[PATCH V16 13/13] PCI: tegra: Add Tegra194 PCIe support

[Vidya Sagar]

On 15-02-2024 21:31, Bjorn Helgaas wrote:
> External email: Use caution opening links or attachments
>
>
> On Thu, Feb 15, 2024 at 04:55:47PM +0530, Vidya Sagar wrote:
>> On 15-02-2024 00:42, Bjorn Helgaas wrote:
>>> Hi Vidya, question about ancient history:
>>>
>>> On Tue, Aug 13, 2019 at 05:06:27PM +0530, Vidya Sagar wrote:
>>>> Add support for Synopsys DesignWare core IP based PCIe host controller
>>>> present in Tegra194 SoC.
>>>> ...
>>>> +static int tegra_pcie_dw_host_init(struct pcie_port *pp)
>>>> +{
>>>> +     struct dw_pcie *pci = to_dw_pcie_from_pp(pp);
>>>> +     struct tegra_pcie_dw *pcie = to_tegra_pcie(pci);
>>>> +     u32 val, tmp, offset, speed;
>>>> +
>>>> +     tegra_pcie_prepare_host(pp);
>>>> +
>>>> +     if (dw_pcie_wait_for_link(pci)) {
>>>> +             /*
>>>> +              * There are some endpoints which can't get the link up if
>>>> +              * root port has Data Link Feature (DLF) enabled.
>>>> +              * Refer Spec rev 4.0 ver 1.0 sec 3.4.2 & 7.7.4 for more info
>>>> +              * on Scaled Flow Control and DLF.
>>>> +              * So, need to confirm that is indeed the case here and attempt
>>>> +              * link up once again with DLF disabled.
>>> This comment suggests that there's an issue with *Endpoints*, not an
>>> issue with the Root Port.  If so, it seems like this problem could
>>> occur with all Root Ports, not just Tegra194.  Do you remember any
>>> details about this?
>>>
>>> I don't remember hearing about any similar issues, and this driver is
>>> the only place PCI_EXT_CAP_ID_DLF is referenced, so maybe it is
>>> actually something related to Tegra194?
>> We noticed PCIe link-up issues with some endpoints. link-up at the physical
>> layer level but NOT at the Data link layer level precisely. We further
>> figured out that it is the DLFE DLLPs that the root port sends during the
>> link up process which are causing the endpoints get confused and preventing
>> them from sending the InitFC DLLPs leading to the link not being up at
>> Data Link Layer level.
> Do you happen to remember any of the endpoints that have issues?
We observed this issue with an ASMedia USB controller card. Unfortunately,
I didn't keep a record of the Vendor-ID and Device-ID of that card.
>
> Could save some painful debugging if we trip over this issue on other
> systems.  We have seen a few cases where links wouldn't train at full
> speed unless they trained at a lower speed first, e.g.,
> imx6_pcie_start_link(), fu740_pcie_start_link().  I guess there are
> probably lots of edge cases that can cause link failures.
>
> Bjorn