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

[Liviu Dudau]

On Tue, Mar 21, 2017 at 11:15:16AM +0100, 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?

Depends where the ATS (Address translation service) lives and what gets
programmed there, but usually one can restrict the RAM (or system address
space, to be more precise) that can be accessed from the PCI bus masters
by only providing PCI(e)-to-system bus mappings that make sense. Remember
that on any PCI bus you can only see addresses that are a subset of your
parent bus addresses. If the root PCI bus only allows access to a predefined
range of addresses, then that is all you can access in the system.

> 
> 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 sure I understand the quirk. What BAR are you referring to when you say
"within the BAR"? The bus master's? Are you saying that only if you write
to a PCI bus address XXXXXXX that is in the range assigned by the parent
bus then you can escape your bus hierarchy and target the system RAM? That
sounds backwards to me.

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

And that is usually done by either having a TrustZone controller on top of
your PCI host bridge or by putting an IOMMU.

Best regards,
Liviu

> 
> Regards.

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