linux-kernel - Re: [PATCH] RFC: arm64: dts: qcom: Disable USB U1/U2 entry for QC targets

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Message-ID: <f15c4760-b52f-41d8-9dc7-3896879b3ca8@oss.qualcomm.com>
Date: Fri, 8 Nov 2024 21:52:04 +0100
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Krishna Kurapati <quic_kriskura@...cinc.com>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Rob Herring <robh@...nel.org>, Conor Dooley <conor+dt@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konradybcio@...nel.org>
Cc: linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, quic_ppratap@...cinc.com,
        quic_jackp@...cinc.com
Subject: Re: [PATCH] RFC: arm64: dts: qcom: Disable USB U1/U2 entry for QC
 targets

On 7.11.2024 8:36 AM, Krishna Kurapati wrote:
> Enabling U1 and U2 power-saving states can lead to stability and
> performance issues, particularly for latency-sensitive or high-
> throughput applications. These low-power link states are intended
> to reduce power consumption by allowing the device to enter partial
> low-power modes during idle periods. However, they can sometimes
> result in unexpected behavior. Over the years, some of the issues
> seen are as follows:
> 
> 1. In device mode of operation, when UVC is active, enabling U1/U2
> is sometimes causing packets drops due to delay in entry/exit of
> intermittent low power states. These packet drops are often reflected
> as Missed Isochronous transfers as the controller was not able to
> send the packet in that microframe interval and hence glitches are
> seen on the final transmitted video output.
> 
> 2. On QCS6490-Rb3Gen2 Vision kit, ADB connection is heavily unstable
> when U1/U2 is enabled. Often when link enters U2, there is a re-
> enumeration seen and device is unusable for many use cases.

Would be nice to enable it on there too, then ;)

Generally per-file commits are preferred to make potential reverts
easier down the road

Konrad