Re: [PATCH] PCI: Disable LTR on Qualcomm WCN7850 [17cb:1107] to allow ASPM L1.2

[Anthony Ruhier]

On Fri, Oct 23, 2026 at 01:20:04PM -0500, Bjorn Helgaas wrote:
> By "firmware reports LTR value", do you mean that the WCN7850 sends an
> LTR Message with snooped/non-snooped values just below
> LTR_L1.2_THRESHOLD?
>
> And since the reported LTR values are never greater than or equal to
> LTR_L1.2_THRESHOLD, the link never enters L1.2?

Not exactly. The firmware doesn't report a max snoop/no snoop that I could see
in lspci (it's set at 0ns), but if I set one with setpci, the card still
doesn't reach L1.2.
However, if I lower after boot the t-power-on-us of the pcie port by 10µs, the
card goes to L1.2.

I suspect that the firmware dynamically sets an internal threshold from the
reported LTR_L1.2_THRESHOLD because lowering the t-power-on-us in the
devicetree by 10µs doesn't mitigate the problem. Doing this seems to shift the
internal threshold to 10µs lower, because lowering the t-power-on-us by anoter
10µs is still needed.

> If so that sounds like a clue that the firmware went to some lengths
> to prevent L1.2 from being used, which is "interesting."

By disassembling the firmware with an LLM, I discovered that when wake-on-lan
is active, it stops this behavior so that the card can go to L1.2. But I
couldn't find anything else of why it would do this.

So I don't know why the firmware tries to prevent L1.2 when LTR is set,
considering that Windows runs the card fine with L1.2, and this is a Windows on
ARM laptop. I haven't seen any degradation of performance or packet loss in
Linux with L1.2 enabled.

> I don't think we can guarantee that the PCI core ASPM support will
> leave LTR disabled.  I don't see anything there that looks like "leave
> PCI_EXP_DEVCTL2_LTR_EN unchanged."

I don't understand this, can you clarify what you mean and what you would like
me to change?

Thanks,
--
Anthony Ruhier