't change the driver but find a better platform for
> > > your workload, or talk to the people that are responsible for
> > > the platform and get them to fix it.
> >
> > Platform does have an IOMMU. No issues there. I am trying to clean out
> > the patch pipe I have in order to get this card working with and without
> > IOMMU.
>
> On PowerPC, I think we automatically enable the IOMMU whenever a DMA
> mask is set that doesn't cover all of the RAM. We could think about
> doing the same thing on ARM64 to make all devices work out of the box.

The ACPI IORT table declares whether you enable IOMMU for a particular 
device or not. The placement of IOMMU HW is system specific. The IORT 
table gives the IOMMU HW topology to the operating system.

> > > If the platform also doesn't have an IOMMU, you can probably work
> > > around it by setting up the dma-ranges property of the PCI host
> > > to map the low PCI addresses to the start of RAM. This will also
> > > require changes in the bootloader to set up the PCI outbound translation,
> > > and it will require implementing the DMA offset on ARM64, which I was
> > > hoping to avoid.
> >
> >   From the email thread, it looks like this was introduced to support
> > some legacy card that has 64 bit addressing limitations and is being
> > carried around ("rotted") since then.
> >
> > I'm the second guy after the powerpc architecture complaining about the
> > very same issue. Any red flags?
>
> 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.

Maybe, maybe not. This is the only card that I had problems with.

> > I can't change the address map for PCIe. SBSA requires all inbound PCIe
> > addresses to be non-translated.
>
> What about changing the memory map? I suspect there will be more
> problems for you in the future when all of your RAM is at high
> addresses.  Is this something you could fix in the bootloader by
> moving the first 2GB to a different CPU physical address?

I'm thinking about this.

> > I'll just have to stick with IOMMU for this card.
>
> Ok. But how do you currently decide whether to use the IOMMU or not?

ACPI table. I wanted to get this fix in so that all operating systems 
whether they have IOMMU driver enabled or not would work.

> 	Arnd

--
Sinan Kaya
Qualcomm Technologies, Inc. on behalf of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a 
Linux Foundation Collaborative Project
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