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

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Message-ID: <571d5733-be1b-449c-ba5a-414e0f950463@oss.qualcomm.com>
Date: Tue, 4 Nov 2025 17:43:05 +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 5:38 PM, Krishna Chaitanya Chundru wrote:
> 
> On 11/4/2025 5:59 PM, Konrad Dybcio wrote:
>> 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"?
> 
> This value is given by hardware team.

Sorry I asked the wrong question

Is it something that comes from the spec (PCI or DWC), or is it qc
specific?

Konrad