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SubjectRE: [PATCH v5 05/14] PCI: Add pcie_print_link_status() to log link speed and whether it's limited
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> -----Original Message-----
> From: Bjorn Helgaas [mailto:helgaas@kernel.org]
> Sent: Monday, April 02, 2018 12:58 PM
> To: Keller, Jacob E <jacob.e.keller@intel.com>
> Cc: Tal Gilboa <talgi@mellanox.com>; Tariq Toukan <tariqt@mellanox.com>; Ariel
> Elior <ariel.elior@cavium.com>; Ganesh Goudar <ganeshgr@chelsio.com>;
> Kirsher, Jeffrey T <jeffrey.t.kirsher@intel.com>; everest-linux-l2@cavium.com;
> intel-wired-lan@lists.osuosl.org; netdev@vger.kernel.org; linux-
> kernel@vger.kernel.org; linux-pci@vger.kernel.org
> Subject: Re: [PATCH v5 05/14] PCI: Add pcie_print_link_status() to log link speed
> and whether it's limited
>
> On Mon, Apr 02, 2018 at 04:25:17PM +0000, Keller, Jacob E wrote:
> > > -----Original Message-----
> > > From: Bjorn Helgaas [mailto:helgaas@kernel.org]
> > > Sent: Friday, March 30, 2018 2:05 PM
> > > To: Tal Gilboa <talgi@mellanox.com>
> > > Cc: Tariq Toukan <tariqt@mellanox.com>; Keller, Jacob E
> > > <jacob.e.keller@intel.com>; Ariel Elior <ariel.elior@cavium.com>; Ganesh
> > > Goudar <ganeshgr@chelsio.com>; Kirsher, Jeffrey T
> > > <jeffrey.t.kirsher@intel.com>; everest-linux-l2@cavium.com; intel-wired-
> > > lan@lists.osuosl.org; netdev@vger.kernel.org; linux-kernel@vger.kernel.org;
> > > linux-pci@vger.kernel.org
> > > Subject: [PATCH v5 05/14] PCI: Add pcie_print_link_status() to log link speed
> and
> > > whether it's limited
> > >
> > > From: Tal Gilboa <talgi@mellanox.com>
> > >
> > > Add pcie_print_link_status(). This logs the current settings of the link
> > > (speed, width, and total available bandwidth).
> > >
> > > If the device is capable of more bandwidth but is limited by a slower
> > > upstream link, we include information about the link that limits the
> > > device's performance.
> > >
> > > The user may be able to move the device to a different slot for better
> > > performance.
> > >
> > > This provides a unified method for all PCI devices to report status and
> > > issues, instead of each device reporting in a different way, using
> > > different code.
> > >
> > > Signed-off-by: Tal Gilboa <talgi@mellanox.com>
> > > [bhelgaas: changelog, reword log messages, print device capabilities when
> > > not limited]
> > > Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
> > > ---
> > > drivers/pci/pci.c | 29 +++++++++++++++++++++++++++++
> > > include/linux/pci.h | 1 +
> > > 2 files changed, 30 insertions(+)
> > >
> > > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > > index e00d56b12747..cec7aed09f6b 100644
> > > --- a/drivers/pci/pci.c
> > > +++ b/drivers/pci/pci.c
> > > @@ -5283,6 +5283,35 @@ u32 pcie_bandwidth_capable(struct pci_dev *dev,
> > > enum pci_bus_speed *speed,
> > > return *width * PCIE_SPEED2MBS_ENC(*speed);
> > > }
> > >
> > > +/**
> > > + * pcie_print_link_status - Report the PCI device's link speed and width
> > > + * @dev: PCI device to query
> > > + *
> > > + * Report the available bandwidth at the device. If this is less than the
> > > + * device is capable of, report the device's maximum possible bandwidth and
> > > + * the upstream link that limits its performance to less than that.
> > > + */
> > > +void pcie_print_link_status(struct pci_dev *dev)
> > > +{
> > > + enum pcie_link_width width, width_cap;
> > > + enum pci_bus_speed speed, speed_cap;
> > > + struct pci_dev *limiting_dev = NULL;
> > > + u32 bw_avail, bw_cap;
> > > +
> > > + bw_cap = pcie_bandwidth_capable(dev, &speed_cap, &width_cap);
> > > + bw_avail = pcie_bandwidth_available(dev, &limiting_dev, &speed,
> > > &width);
> > > +
> > > + if (bw_avail >= bw_cap)
> > > + pci_info(dev, "%d Mb/s available bandwidth (%s x%d link)\n",
> > > + bw_cap, PCIE_SPEED2STR(speed_cap), width_cap);
> > > + else
> > > + pci_info(dev, "%d Mb/s available bandwidth, limited by %s x%d
> > > link at %s (capable of %d Mb/s with %s x%d link)\n",
> > > + bw_avail, PCIE_SPEED2STR(speed), width,
> > > + limiting_dev ? pci_name(limiting_dev) : "<unknown>",
> > > + bw_cap, PCIE_SPEED2STR(speed_cap), width_cap);
> > > +}
> >
> > Personally, I would make thic last one a pci_warn() to indicate it at a
> > higher log level, but I'm ok with the wording, and if consensus is that
> > this should be at info, I'm ok with that.
>
> Tal's original patch did have a pci_warn() here, and we went back and
> forth a bit. They get bug reports when a device doesn't perform as
> expected, which argues for pci_warn(). But they also got feedback
> saying warnings are a bit too much, which argues for pci_info() [1]
>
> I don't have a really strong opinion either way. I have a slight
> preference for info because the user may not be able to do anything
> about it (there may not be a faster slot available), and I think
> distros are usually configured so a warning interrupts the smooth
> graphical boot.
>
> It looks like mlx4, fm10k, and ixgbe currently use warnings, while
> bnx2x, bnxt_en, and cxgb4 use info. It's a tie so far :)
>
> [1] https://lkml.kernel.org/r/e47f3628-b56c-4d0a-f18b-
> 5ffaf261ccc0@mellanox.com
>

With that information, I'm fine with the proposal to display this as only an info. The message is still printed and can be used for debugging purposes, and I think that's really enough.

> Here's a proposal for printing the bandwidth as "x.xxx Gb/s":

Nice, I like that also.

Regards,
Jake

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