linux-kernel - Re: [PATCH 4/4] PCI/sysfs: Allow userspace to query and set device reset mechanism

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Message-ID: <20210315090339.54546e91@x1.home.shazbot.org>
Date:   Mon, 15 Mar 2021 09:03:39 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Pali Rohár <pali@...nel.org>
Cc:     Amey Narkhede <ameynarkhede03@...il.com>, bhelgaas@...gle.com,
        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 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