linux-kernel - Re: [PATCH] PCI: qcom: Program correct T_POWER

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Message-ID: <29b32098-39ca-440d-9b51-915157b752b5@oss.qualcomm.com>
Date: Tue, 4 Nov 2025 13:29:08 +0100
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Krishna Chaitanya Chundru <krishna.chundru@....qualcomm.com>,
        Manivannan Sadhasivam <mani@...nel.org>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Krzysztof Wilczyński <kwilczynski@...nel.org>,
        Rob Herring <robh@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>
Cc: linux-pci@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org, mayank.rana@....qualcomm.com,
        quic_vbadigan@...cinc.com
Subject: Re: [PATCH] PCI: qcom: Program correct T_POWER_ON value for L1.2 exit
 timing

On 11/4/25 1:12 PM, Krishna Chaitanya Chundru wrote:
> The T_POWER_ON indicates the time (in μs) that a Port requires the port
> on the opposite side of Link to wait in L1.2.Exit after sampling CLKREQ#
> asserted before actively driving the interface. This value is used by
> the ASPM driver to compute the LTR_L1.2_THRESHOLD.
> 
> Currently, the root port exposes a T_POWER_ON value of zero in the L1SS
> capability registers, leading to incorrect LTR_L1.2_THRESHOLD calculations.
> This can result in improper L1.2 exit behavior and can trigger AER's.
> 
> To address this, program the T_POWER_ON value to 80us (scale = 1,
> value = 8) in the PCI_L1SS_CAP register during host initialization. This
> ensures that ASPM can take the root port's T_POWER_ON value into account
> while calculating the LTR_L1.2_THRESHOLD value.

Is 80us a meaningful value, or "just happens to work"?

Konrad