ACK: [SRU][O][PATCH 0/1] PCI: Wait for device readiness with Configuration RRS

[Massimiliano Pellizzer]

On Sun, 6 Apr 2025 at 11:09, Kuba Pawlak <kuba.pawlak at canonical.com> wrote:
>
> On 4.04.2025 19:14, Tim Whisonant wrote:
> > BugLink: https://bugs.launchpad.net/bugs/2106251
> >
> > SRU Justification:
> >
> > [Impact]
> >
> > PCI: Wait for device readiness with Configuration RRS
> >
> > After a device reset, delays are required before the device can
> > successfully complete config accesses.  PCIe r6.0, sec 6.6, specifies some
> > delays required before software can perform config accesses.  Devices that
> > require more time after those delays may respond to config accesses with
> > Configuration Request Retry Status (RRS) completions.
> >
> > Callers of pci_dev_wait() are responsible for delays until the device can
> > respond to config accesses.  pci_dev_wait() waits any additional time until
> > the device can successfully complete config accesses.
> >
> > Reading config space of devices that are not present or not ready typically
> > returns ~0 (PCI_ERROR_RESPONSE).  Previously we polled the Command register
> > until we got a value other than ~0.  This is sometimes a problem because
> > Root Complex handling of RRS completions may include several retries and
> > implementation-specific behavior that is invisible to software (see sec
> > 2.3.2), so the exponential backoff in pci_dev_wait() may not work as
> > intended.
> >
> > Linux enables Configuration RRS Software Visibility on all Root Ports that
> > support it.  If it is enabled, read the Vendor ID instead of the Command
> > register.  RRS completions cause immediate return of the 0x0001 reserved
> > Vendor ID value, so the pci_dev_wait() backoff works correctly.
> >
> > When a read of Vendor ID eventually completes successfully by returning a
> > non-0x0001 value (the Vendor ID or 0xffff for VFs), the device should be
> > initialized and ready to respond to config requests.
> >
> > For conventional PCI devices or devices below Root Ports that don't support
> > Configuration RRS Software Visibility, poll the Command register as before.
> >
> > This was developed independently, but is very similar to Stanislav
> > Spassov's previous work at
> > https://lore.kernel.org/linux-pci/20200223122057.6504-1-stanspas@amazon.com
> >
> > [Fix]
> >
> > Oracular: cherry picked from upstream
> > Noble:    backported from upstream
> > Jammy:    not required
> > Focal:    not required
> > Bionic:   not required
> > Xenial:   not required
> > Trusty:   not required
> >
> > [Test Plan]
> >
> > Compile and boot tested.
> >
> > [Where problems could occur]
> >
> > The PCI core is affected in the device wait logic. Latency between
> > device reset and configuration accesses may be altered.
> >
> > Bjorn Helgaas (1):
> >    PCI: Wait for device readiness with Configuration RRS
> >
> >   drivers/pci/pci.c   | 41 ++++++++++++++++++++++++++++-------------
> >   drivers/pci/pci.h   |  5 +++++
> >   drivers/pci/probe.c |  9 +++------
> >   include/linux/pci.h |  1 +
> >   4 files changed, 37 insertions(+), 19 deletions(-)
> >
> Acked-by: Kuba Pawlak <kuba.pawlak at canonical.com>
> --
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> kernel-team at lists.ubuntu.com
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Acked-by: Massimiliano Pellizzer <massimiliano.pellizzer at canonical.com>

-- 
Massimiliano Pellizzer