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Date:   Wed, 17 Mar 2021 21:13:46 +0100
From:   Pali =?utf-8?B?Um9ow6Fy?= <pali@kernel.org>
To:     Alex Williamson <alex.williamson@redhat.com>
Cc:     Amey Narkhede <ameynarkhede03@gmail.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 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.