On Sun, 26 Apr 2020 at 11:08, Christian König <christian.koenig@xxxxxxx> wrote:
>
> Am 25.04.20 um 19:32 schrieb Ard Biesheuvel:
> > On Tue, 21 Apr 2020 at 19:07, Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
> >> On Tue, 21 Apr 2020 at 18:43, Christian König <christian.koenig@xxxxxxx> wrote:
> >>> Am 21.04.20 um 18:22 schrieb Ard Biesheuvel:
> >>>> When resizing a BAR, pci_reassign_bridge_resources() is invoked to
> >>>> bring the bridge windows of parent bridges in line with the new BAR
> >>>> assignment.
> >>>>
> >>>> This assumes that the device whose BAR is being resized lives on a
> >>>> subordinate bus, but this is not necessarily the case. A device may
> >>>> live on the root bus, in which case dev->bus->self is NULL, and
> >>>> passing a NULL pci_dev pointer to pci_reassign_bridge_resources()
> >>>> will cause it to crash.
> >>>>
> >>>> So let's make the call to pci_reassign_bridge_resources() conditional
> >>>> on whether dev->bus->self is non-NULL in the first place.
> >>>>
> >>>> Fixes: 8bb705e3e79d84e7 ("PCI: Add pci_resize_resource() for resizing BARs")
> >>>> Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> >>> Sounds like it makes sense, patch is
> >>> Reviewed-by: Christian König <christian.koenig@xxxxxxx>.
> >> Thanks Christian.
> >>
> >>> May I ask where you found that condition?
> >>>
> >> In this particular case, it was on an ARM board with funky PCIe IP
> >> that does not expose a root port in its bus hierarchy.
> >>
> >> But in the general case, PCIe endpoints can be integrated into the
> >> root complex, in which case they appear on the root bus, and there is
> >> no reason such endpoints shouldn't be allowed to have resizable BARs.
> > Actually, looking at this more carefully, I think
> > pci_reassign_bridge_resources() needs to do /something/ to ensure that
> > the resources are reshuffled if needed when the resized BAR overlaps
> > with another one.
>
> The resized BAR never overlaps with an existing one since to resize a
> BAR it needs to be deallocated and disabled. This is done as a
> precaution to avoid potential incorrect routing and decode of memory access.
>
> The call to pci_reassign_bridge_resources() is only there to change the
> resources of the upstream bridge to the device which is resized and not
> to adjust the resources of the device itself.
>
So does that mean that BAR resizing is only possible if no other BARs,
either on the same device or on other ones, need to be moved?
