The PCIe active link power state is L0. ASPM defines two low-power states: L0s and L1. The L1 PM Substates feature defines two additional low-power states: L1.1 and L2.2. The L1.2 state may have substantial entry/exit latency. Downstream devices can use the Latency Tolerance Reporting (LTR) feature to report how much latency they can tolerate. The L1 PM Substates capability includes a programmable L1.2 threshold register. If the downstream devices can tolerate the latency indicated by the threshold, L1.2 may be used. If LTR is not enabled, the L1.2 threshold is ignored and L1.2 can be used whenever it is enabled and CLKREQ# is de-asserted. Linux currently never enables LTR, but firmware may have done so. The current L1 PM substates support in Linux sets the L1.2 threshold to a fixed value (163.84us) copied from coreboot [1]. I don't know where that value came from, and it is incorrect for some platforms, e.g., Tegra [2]. These patches change that so we calculate the L1.2 threshold based on values reported by the hardware, and we enable LTR whenever possible. This is all just my understanding from reading the spec. I'd be glad to be corrected if I'm going wrong. I'm particularly puzzled about the coreboot code in pciexp_L1_substate_commit() that sets LTR_L1.2_THRESHOLD: + pcie_update_cfg(root, root_cap + 0x08, ~0xe3ff0000, + (1 << 21) | (1 << 23) | (1 << 30)); This is writing the L1 PM Substates Control 1 register, but the shifts don't match up with any of the fields in the register, so this is an awfully funny way to write the threshold. [1] https://mail.coreboot.org/pipermail/coreboot-gerrit/2015-March/021134.html [2] https://lkml.kernel.org/r/1510492674-12786-1-git-send-email-vidyas@xxxxxxxxxx --- Bjorn Helgaas (2): PCI/ASPM: Calculate LTR_L1.2_THRESHOLD from device characteristics PCI/ASPM: Enable Latency Tolerance Reporting when supported drivers/pci/pcie/aspm.c | 71 +++++++++++++++++++++++++++++++---------------- drivers/pci/probe.c | 33 ++++++++++++++++++++++ include/linux/pci.h | 2 + 3 files changed, 82 insertions(+), 24 deletions(-)
