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

James Bottomley
2015-11-10T18:27:14Z

On Tue, 2015-11-10 at 12:19 -0500, Sinan Kaya wrote:
> 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:
> >> 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.

Your characterisation of "some legacy card" isn't entirely correct.
Just to clarify how this happens, most I/O cards today are intelligent
offload engines which means they have some type of embedded CPU (it can
even be a specially designed asic). This CPU is driven by firmware
which is mostly (but not always) in the machine language of the CPU.
DMA transfers are sometimes run by this CPU, but mostly handed off to a
separate offload engine. When the board gets revised, it's often easier
to update the offload engine to 64 bits and keep the CPU at 32 (or even
16) bits. This means that all the internal addresses in the firmware
are 32 bit only. As I read the comments in the original thread, it
looks like the mpt people tried to mitigate this by using segment
registers for external addresses firmware uses ... that's why they say
that they don't have to have all the addresses in DMA32 ... they just
need the upper 32 bits to be constant so they can correctly program the
segment register. Unfortunately, we have no way to parametrise this to
the DMA allocation code.

You'll find the same thing with Adaptec SPI cards. Their route to 64
bits was via an initial 39 bit extension that had them layering the
additional 7 bits into the unused lower region of the page descriptors
for the firmware (keeping the actual pointers to DMA at 32 bits because
they're always parametrised as address, offset, length and the address
is always a 4k page).

Eventually, everything will rev to 64 bits and this problem will go
away, but, as I suspect you know, it takes time for the embedded world
to get to where everyone else already is.

As Arnd said, if you failed to allow for this in your platform, then
oops, just don't use the card. I think this solution would be better
than trying to get the driver to work out which cards can support 64 bit
firmware descriptors and only failing on your platform for those that
can't.

James