Re: [Linux-kernel-mentees] [PATCH v6 2/2] PCI/ASPM: Add support for LTR _DSM

[Bjorn Helgaas <helgaas@kernel.org>]

On Thu, Oct 01, 2020 at 04:44:36PM -0500, Bjorn Helgaas wrote:
> From: Puranjay Mohan <puranjay12@gmail.com>
> 
> Latency Tolerance Reporting (LTR) is required for the ASPM L1.2 PM
> substate.  Devices with Upstream Ports (Endpoints and Switches) may support
> the optional LTR Capability.  When LTR is enabled, devices transmit LTR
> messages containing Snoop and No-Snoop Latencies upstream.  The L1.2
> substate may be entered if the most recent LTR values are greater than or
> equal to the LTR_L1.2_THRESHOLD from the L1 PM Substates Control 1
> register.
> 
> Add a new function pci_ltr_init() which will be called from
> pci_init_capabilities() to initialize every PCIe device's LTR values.
> Add code in probe.c to evaluate LTR _DSM and save the latencies in pci_dev.

> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -2106,6 +2106,9 @@ static void pci_configure_ltr(struct pci_dev *dev)
>  	if (!pci_is_pcie(dev))
>  		return;
>  
> +	/* Read latency values (if any) from platform */
> +	pci_acpi_evaluate_ltr_latency(dev);
> +
>  	pcie_capability_read_dword(dev, PCI_EXP_DEVCAP2, &cap);
>  	if (!(cap & PCI_EXP_DEVCAP2_LTR))
>  		return;
> @@ -2400,6 +2403,7 @@ static void pci_init_capabilities(struct pci_dev *dev)
>  	pci_ptm_init(dev);		/* Precision Time Measurement */
>  	pci_aer_init(dev);		/* Advanced Error Reporting */
>  	pci_dpc_init(dev);		/* Downstream Port Containment */
> +	pci_ltr_init(dev);		/* Latency Tolerance Reporting */

I don't think we're doing this in quite the right order.  If I
understand correctly, we have this:

  pci_device_add
    pci_configure_device
      pci_configure_ltr
        pci_acpi_evaluate_ltr_latency
          acpi_evaluate_dsm(DSM_PCI_LTR_MAX_LATENCY)
        pcie_capability_set_word(PCI_EXP_DEVCTL2_LTR_EN)  <-- enable LTR
    pci_init_capabilities
      pci_ltr_init
        pci_write_config_word(PCI_LTR_MAX_SNOOP_LAT)  <-- set LTR msg content

So we enable LTR messages before we set the latency values in the LTR
capability.  I think we need to set the values *first*.

>  	pcie_report_downtraining(dev);
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