On Tue, Oct 07, 2014 at 10:39:47PM +0100, Arnd Bergmann wrote: > On Tuesday 07 October 2014 15:47:50 Lorenzo Pieralisi wrote: > > On Tue, Oct 07, 2014 at 02:52:27PM +0100, Arnd Bergmann wrote: > > > On Tuesday 07 October 2014 13:06:59 Lorenzo Pieralisi wrote: > > > > On Wed, Oct 01, 2014 at 10:38:45AM +0100, Arnd Bergmann wrote: > > > > > > > > [...] > > > > > > > > > pci_mmap_page_range could either get generalized some more in an attempt > > > > > to have a __weak default implementation that works on ARM, or it could > > > > > be changed to lose the dependency on pci_sys_data instead. In either > > > > > case, the change would involve using the generic pci_host_bridge_window > > > > > list. > > > > > > > > On ARM pci_mmap_page_range requires pci_sys_data to retrieve its > > > > mem_offset parameter. I had a look, and I do not understand *why* > > > > it is required in that function, so I am asking. That function > > > > is basically used to map PCI resources to userspace, IIUC, through > > > > /proc or /sysfs file mappings. As far as I understand those mappings > > > > expect VMA pgoff to be the CPU address when files representing resources > > > > are mmapped from /proc and 0 when mmapped from /sys (I mean from > > > > userspace, then VMA pgoff should be updated by the kernel to map the > > > > resource). > > > > > > Applying the mem_offset is certainly the more intuitive way, since > > > that lets you read the PCI BAR values from a device and access the > > > device with the appropriate offsets. > > > > Ok, but I am referring to this snippet (drivers/pci/pci-sysfs.c): > > > > /* pci_mmap_page_range() expects the same kind of entry as coming > > * from /proc/bus/pci/ which is a "user visible" value. If this is > > * different from the resource itself, arch will do necessary fixup. > > */ > > pci_resource_to_user(pdev, i, res, &start, &end); > > > > --> Here start represents a CPU physical address, if pci_resource_to_user() > > does not fix it up, correct ? > > > > vma->vm_pgoff += start >> PAGE_SHIFT; > > > > [...] > > > > return pci_mmap_page_range(...); > > > > pci_mmap_page_range() applies (mem_offset >> PAGE_SHIFT) to pgoff in the > > ARM implemention. > > > > Is not there a mismatch here on platforms where mem_offset != 0 ? > > Yes, I think that's right: ARM never gained its own pci_resource_to_user() > implementation, presumably because nobody ran into this problem and > debugged it all the way. 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). 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. 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. Lorenzo -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
