Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Wed, 17 Mar 2021 13:32:45 -0600
From:   Alex Williamson <alex.williamson@redhat.com>
To:     Pali =?UTF-8?B?Um9ow6Fy?= <pali@kernel.org>
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
Message-ID: <20210317133245.7d95909c@omen.home.shazbot.org>
In-Reply-To: <20210317192424.kpfybcrsen3ivr4f@pali>
References: <20210312173452.3855-1-ameynarkhede03@gmail.com>
        <20210312173452.3855-5-ameynarkhede03@gmail.com>
        <20210314235545.girtrazsdxtrqo2q@pali>
        <20210315134323.llz2o7yhezwgealp@archlinux>
        <20210315135226.avwmnhkfsgof6ihw@pali>
        <20210315083409.08b1359b@x1.home.shazbot.org>
        <20210315145238.6sg5deblr2z2pupu@pali>
        <20210315090339.54546e91@x1.home.shazbot.org>
        <20210317190206.zrtzwgskxdogl7dz@pali>
        <20210317131536.38f398b0@omen.home.shazbot.org>
        <20210317192424.kpfybcrsen3ivr4f@pali>
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Wed, 17 Mar 2021 20:24:24 +0100
Pali Roh=C3=A1r <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 Roh=C3=A1r <pali@kernel.org> wrote:
> >  =20
> > > On Monday 15 March 2021 09:03:39 Alex Williamson wrote: =20
> > > > On Mon, 15 Mar 2021 15:52:38 +0100
> > > > Pali Roh=C3=A1r <pali@kernel.org> wrote:
> > > >    =20
> > > > > On Monday 15 March 2021 08:34:09 Alex Williamson wrote:   =20
> > > > > > On Mon, 15 Mar 2021 14:52:26 +0100
> > > > > > Pali Roh=C3=A1r <pali@kernel.org> wrote:
> > > > > >      =20
> > > > > > > On Monday 15 March 2021 19:13:23 Amey Narkhede wrote:     =20
> > > > > > > > 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.       =20
> > > > > > >=20
> > > > > > > No. PCI secondary bus reset =3D PCIe Hot Reset. Slot reset is=
 just another
> > > > > > > type of reset, which is currently implemented only for PCIe h=
ot 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).     =20
> > > > > >=20
> > > > > > Warm reset is beyond the scope of this series, but could be imp=
lemented
> > > > > > in a compatible way to fit within the pci_reset_fn_methods[] ar=
ray
> > > > > > defined here.     =20
> > > > >=20
> > > > > Ok!
> > > > >    =20
> > > > > > Note that with this series the resets available through
> > > > > > pci_reset_function() and the per device reset attribute is sysf=
s remain
> > > > > > exactly the same as they are currently.  The bus and slot reset
> > > > > > methods used here are limited to devices where only a single fu=
nction is
> > > > > > affected by the reset, therefore it is not like the patch you p=
roposed
> > > > > > which performed a reset irrespective of the downstream devices.=
  This
> > > > > > series only enables selection of the existing methods.  Thanks,
> > > > > >=20
> > > > > > Alex
> > > > > >      =20
> > > > >=20
> > > > > But with this patch series, there is still an issue with PCI seco=
ndary
> > > > > 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.   =20
> > > >=20
> > > > No, there's not:
> > > >=20
> > > > int pci_reset_function(struct pci_dev *dev)
> > > > {
> > > >         int rc;
> > > >=20
> > > >         if (!dev->reset_fn)
> > > >                 return -ENOTTY;
> > > >=20
> > > >         pci_dev_lock(dev);   =20
> > > > >>>     pci_dev_save_and_disable(dev);   =20
> > > >=20
> > > >         rc =3D __pci_reset_function_locked(dev);
> > > >    =20
> > > > >>>     pci_dev_restore(dev);   =20
> > > >         pci_dev_unlock(dev);
> > > >=20
> > > >         return rc;
> > > > }
> > > >=20
> > > > The remove/re-scan was discussed primarily because your patch perfo=
rmed
> > > > a bus reset regardless of what devices were affected by that reset =
and
> > > > it's difficult to manage the scope where multiple devices are affec=
ted.
> > > > Here, the bus and slot reset functions will fail unless the scope is
> > > > limited to the single device triggering this reset.  Thanks,
> > > >=20
> > > > Alex
> > > >    =20
> > >=20
> > > I was thinking a bit more about it and I'm really sure how it would
> > > behave with hotplugging PCIe bridge.
> > >=20
> > > 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).
> > >=20
> > > But I'm not sure how it would behave on real HW PCIe hotplugging brid=
ge.
> > > Kernel has already code which removes PCIe device if it changes prese=
nce
> > > bit (and inform via interrupt). And Link Down event triggers this
> > > change. =20
> >=20
> > 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, =20
>=20
> Yes, but I'm talking about bus reset, not about slot reset.
>=20
> I mean: to use bus reset via sysfs on hardware which supports slots and
> hotplugging.
>=20
> 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.  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,

Alex