Message-ID: <20210323101538.0f683e53@omen.home.shazbot.org>
Date:   Tue, 23 Mar 2021 10:15:38 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Amey Narkhede <ameynarkhede03@...il.com>
Cc:     bhelgaas@...gle.com, pali@...nel.org, raphael.norwitz@...anix.com,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org
Subject: Re: [PATCH 4/4] PCI/sysfs: Allow userspace to query and set device
 reset mechanism

On Tue, 23 Mar 2021 10:06:25 -0600
Alex Williamson <alex.williamson@...hat.com> wrote:

> On Tue, 23 Mar 2021 21:02:21 +0530
> Amey Narkhede <ameynarkhede03@...il.com> wrote:
> 
> > On 21/03/23 08:44AM, Alex Williamson wrote:  
> > > On Tue, 23 Mar 2021 15:34:19 +0100
> > > Pali Rohár <pali@...nel.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 <pali@...nel.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@...nel.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@...nel.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@...nel.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@...nel.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().

I'm already confusing them, s/bus_reset/reset_bus/ in the last sentence
above.  Thanks,

Alex

> 
> 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
> 

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