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

[Bjorn Helgaas <helgaas@kernel.org>]

On Mon, Apr 02, 2018 at 10:34:58AM +0300, Tal Gilboa wrote:
> On 4/2/2018 3:40 AM, Bjorn Helgaas wrote:
> > 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@mellanox.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@mellanox.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@google.com>
> > > > Reviewed-by: Tariq Toukan <tariqt@mellanox.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.
> > 
> 
> I meant that PCIE_SPEED_16_0GT will return 0 from this macro since it wasn't
> added. Need to return 15754.

Oh, duh, of course!  Sorry for being dense.  What about the following?
I included the calculation as opposed to just the magic numbers to try
to make it clear how they're derived.  This has the disadvantage of
truncating the result instead of rounding, but I doubt that's
significant in this context.  If it is, we could use the magic numbers
and put the computation in a comment.

Another question: we currently deal in Mb/s, not MB/s.  Mb/s has the
advantage of sort of corresponding to the GT/s numbers, but using MB/s
would have the advantage of smaller numbers that match the table here:
https://en.wikipedia.org/wiki/PCI_Express#History_and_revisions,
but I don't know what's most typical in user-facing situations.
What's better?


commit 946435491b35b7782157e9a4d1bd73071fba7709
Author: Tal Gilboa <talgi@mellanox.com>
Date:   Fri Mar 30 08:32:03 2018 -0500

    PCI: Add pcie_bandwidth_capable() to compute max supported link bandwidth
    
    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_width * max_link_speed * (1 - encoding_overhead)
    
    2.5 and 5.0 GT/s links use 8b/10b encoding, which reduces the raw bandwidth
    available by 20%; 8.0 GT/s and faster links use 128b/130b encoding, which
    reduces it by about 1.5%.
    
    The result is in Mb/s, i.e., megabits/second, of raw bandwidth.
    
    Signed-off-by: Tal Gilboa <talgi@mellanox.com>
    [bhelgaas: add 16 GT/s, adjust for pcie_get_speed_cap() and
    pcie_get_width_cap() signatures, don't export outside drivers/pci]
    Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
    Reviewed-by: Tariq Toukan <tariqt@mellanox.com>

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 43075be79388..ff1e72060952 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -5208,6 +5208,28 @@ enum pcie_link_width pcie_get_width_cap(struct pci_dev *dev)
 	return PCIE_LNK_WIDTH_UNKNOWN;
 }
 
+/**
+ * pcie_bandwidth_capable - calculate 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.  The result
+ * is in Mb/s, i.e., megabits/second of raw bandwidth.
+ */
+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..37f9299ed623 100644
--- a/drivers/pci/pci.h
+++ b/drivers/pci/pci.h
@@ -261,8 +261,18 @@ 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 reduced by encoding overhead */
+#define PCIE_SPEED2MBS_ENC(speed) \
+	((speed) == PCIE_SPEED_16_0GT ? (16000*(128/130)) : \
+	 (speed) == PCIE_SPEED_8_0GT  ?  (8000*(128/130)) : \
+	 (speed) == PCIE_SPEED_5_0GT  ?  (5000*(8/10)) : \
+	 (speed) == PCIE_SPEED_2_5GT  ?  (2500*(8/10)) : \
+	 0)
+
 enum pci_bus_speed pcie_get_speed_cap(struct pci_dev *dev);
 enum pcie_link_width pcie_get_width_cap(struct pci_dev *dev);
+u32 pcie_bandwidth_capable(struct pci_dev *dev, enum pci_bus_speed *speed,
+			   enum pcie_link_width *width);
 
 /* Single Root I/O Virtualization */
 struct pci_sriov {