On Wed, Jul 9, 2014 at 3:31 AM, Arnd Bergmann <arnd@xxxxxxxx> 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. > >> 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. What is io_offset supposed to be and be based on? > 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. > > 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. > >> 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. FWIW, I test i/o accesses with Versatile QEMU. The LSI53xxxx device in the model has a kconfig option to use i/o accesses. However, I have seen in the past this is an area where 2 wrongs can make a right. Rob -- To unsubscribe from this list: send the line "unsubscribe linux-pci" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
