[RFC PATCH v0.001] PCI: Add support for tango PCIe controller

[Robin Murphy]

On 21/03/17 10:15, Mason wrote:
> [ Adding iommu ML ]
> 
> On 17/03/2017 17:11, Mason wrote:
> 
>> + * QUIRK #5
>> + * Only transfers within the BAR are forwarded to the host.
>> + * By default, the DMA framework expects that
>> + * PCI address 0x8000_0000 -> CPU address 0x8000_0000
>> + * which is where DRAM0 is mapped.
> 
> I have an additional question on this topic.
> 
> In a typical system, PCI bus masters are able to access all of RAM,
> unless there is some kind of IOMMU, right?

Typically, yes (as always, exceptions exist, but it happens that we
still need some work to actually support those cases properly).

> I suppose one may consider the above limitation ("Only transfers
> within the BAR are forwarded to the host") as some form of weird
> IOMMU? (There is, in fact, some remapping logic in the controller
> setup which I haven't discussed so far.)

Not really. If it's the 8x128MB region thing mentioned elsewhere, that's
far too coarse a granularity to be much use with the existing IOMMU API
(this vaguely reminds me of a similar discussion about programmable
interconnects ages ago). Unless it's actually got some sort of GART-type
thing or better capable of page-granularity mappings within a
significantly-sized region, I'd put that idea to bed.

> Since this SoC is used for TV, the media cartels mandate some way
> to limit where PCI bus masters can peek/poke in RAM.

FWIW, since that sounds like more of a box-ticking exercise than a real
practical concern, I'd point out that content protection is more or less
the poster child for TrustZone, and your TZASC should help tick that box
regardless.

Robin.

> 
> Regards.
>