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Date:   Tue, 23 Mar 2021 15:34:19 +0100
From:   Pali =?utf-8?B?Um9ow6Fy?= <pali@kernel.org>
To:     Amey Narkhede <ameynarkhede03@gmail.com>
Cc:     alex.williamson@redhat.com, bhelgaas@google.com,
        raphael.norwitz@nutanix.com, linux-kernel@vger.kernel.org,
        linux-pci@vger.kernel.org
Subject: Re: [PATCH 4/4] PCI/sysfs: Allow userspace to query and set device
 reset mechanism
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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 <pali@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 <pali@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 <pali@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 <pali@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 <pali@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).