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 ? > > Question is: why pci_mmap_page_range() should apply an additional > > shift to the VMA pgoff based on pci_sys_data.mem_offset, which represents > > the offset from cpu->bus offset. I do not understand that. PowerPC > > does not seem to apply that fix-up (in PowerPC __pci_mmap_make_offset there > > is commented out code which prevents the pci_mem_offset shift to be > > applied). I think it all boils down to what the userspace interface is > > expecting when the memory areas are mmapped, if anyone has comments on > > this that is appreciated. > > The important part is certainly that whatever transformation is done > by pci_resource_to_user() gets undone by __pci_mmap_make_offset(). Exactly, it does not seem to be the case above, that's why I asked. > In case of PowerPC and Microblaze, the mem_offset handling is commented > out in both, to work around X11 trying to use the same values on > /dev/mem. However, they do have the respective fixup for io_offset. > > sparc applies the offset in both places for both io_offset and mem_offset. > xtensa applies only io_offset in __pci_mmap_make_offset but neither > in pci_resource_to_user. This probably works because the mem_offset is > always zero there. > mips applies a different fixup (for 36-bit addressing), but not the > mem_offset. > > Every other architecture applies no offset here, neither in __pci_mmap_make_offset/pci_mmap_page_range nor in pci_resource_to_user > > The only hint I could find for how the ARM version came to be is > from the historic kernel tree git log for linux-2.5.42, which added > the current code as > > 2002/10/13 11:05:47+01:00 rmk > [ARM] Update pcibios_enable_device, supply pci_mmap_page_range() > Update pcibios_enable_device to only enable requested resources, > mainly for IDE. Supply a pci_mmap_page_range() function to allow > user space to mmap PCI regions. > > At that point, only two platforms had a nonzero mem_offset: > footbridge/dc21285 and integrator/pci_v3. Both were using VGA, > and presumably used this to make X work. (rmk might remember > details). I think that, as I mentioned, it boils down to what the userspace interface (proc/sys and they seem to differ) is supposed to be passed from userspace processes upon mmap. > The code at the time matched what powerpc and sparc did, but then > both implemented pci_resource_to_user() in order for libpciaccess > to work correctly (bcea1db16b for sparc, 463ce0e103f for powerpc), > and later powerpc changed it again to not apply the offset in > pci_resource_to_user or pci_mmap_page_range in 396a1a5832ae. I will keep investigating, thank you for your help, any further comments appreciated. Lorenzo -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html