On Tue, 23 Mar 2021 21:02:21 +0530
Amey Narkhede <ameynarkhed...@gmail.com> wrote:
> On 21/03/23 08:44AM, Alex Williamson wrote:
> > On Tue, 23 Mar 2021 15:34:19 +0100
> > Pali Rohár <p...@kernel.org> wrote:
> >
> > > On Thursday 18 March 2021 20:01:55 Amey Narkhede wrote:
> > > > On 21/03/17 09:13PM, Pali Rohár wrote:
> > > > > On Wednesday 17 March 2021 14:00:20 Alex Williamson wrote:
> > > > > > On Wed, 17 Mar 2021 20:40:24 +0100
> > > > > > Pali Rohár <p...@kernel.org> wrote:
> > > > > >
> > > > > > > On Wednesday 17 March 2021 13:32:45 Alex Williamson wrote:
> > > > > > > > On Wed, 17 Mar 2021 20:24:24 +0100
> > > > > > > > Pali Rohár <p...@kernel.org> wrote:
> > > > > > > >
> > > > > > > > > On Wednesday 17 March 2021 13:15:36 Alex Williamson wrote:
> > > > > > > > > > On Wed, 17 Mar 2021 20:02:06 +0100
> > > > > > > > > > Pali Rohár <p...@kernel.org> wrote:
> > > > > > > > > >
> > > > > > > > > > > On Monday 15 March 2021 09:03:39 Alex Williamson wrote:
> > > > > > > > > > > > On Mon, 15 Mar 2021 15:52:38 +0100
> > > > > > > > > > > > Pali Rohár <p...@kernel.org> wrote:
> > > > > > > > > > > >
> > > > > > > > > > > > > On Monday 15 March 2021 08:34:09 Alex Williamson
> > > > > > > > > > > > > wrote:
> > > > > > > > > > > > > > On Mon, 15 Mar 2021 14:52:26 +0100
> > > > > > > > > > > > > > Pali Rohár <p...@kernel.org> wrote:
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > > On Monday 15 March 2021 19:13:23 Amey Narkhede
> > > > > > > > > > > > > > > wrote:
> > > > > > > > > > > > > > > > slot reset (pci_dev_reset_slot_function) and
> > > > > > > > > > > > > > > > secondary bus
> > > > > > > > > > > > > > > > reset(pci_parent_bus_reset) which I think are
> > > > > > > > > > > > > > > > hot reset and
> > > > > > > > > > > > > > > > warm reset respectively.
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > No. PCI secondary bus reset = PCIe Hot Reset.
> > > > > > > > > > > > > > > Slot reset is just another
> > > > > > > > > > > > > > > type of reset, which is currently implemented
> > > > > > > > > > > > > > > only for PCIe hot plug
> > > > > > > > > > > > > > > bridges and for PowerPC PowerNV platform and it
> > > > > > > > > > > > > > > just call PCI secondary
> > > > > > > > > > > > > > > bus reset with some other hook. PCIe Warm Reset
> > > > > > > > > > > > > > > does not have API in
> > > > > > > > > > > > > > > kernel and therefore drivers do not export this
> > > > > > > > > > > > > > > type of reset via any
> > > > > > > > > > > > > > > kernel function (yet).
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > Warm reset is beyond the scope of this series, but
> > > > > > > > > > > > > > could be implemented
> > > > > > > > > > > > > > in a compatible way to fit within the
> > > > > > > > > > > > > > pci_reset_fn_methods[] array
> > > > > > > > > > > > > > defined here.
> > > > > > > > > > > > >
> > > > > > > > > > > > > Ok!
> > > > > > > > > > > > >
> > > > > > > > > > > > > > Note that with this series the resets available
> > > > > > > > > > > > > > through
> > > > > > > > > > > > > > pci_reset_function() and the per device reset
> > > > > > > > > > > > > > attribute is sysfs remain
> > > > > > > > > > > > > > exactly the same as they are currently. The bus
> > > > > > > > > > > > > > and slot reset
> > > > > > > > > > > > > > methods used here are limited to devices where only
> > > > > > > > > > > > > > a single function is
> > > > > > > > > > > > > > affected by the reset, therefore it is not like the
> > > > > > > > > > > > > > patch you proposed
> > > > > > > > > > > > > > which performed a reset irrespective of the
> > > > > > > > > > > > > > downstream devices. This
> > > > > > > > > > > > > > series only enables selection of the existing
> > > > > > > > > > > > > > methods. Thanks,
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > Alex
> > > > > > > > > > > > > >
> > > > > > > > > > > > >
> > > > > > > > > > > > > But with this patch series, there is still an issue
> > > > > > > > > > > > > with PCI secondary
> > > > > > > > > > > > > bus reset mechanism as exported sysfs attribute does
> > > > > > > > > > > > > not do that
> > > > > > > > > > > > > remove-reset-rescan procedure. As discussed in other
> > > > > > > > > > > > > thread, this reset
> > > > > > > > > > > > > let device in unconfigured / broken state.
> > > > > > > > > > > >
> > > > > > > > > > > > No, there's not:
> > > > > > > > > > > >
> > > > > > > > > > > > int pci_reset_function(struct pci_dev *dev)
> > > > > > > > > > > > {
> > > > > > > > > > > > int rc;
> > > > > > > > > > > >
> > > > > > > > > > > > if (!dev->reset_fn)
> > > > > > > > > > > > return -ENOTTY;
> > > > > > > > > > > >
> > > > > > > > > > > > pci_dev_lock(dev);
> > > > > > > > > > > > >>> pci_dev_save_and_disable(dev);
> > > > > > > > > > > >
> > > > > > > > > > > > rc = __pci_reset_function_locked(dev);
> > > > > > > > > > > >
> > > > > > > > > > > > >>> pci_dev_restore(dev);
> > > > > > > > > > > > pci_dev_unlock(dev);
> > > > > > > > > > > >
> > > > > > > > > > > > return rc;
> > > > > > > > > > > > }
> > > > > > > > > > > >
> > > > > > > > > > > > The remove/re-scan was discussed primarily because your
> > > > > > > > > > > > patch performed
> > > > > > > > > > > > a bus reset regardless of what devices were affected by
> > > > > > > > > > > > that reset and
> > > > > > > > > > > > it's difficult to manage the scope where multiple
> > > > > > > > > > > > devices are affected.
> > > > > > > > > > > > Here, the bus and slot reset functions will fail unless
> > > > > > > > > > > > the scope is
> > > > > > > > > > > > limited to the single device triggering this reset.
> > > > > > > > > > > > Thanks,
> > > > > > > > > > > >
> > > > > > > > > > > > Alex
> > > > > > > > > > > >
> > > > > > > > > > >
> > > > > > > > > > > I was thinking a bit more about it and I'm really sure
> > > > > > > > > > > how it would
> > > > > > > > > > > behave with hotplugging PCIe bridge.
> > > > > > > > > > >
> > > > > > > > > > > On aardvark PCIe controller I have already tested that
> > > > > > > > > > > secondary bus
> > > > > > > > > > > reset bit is triggering Hot Reset event and then also
> > > > > > > > > > > Link Down event.
> > > > > > > > > > > These events are not handled by aardvark driver yet
> > > > > > > > > > > (needs to
> > > > > > > > > > > implemented into kernel's emulated root bridge code).
> > > > > > > > > > >
> > > > > > > > > > > But I'm not sure how it would behave on real HW PCIe
> > > > > > > > > > > hotplugging bridge.
> > > > > > > > > > > Kernel has already code which removes PCIe device if it
> > > > > > > > > > > changes presence
> > > > > > > > > > > bit (and inform via interrupt). And Link Down event
> > > > > > > > > > > triggers this
> > > > > > > > > > > change.
> > > > > > > > > >
> > > > > > > > > > This is the difference between slot and bus resets, the
> > > > > > > > > > slot reset is
> > > > > > > > > > implemented by the hotplug controller and disables presence
> > > > > > > > > > detection
> > > > > > > > > > around the bus reset. Thanks,
> > > > > > > > >
> > > > > > > > > Yes, but I'm talking about bus reset, not about slot reset.
> > > > > > > > >
> > > > > > > > > I mean: to use bus reset via sysfs on hardware which supports
> > > > > > > > > slots and
> > > > > > > > > hotplugging.
> > > > > > > > >
> > > > > > > > > And if I'm reading code correctly, this combination is
> > > > > > > > > allowed, right?
> > > > > > > > > Via these new patches it is possible to disable slot reset
> > > > > > > > > and enable
> > > > > > > > > bus reset.
> > > > > > > >
> > > > > > > > That's true, a slot reset is simply a bus reset wrapped around
> > > > > > > > code
> > > > > > > > that prevents the device from getting ejected.
> > > > > > >
> > > > > > > Yes, this makes slot reset "safe". But bus reset is "unsafe".
> > > > > > >
> > > > > > > > Maybe it would make
> > > > > > > > sense to combine the two as far as this interface is concerned,
> > > > > > > > ie. a
> > > > > > > > single "bus" reset method that will always use slot reset when
> > > > > > > > available. Thanks,
> > > > > > >
> > > > > > > That should work when slot reset is available.
> > > > > > >
> > > > > > > Other option is that mentioned remove-reset-rescan procedure.
> > > > > >
> > > > > > That's not something we can introduce to the pci_reset_function()
> > > > > > path
> > > > > > without a fair bit of collateral in using it through vfio-pci.
> > > > > >
> > > > > > > But quick search in drivers/pci/hotplug/ results that not all
> > > > > > > hotplug
> > > > > > > drivers implement reset_slot method.
> > > > > > >
> > > > > > > So there is a possible issue with hotplug driver which may eject
> > > > > > > device
> > > > > > > during bus reset (because e.g. slot reset is not implemented)?
> > > > > >
> > > > > > People aren't reporting it, so maybe those controllers aren't being
> > > > > > used for this use case. Or maybe introducing this patch will make
> > > > > > these reset methods more readily accessible for testing. We can
> > > > > > fix or
> > > > > > blacklist those controllers for bus reset when reports come in.
> > > > > > Thanks,
> > > > >
> > > > > Ok! I do not know neither if those controllers are used, but looks
> > > > > like
> > > > > that there are still changes in hotplug code.
> > > > >
> > > > > So I guess with these patches people can test it and report issues
> > > > > when
> > > > > such thing happen.
> > > > So after a bit research as I understood we need to group slot
> > > > and bus reset together in a single category of reset methods and
> > > > then implicitly use slot reset if it is available when bus reset is
> > > > enabled by the user.
> > > > Is that right?
> > >
> > > Yes, I understand it in same way. Just I do not know which name to
> > > choose for this reset category. In PCI spec it is called Secondary Bus
> > > Reset (as it resets whole bus with all devices; but we allow this reset
> > > in this patch series only if on the bus is connected exactly one device).
> > > In PCIe spec it is called Hot Reset. And if kernel detects Slot support
> > > then kernel currently calls it Slot reset. But it is still same thing.
> > > Any opinion? I think that we could call it Hot Reset as this patch
> > > series exports it only for single device (so calling it _bus_ is not the
> > > best match).
> >
> > A similar abstraction where our scope is not limited to a single
> > function calls this a bus reset:
> >
> > int pci_reset_bus(struct pci_dev *pdev)
> > {
> > return (!pci_probe_reset_slot(pdev->slot)) ?
> > __pci_reset_slot(pdev->slot) : __pci_reset_bus(pdev->bus);
> > }
> >
> > Thanks,
> > Alex
> >
> I was going to use similar function
>
> int pci_bus_reset(struct pci_dev *dev, int probe)
> {
> return pci_dev_reset_slot_function(dev, probe) ?
> pci_parent_bus_reset(dev, probe) : 0;
>
> }
I think via the sysfs attribute we can simply call this "bus" reset,
but internally having both pci_reset_bus() and pci_bus_reset() would be
really confusing. We're doing the same thing as pci_bus_reset() but
with a different scope, so I'd probably suggest
pci_bus_reset_function().
Also, the above ternary form isn't true to the original, only -ENOTTY
allows fall-through, so something more like:
int pci_reset_bus_function(struct pci_dev *dev, int probe)
{
int rc = pci_dev_reset_slot_function(dev, probe);
return (rc == -ENOTTY) ? pci_parent_bus_reset(dev, probe) : rc;
}
Thanks,
Alex
