linux-kernel - Re: [PATCH v5 03/14] PCI: Add pcie_bandwidth_capable() to compute max supported link bandwidth

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Date:   Sun, 1 Apr 2018 19:40:49 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Tal Gilboa <talgi@...lanox.com>
Cc:     Tariq Toukan <tariqt@...lanox.com>,
        Jacob Keller <jacob.e.keller@...el.com>,
        Ariel Elior <ariel.elior@...ium.com>,
        Ganesh Goudar <ganeshgr@...lsio.com>,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        everest-linux-l2@...ium.com, intel-wired-lan@...ts.osuosl.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-pci@...r.kernel.org
Subject: Re: [PATCH v5 03/14] PCI: Add pcie_bandwidth_capable() to compute
 max supported link bandwidth

On Sun, Apr 01, 2018 at 11:38:53PM +0300, Tal Gilboa wrote:
> On 3/31/2018 12:05 AM, Bjorn Helgaas wrote:
> > From: Tal Gilboa <talgi@...lanox.com>
> > 
> > Add pcie_bandwidth_capable() to compute the max link bandwidth supported by
> > a device, based on the max link speed and width, adjusted by the encoding
> > overhead.
> > 
> > The maximum bandwidth of the link is computed as:
> > 
> >    max_link_speed * max_link_width * (1 - encoding_overhead)
> > 
> > The encoding overhead is about 20% for 2.5 and 5.0 GT/s links using 8b/10b
> > encoding, and about 1.5% for 8 GT/s or higher speed links using 128b/130b
> > encoding.
> > 
> > Signed-off-by: Tal Gilboa <talgi@...lanox.com>
> > [bhelgaas: adjust for pcie_get_speed_cap() and pcie_get_width_cap()
> > signatures, don't export outside drivers/pci]
> > Signed-off-by: Bjorn Helgaas <bhelgaas@...gle.com>
> > Reviewed-by: Tariq Toukan <tariqt@...lanox.com>
> > ---
> >   drivers/pci/pci.c |   21 +++++++++++++++++++++
> >   drivers/pci/pci.h |    9 +++++++++
> >   2 files changed, 30 insertions(+)
> > 
> > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > index 43075be79388..9ce89e254197 100644
> > --- a/drivers/pci/pci.c
> > +++ b/drivers/pci/pci.c
> > @@ -5208,6 +5208,27 @@ enum pcie_link_width pcie_get_width_cap(struct pci_dev *dev)
> >   	return PCIE_LNK_WIDTH_UNKNOWN;
> >   }
> > +/**
> > + * pcie_bandwidth_capable - calculates a PCI device's link bandwidth capability
> > + * @dev: PCI device
> > + * @speed: storage for link speed
> > + * @width: storage for link width
> > + *
> > + * Calculate a PCI device's link bandwidth by querying for its link speed
> > + * and width, multiplying them, and applying encoding overhead.
> > + */
> > +u32 pcie_bandwidth_capable(struct pci_dev *dev, enum pci_bus_speed *speed,
> > +			   enum pcie_link_width *width)
> > +{
> > +	*speed = pcie_get_speed_cap(dev);
> > +	*width = pcie_get_width_cap(dev);
> > +
> > +	if (*speed == PCI_SPEED_UNKNOWN || *width == PCIE_LNK_WIDTH_UNKNOWN)
> > +		return 0;
> > +
> > +	return *width * PCIE_SPEED2MBS_ENC(*speed);
> > +}
> > +
> >   /**
> >    * pci_select_bars - Make BAR mask from the type of resource
> >    * @dev: the PCI device for which BAR mask is made
> > diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
> > index 66738f1050c0..2a50172b9803 100644
> > --- a/drivers/pci/pci.h
> > +++ b/drivers/pci/pci.h
> > @@ -261,8 +261,17 @@ void pci_disable_bridge_window(struct pci_dev *dev);
> >   	 (speed) == PCIE_SPEED_2_5GT ? "2.5 GT/s" : \
> >   	 "Unknown speed")
> > +/* PCIe speed to Mb/s with encoding overhead: 20% for gen2, ~1.5% for gen3 */
> > +#define PCIE_SPEED2MBS_ENC(speed) \
> 
> Missing gen4.

I made it "gen3+".  I think that's accurate, isn't it?  The spec
doesn't seem to actually use "gen3" as a specific term, but sec 4.2.2
says rates of 8 GT/s or higher (which I think includes gen3 and gen4)
use 128b/130b encoding.