Re: [PATCH v8 4/9] pci: OF: Fix the conversion of IO ranges into IO resources.

[Liviu Dudau <Liviu.Dudau@arm.com>]

On Wed, Jul 09, 2014 at 09:31:50AM +0100, Arnd Bergmann wrote:
> On Tuesday 08 July 2014, Liviu Dudau wrote:
> > On Mon, Jul 07, 2014 at 10:22:00PM +0100, Arnd Bergmann wrote:
> > > 
> > > I looked at the other drivers briefly, and I think you indeed fix the Tegra
> > > driver with this but break the integrator driver as mentioned above.
> > > The other callers of of_pci_range_to_resource() are apparently not
> > > impacted as they recalculate the values they get.
> > 
> > I would argue that integrator version is having broken assumptions. If it would
> > try to allocate that IO range or request the resource as returned currently by
> > of_pci_range_to_resource (without my patch) it would fail. I know because I did
> > the same thing in my host bridge driver and it failed miserably. That's why I
> > tried to patch it.
> 
> The integrator code was just introduced and the reason for how it does things
> is the way that of_pci_range_to_resource() works today. We tried to cope with
> it and not change the existing behavior in order to not break any other drivers.
> 
> It's certainly not fair to call the integrator version broken, it just works
> around the common code having a quirky interface. We should probably have
> done of_pci_range_to_resource better than it is today (I would have argued
> for it to return an IORESOURCE_MEM with the CPU address), but it took long
> enough to get that merged and I was sick of arguing about it.

Understood. That is why I have carefully worded my email as not to diss anyone.
I didn't say the code is broken, I've said it has broken assumptions.

> 
> > If the IO space is memory mapped, then we use the port number, the io_offset
> > and the PCI_IOBASE to get to the virtual address that, when accessed, will
> > generate the correct addresses on the bus, based on what the host bridge has
> > been configured.
> > 
> > This is the current level of my understanding of PCI IO.
> 
> Your understanding is absolutely correct, and that's great because very few
> people get that right. What I think we're really arguing about is what the
> of_pci_range_to_resource is supposed to return. As you and Bjorn both pointed
> out earlier, there are in fact two resources associated with the I/O window
> and the flaw in the current implementation is that of_pci_range_to_resource
> returns the numeric values for the IORESOURCE_MEM resource, but sets the
> type to IORESOURCE_IO, which is offset from that by PCI_IOBASE.
> 
> You try to fix that by making it return the correct IORESOURCE_IO resource,
> which is a reasonable approach but you must not break drivers that rely
> on the broken resource while doing that.

Or I need to fix the existing drivers that rely on the old behaviour.

> 
> The approach that I would have picked is to return the IORESOURCE_MEM
> resource associated with the I/O window and pick a (basically random)
> IORESOURCE_IO resource struct based on what hasn't been used and then
> compute the appropriate io_offset from that. This approach of course
> would also have required fixing up all drivers relying on the current
> behavior.
> 
> To be clear, I'm fine with you (and Bjorn if he cares) picking the
> approach you like here, either one of these works fine as long as the
> host drivers use the interface in the way it is defined.

OK. Thanks for that. It does look like either way some existing code needs
fixing, so I will have a look at that. Unless Bjorn votes for making a new
version of pci_range_to_resource().

> 
> > Now, I believe Rob has switched entirely to using my series in some test that
> > he has run and he hasn't encountered any issues, as long as one remembers in
> > the host bridge driver to add the io_base offset to the .start resource. If
> > not then I need to patch pci_v3.c.
> 
> The crazy part of all these discussions is that basically nobody ever uses
> I/O port access, so it's very hard to test and we don't even notice when
> we get it wrong, but we end up spending most of the time for PCI host controller
> reviews trying to get these right.
> 
> I'm very thankful that you are doing this work to get it moved into common
> code so hopefully this is the last time we ever have to worry about it because
> all future drivers will be able to use the common implemnetation.

Ahh, we humans! We always hope for the best! :)

My only chance of succeeding is if I make it a no brainer for people to use the
code. At the moment the interface for host bridge drivers is not too bad, but
it looks like the internals are still hard to comprehend.

Best regards,
Liviu

> 
> 	Arnd
> 

-- 
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| but they're not |
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