Re: [PATCH v1 1/1] PCI/ERR: Handle fatal error recovery for non-hotplug capable devices

From: "Oliver O'Halloran" <oohall@gmail.com>
To: "Kuppuswamy,
	Sathyanarayanan"  <sathyanarayanan.kuppuswamy@linux.intel.com>
Cc: Yicong Yang <yangyicong@hisilicon.com>,
	Bjorn Helgaas <bhelgaas@google.com>,
	jay.vosburgh@canonical.com, linux-pci@vger.kernel.org,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	ashok.raj@intel.com
Subject: Re: [PATCH v1 1/1] PCI/ERR: Handle fatal error recovery for non-hotplug capable devices
Date: Wed, 27 May 2020 13:00:32 +1000	[thread overview]
Message-ID: <CAOSf1CHTUyQ5o_ThkaPUkGjtTSK1UOkxSmKAWY3n3bdrVcjacA@mail.gmail.com> (raw)
In-Reply-To: <f9a46300-ef4b-be19-b8cf-bcb876c75d62@linux.intel.com>

On Wed, May 27, 2020 at 12:00 PM Kuppuswamy, Sathyanarayanan
<sathyanarayanan.kuppuswamy@linux.intel.com> wrote:
>
> Hi,
>
> On 5/21/20 7:56 PM, Yicong Yang wrote:
> >
> >
> > On 2020/5/22 3:31, Kuppuswamy, Sathyanarayanan wrote:
> >>
> > Not exactly. In pci_bus_error_reset(), we call pci_slot_reset() only if it's
> > hotpluggable. But we always call pci_bus_reset() to perform a secondary bus
> > reset for the bridge. That's what I think is unnecessary for a normal link,
> > and that's what reset link indicates us to do. The slot reset is introduced
> > in the process only to solve side effects. (c4eed62a2143, PCI/ERR: Use slot reset if available)
>
> IIUC, pci_bus_reset() will do slot reset if its supported (hot-plug
> capable slots). If its not supported then it will attempt secondary
> bus reset. So secondary bus reset will be attempted only if slot
> reset is not supported.
>
> Since reported_error_detected() requests us to do reset, we will have
> to attempt some kind of reset before we call ->slot_reset() right?

Yes, the driver returns PCI_ERS_RESULT_NEED_RESET from
->error_detected() to indicate that it doesn't know how to recover
from the error. How that reset is performed doesn't really matter, but
it does need to happen.

> > PCI_ERS_RESULT_NEED_RESET indicates that the driver
> > wants a platform-dependent slot reset and its ->slot_reset() method to be called then.
> > I don't think it's same as slot reset mentioned above, which is only for hotpluggable
> > ones.
> What you think is the correct reset implementation ? Is it something
> like this?
>
> if (hotplug capable)
>     try_slot_reset()
> else
>     do_nothing()

Looks broken to me, but all the reset handling is a rat's nest so
maybe I'm missing something. In the case of a DPC trip the link is
disabled which has the side-effect of hot-resetting the downstream
device. Maybe it's fine?

As an aside, why do we have both ->slot_reset() and ->reset_done() in
the error handling callbacks? Seems like their roles are almost
identical.

Oliver