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

[Tal Gilboa <talgi@mellanox.com>]

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.