Re: [PATCH v1 0/2] PCI/ASPM: Refine L1 PM Substates support

[Bjorn Helgaas <helgaas@kernel.org>]

On Sat, Dec 02, 2017 at 03:45:38PM -0600, Bjorn Helgaas wrote:
> 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@nvidia.com
> 
> ---
> 
> 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(-)

I applied these with Vidya's Reviewed-by to pci/aspm for v4.16.

I'd still really like to hear from the coreboot folks about the
rationale for the current coreboot code.  You folks are in a much
better position to actually understand the hardware details.

Bjorn