pci_ioremap_set_mem_type(), pci_remap_iospace()

[Thomas Petazzoni]

Hello,

Adding in Cc Arnd Bergmann, since he participated to the discussion
several years ago around the PCI DT binding for the Armada platform.

On Thu, 28 Apr 2016 15:41:17 +0100, Liviu Dudau wrote:

> > See how the address and size are 0 ? 
> 
> PCI or host address? Either are not zero, I'm afraid: PCI is 0x1 0, 0x2 0 ...
> while host address is given by MBUS_ID(...) 0. Anyway ...

The host address is kind of a "fake address". "MBUS_ID(..., ....) 0" is
not something that you can ioremap. See
Documentation/devicetree/bindings/pci/mvebu-pci.txt:

"""
Since the location and size of the different MBus windows is not fixed
in
hardware, and only determined in runtime, those ranges cover the full
first
4 GB of the physical address space, and do not translate into a valid
CPU
address.
"""

> > We don't know at the moment of
> > scanning the DT, what will be the address and size of the different MEM
> > and IO mappings.
> 
> True, but you have bounds on the sizes of each region given the way you
> encode the address translation. Trying to decode what the example above is
> telling me:
>   - each port has 0x80000_00000000 possible IO space allocated based on
>     the MBUS_ID split of address space
>   - same for MEM space.
> 
> And IMO you *should* have address and sizes for the MEM and IO mappings, as
> they act as *upper* boundaries. No one says you need to reserve the whole
> space, you are just describing how the hardware translates addresses between
> the busses.

Hm, not sure what would be the benefit of changing the Device Tree with
this, but I'm probably missing something.

> Just for my own clarification, is the reason why the ranges are declared like
> this due to the fact that each port is a separate entity and multiple ports
> cannot be served by the same MBus window? What stops you from having one
> MBus window assigned to all IO space and the other window(s) assigned to MEM
> for individual ports?

Each port needs its own MBus window for the PCI memory space and PCI
I/O space that is accessed by this port. The MBUS_ID(x, y) thing is
here to encode the information that are needed to create such windows,
which as you can see are different for each port.

So we cannot have a single MBus window for all the IO space, we need
one per PCI port.

Best regards,

Thomas
-- 
Thomas Petazzoni, CTO, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com