Hi Bjorn,
On 1/20/2017 9:20 AM, Sinan Kaya wrote:
> Each PCIe device can issue up to 32 transactions at a time by default.
> Each transaction is tracked by a tag number on the bus. 2.2.6.2.
> Transaction Descriptor – Transaction ID Field section of the PCIe 3.1
> specification describes extended tags.
>
> 32 transaction limit has been extended to 256 on PCI Express. According
> to the specification, all PCIe devices are required to support receiving
> 8-bit Tags (Tag completer). The PCIe-PCI bridges handle the translation
> of 8-bit tags to 5-bit tags.
>
> However, the generation of 8-bit tags is left optional to a particular HW
> implementation. The code needs to check HW support before attempting
> to enable extended tags producer capability.
>
> 32 outstanding transactions is not enough for some performance critical
> applications especially when a lot of small sized frames are transmitted.
>
> Extended tags support increases this number to 256. Devices not
> supporting extended tags tie-off this field to 0. According to ECN, it
> is safe to enable this feature for all PCIe devices.
>
> Signed-off-by: Sinan Kaya <okaya@xxxxxxxxxxxxxx>
> ---
> drivers/pci/probe.c | 16 ++++++++++++++++
> 1 file changed, 16 insertions(+)
>
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index e164b5c..1192475 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -1650,12 +1650,28 @@ static void program_hpp_type2(struct pci_dev *dev, struct hpp_type2 *hpp)
> */
> }
>
> +static void pci_configure_extended_tags(struct pci_dev *pdev)
> +{
> + u32 dev_cap;
> + int ret;
> +
> + ret = pcie_capability_read_dword(pdev, PCI_EXP_DEVCAP, &dev_cap);
> +
> + if (ret)
> + return;
> +
> + if (dev_cap & PCI_EXP_DEVCAP_EXT_TAG)
> + pcie_capability_set_word(pdev, PCI_EXP_DEVCTL,
> + PCI_EXP_DEVCTL_EXT_TAG);
> +}
> +
> static void pci_configure_device(struct pci_dev *dev)
> {
> struct hotplug_params hpp;
> int ret;
>
> pci_configure_mps(dev);
> + pci_configure_extended_tags(dev);
>
> memset(&hpp, 0, sizeof(hpp));
> ret = pci_get_hp_params(dev, &hpp);
>
Any feedback?
Sinan
--
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.
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