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SubjectRe: [PATCH v5 03/14] PCI: Add pcie_bandwidth_capable() to compute max supported link bandwidth
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On 4/2/2018 5:05 PM, Bjorn Helgaas wrote:
> 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.

We can always use DIV_ROUND_UP((speed * enc_nominator),
enc_denominator). I think this is confusing and since this introduces a
bandwidth limit I would prefer to give a wider limit than a wrong one,
even it is by less than 1Mb/s. My vote is for leaving it as you wrote below.

>
> 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?

I don't know what's better but for network devices we measure bandwidth
in Gb/s, so presenting bandwidth in MB/s would mean additional
calculations. The truth is I would have prefer to use Gb/s instead of
Mb/s, but again, don't want to loss up to 1Gb/s.

>
>
> 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 {
>

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