Re: [RFC 2/4] PCI: generic: Add support for ARM64 and MSI(x)

Lorenzo Pieralisi
2014-10-09T09:04:58Z

On Wed, Oct 08, 2014 at 03:47:43PM +0100, Arnd Bergmann wrote:
> On Wednesday 08 October 2014 11:19:43 Lorenzo Pieralisi wrote:
> >
> > Ok. So, unless I am missing something, on platform with mem_offset != 0
> > /proc and /sys interfaces for remapping PCI resources can't work (IIUC
> > the proc interface expects the user to pass in the resource address as
> > seen from /proc/bus/pci/devices - which are not BAR values. Even if the
> > user passed the BAR value to mmap, pci_mmap_fits() in proc_bus_pci_mmap()
> > would fail since it compares the pgoff to resource values, which are not
> > BAR values).
>
> I think you are right for the sysfs interface, that one can't possibly
> work because of the incorrect address computation.
>
> For the /procfs interface, I think it can work as long as the offsets
> used there are coming from the config space dump in /proc/bus/pci/*
> rather than from the /sys/bus/pci/devices/*/resource file.
>
> > As things stand I think we can safely remove the mem_offset (and
> > pci_sys_data dependency) from pci_mmap_page_range(). I do not think
> > we can break userspace in any way, basically because it can't work at
> > the moment, again, happy to be corrected if I am wrong, please shout.
>
> Please look at the procfs interface again. That one can be defined
> in two ways (either like sparc and arm, or like powerpc and microblaze)
> but either one should be able to work with user space that expects
> that interface and break with user space that expects the other one.

I agree as long as pci_mmap_page_range() is concerned, but I am
referring to the pci_mmap_fits() implementation here:

start = vma->vm_pgoff;
size = ((pci_resource_len(pdev, resno) - 1) >> PAGE_SHIFT) + 1;
pci_start = (mmap_api == PCI_MMAP_PROCFS) ?
pci_resource_start(pdev, resno) >> PAGE_SHIFT : 0;
if (start >= pci_start && start < pci_start + size &&
start + nr <= pci_start + size)
return 1;
return 0;

pci_mmap_fits(), when mapping from procfs always check the offset against
resources, which are fixed-up addresses. If we passed the values dumped
from the device config space (as pci_mmap_page_range() expects on arm) IMHO
the check above would fail (always referring to platforms where
mem_offset != 0).

Last changes where introduced by commit 8c05cd08a, whose commit log adds
to my confusion:

"[...] I think what we want here is for pci_start to be 0 when mmap_api ==
PCI_MMAP_PROCFS.[...]"

But that's not what the code does.

I will try to grab an integrator board to give it a go.

> > Or we can add mem_offset to the host bridge (after all architectures like
> > PowerPC and Microblaze have a pci_mem_offset variable in their host
> > controllers), but still, this removes pci_sys_data dependency but does
> > not solve the pci_mmap_page_range() issue.
>
> The host bridge already stores the mem_offset in terms of the resource
> list, so we could readily use that, except that it might break the
> powerpc hack if that is still in use.

Well, yes, I am not saying it can't be done by using the resources list,
I am just trying to understand if that's really useful.

Thank you !
Lorenzo