Re: [PATCH V3 2/4] ARM64 LPC: LPC driver implementation on Hip06

[Arnd Bergmann <arnd@arndb.de>]

On Thursday, September 15, 2016 12:05:51 PM CEST Gabriele Paoloni wrote:
> > -----Original Message-----
> > On Thursday, September 15, 2016 8:02:27 AM CEST Gabriele Paoloni wrote:
> > >
> > > From <<3.1.1. Open Firmware Properties for Bus Nodes>> in
> > > http://www.firmware.org/1275/bindings/isa/isa0_4d.ps
> > >
> > > I quote:
> > > "There shall be an entry in the "ranges" property for each
> > > of the Memory and/or I/O spaces if that address space is
> > > mapped through the bridge."
> > >
> > > It seems to me that it is ok to have 1:1 address mapping and that
> > > therefore of_translate_address() should fail if "ranges" is not
> > > present.
> > 
> > The key here is the definition of "mapped through the bridge".
> > I can only understand this as "directly mapped", i.e. an I/O
> > port of the child bus corresponds directly to a memory address
> > on the parent bus, but this is not the case here.
> > 
> > The problem with adding the mapping here is that it looks
> > like it should be valid to create a page table entry for
> > the address returned from the translation and access it through
> > a pointer dereference, but that is clearly not possible.
> 
> I understand that somehow we are abusing of the ranges property
> here however the point is that with the current implementation ranges
> is needed because otherwise the ipmi driver probe will fail here:
> 
> of_ipmi_probe -> of_address_to_resource -> __of_address_to_resource
> -> of_translate_address -> __of_translate_address
> 
> Now we had a bit of discussion internally and to avoid
> having ranges we came up with two possible solutions:
> 
> 1) Using bit 3 of phys.hi cell in 2.2.1 of
> http://www.firmware.org/1275/bindings/isa/isa0_4d.ps
> This would mean reworking of_bus_isa_get_flags in 
> http://lxr.free-electrons.com/source/drivers/of/address.c#L398
> and setting a new flag to be checked in __of_address_to_resource
> 
> 2) Adding a property in the bindings of each device that is
> a child of our LPC bus and modify __of_address_to_resource
> to check if the property is in the DT and eventually bypass
> of_translate_address
> 
> However in both 1) and 2) there are some issues:
> in 1) we are not complying with the isa binding doc (we use
> a bit that should be zero); in 2) we need to modify the
> bindings documentation of the devices that are connected
> to our LPC controller (therefore modifying other devices
> bindings to fit our special case).
> 
> I think that maybe having the 1:1 range mapping doesn't
> reflect well the reality but it is the less painful
> solution...
> 
> What's your view?

We can check the 'i' bit for I/O space in of_bus_isa_get_flags,
and that should be enough to translate the I/O port number.

The only part we need to change here is to not go through
the crazy conversion all the way from PCI I/O space to a
physical address and back to a (logical) port number
that we do today with of_translate_address/pci_address_to_pio.

I can think of a several of ways to fix __of_address_to_resource
to just do the right thing according to the ISA binding to
make the normal drivers work.

The easiest solution is probably to hook into the
"taddr == OF_BAD_ADDR" case in __of_address_to_resource
and add a lookup for ISA buses there, and instead check
if some special I/O port operations were registered
for the port number, using an architecture specific
function that arm64 implements. Other architectures
like x86 that don't have a direct mapping between I/O
ports and MMIO addresses would implement that same
function differently.

	Arnd