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

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Message-ID: <8c664765-2c5a-4e42-8390-5969f0bce671@oss.qualcomm.com>
Date: Tue, 4 Nov 2025 22:21:13 +0530
From: Krishna Chaitanya Chundru <krishna.chundru@....qualcomm.com>
To: Konrad Dybcio <konrad.dybcio@....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 11/4/2025 10:13 PM, Konrad Dybcio wrote:
> 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?
T power ON value is part of the PCIe spec, expectation is this value 
needs to be
set in the hardware only, In QC this value is Zero so it requires sw support
to program this correctly before enumeration.

- Krishna Chaitanya.
> Konrad