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

[zhichang.yuan]

On 09/22/2016 11:20 PM, Gabriele Paoloni wrote:
> > -----Original Message-----
> > From: Arnd Bergmann [mailto:arnd@xxxxxxxx]
> > Sent: 22 September 2016 15:59
> > To: Gabriele Paoloni
> > Cc: zhichang; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx;
> > devicetree@xxxxxxxxxxxxxxx; lorenzo.pieralisi@xxxxxxx; minyard@xxxxxxx;
> > linux-pci@xxxxxxxxxxxxxxx; gregkh@xxxxxxxxxxxxxxxxxxx; John Garry;
> > will.deacon@xxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Yuanzhichang;
> > Linuxarm; xuwei (O); linux-serial@xxxxxxxxxxxxxxx;
> > benh@xxxxxxxxxxxxxxxxxxx; zourongrong@xxxxxxxxx; liviu.dudau@xxxxxxx;
> > kantyzc@xxxxxxx
> > Subject: Re: [PATCH V3 2/4] ARM64 LPC: LPC driver implementation on
> > Hip06
> >
> > On Thursday, September 22, 2016 2:47:14 PM CEST Gabriele Paoloni wrote:
> > > > >   static int of_empty_ranges_quirk(struct device_node *np)
> > > > >   {
> > > > >          if (IS_ENABLED(CONFIG_PPC)) {
> > > > > @@ -503,7 +512,7 @@ static int of_translate_one(struct
> > device_node
> > > > *parent, struct of_bus *bus,
> > > > >           * This code is only enabled on powerpc. --gcl
> > > > >           */
> > > > >          ranges = of_get_property(parent, rprop, &rlen);
> > > > > -       if (ranges == NULL && !of_empty_ranges_quirk(parent)) {
> > > > > +       if (ranges == NULL && !of_empty_ranges_quirk(parent) &&
> > > > !of_isa_indirect_io(parent)) {
> > > > >                  pr_debug("OF: no ranges; cannot translate\n");
> > > > >                  return 1;
> > > > >          }
> > > > I don't see what effect that would have. What do you want to
> > > > achieve with this?
> > > If I read the code correctly adding the function above would end
> > > up in a 1:1 mapping:
> > > http://lxr.free-electrons.com/source/drivers/of/address.c#L513
> > >
> > > so taddr will be assigned with the cpu address space specified
> > > in the children nodes of LPC and we are not using a quirk function
> > > (we are just checking that we have the indirect io assigned and
> > > that we are on a ISA bus). Now probably there is a nit in my
> > > code sketch where of_isa_indirect_io should be probably an
> > architecture
> > > specific function...
> > But the point is that it would then return an incorrect address,
> > which in the worst case could be the same as another I/O space
> > if that happens to be at CPU address zero.
> If we do not touch __of_address_to_resource after taddr is returned
> by of_translate_address we will check for (flags & IORESOURCE_IO),
> then we call pci_address_to_pio to retrieve the unique token (remember
> that LPC driver will register the LPC io range to pci io_range_list).
>
> I do not think that we can have any conflict with any other I/O space
> as pci_register_io_range will guarantee that the LPC range does not
> overlap with any other I/O range...
If we don't bypass the calling of pci_address_to_pio after 
of_translate_address,
there should no conflict between LPC logical IO range and other logical 
IO ranges
of other devices.
I guess Arnd want to skip all the translation for our LPC IO address. 
But if we do it
like that, it seems we can't avoid the possible conflict with the 
logical IO ranges of
PCI host bridges without any changes on the pci_register_io_range and 
pci_address_to_pio.
Because two completely separate I/O spaces are created without 
synchronization.

Best,
Zhichang
> > > > I think all we need from this function is to return '1' if
> > > > we hit an ISA I/O window, and that should happen for the two
> > > > interesting cases, either no 'ranges' at all, or no translation
> > > > for the range in question, so that __of_translate_address can
> > > > return OF_BAD_ADDR, and we can enter the special case
> > > > handling in the caller, that handles it like
> > > I don't think this is very right as you may fail for different
> > > reasons other than a missing range property, e.g:
> > > http://lxr.free-electrons.com/source/drivers/of/address.c#L575
> > >
> > > And even if the only failure case was a missing range if in the
> > > future __of_translate_address had to be reworked we would again
> > > make a wrong assumption...you get my point?
> > The newly introduced function would clearly have to make
> > some sanity checks. The idea is that treat the case of
> > not being able to translate a bus specific I/O address
> > into a CPU address literally and fall back to another method
> > of translating that address.
> >
> > This matches my mental model of how we find the resource:
> >
> > - start with the bus address
> > - try to translate that into a CPU address
> > - if we arrive at a CPU physical address for IORESOURCE_MEM, use that
> > - if we arrive at a CPU physical address for IORESOURCE_IO, translate
> >    that into a Linux IORESOURCE_IO token
> > - if there is no valid CPU physical address, try to translate
> >    the address into an IORESOURCE_IO using the ISA accessor
> > - if that fails too, give up.
> >
> > If you try to fake a CPU physical address inbetween, it just
> > gets more confusing.
> >
> > 	Arnd