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Sashiko AI review found 2 potential issue(s) to consider: > - [Medium] Modifying device tree hardware descriptions to work around a mathematical bug in the driver's latency encoding logic. > - [Low] The commit message incorrectly states that the driver converts aspm-l1-entry-delay-ns to 'whole ms'. > -- > > commit bbff94096c8fb5346fe688ef1d463490a2efe6df > Author: Manikanta Maddireddy > > arm64: tegra: fix aspm-l1-entry-delay-ns L1 latency cells > > This commit updates the aspm-l1-entry-delay-ns property in Tegra194 and > Tegra234 device trees to larger values to adjust the L1 entrance latency > codes programmed by the driver. > >> The Tegra194 PCIe driver converts aspm-l1-entry-delay-ns to whole ms >> with ceiling division, then derives the Synopsys DesignWare PORT_AFR L1 > > This isn't a bug, but there's a typo in the commit message. The driver > uses DIV_ROUND_UP(val, 1000) which converts nanoseconds to microseconds, > not milliseconds. > >> diff --git a/arch/arm64/boot/dts/nvidia/tegra194.dtsi b/arch/arm64/boot/dts/nvidia/tegra194.dtsi >> index 1d659454a6f9f..7bbf0e892724d 100644 >> --- a/arch/arm64/boot/dts/nvidia/tegra194.dtsi >> +++ b/arch/arm64/boot/dts/nvidia/tegra194.dtsi >> @@ -2382,7 +2382,7 @@ pcie@14100000 { >> nvidia,aspm-cmrt-us = <60>; >> nvidia,aspm-pwr-on-t-us = <20>; >> nvidia,aspm-l0s-entrance-latency-us = <3>; >> - aspm-l1-entry-delay-ns = <4000>; >> + aspm-l1-entry-delay-ns = <8000>; > > Does this work around a driver calculation bug by modifying the device tree > hardware descriptions? > > Device tree properties are generally expected to accurately describe physical > hardware in an OS-agnostic way. The issue appears to stem from the driver > using order_base_2() to determine the latency bucket. For exact powers of > two, this returns the exact log2 value. For example, order_base_2(4) is 2. > Code 2 maps to the "2 us to <4 us" bucket, which is insufficient for exactly > 4 us. > > Could the driver's calculation logic be updated to handle boundary conditions > properly, rather than artificially inflating the hardware values in the > device tree files? > PORT_AFR L1 entrance latency uses the same style of stepped microsecond buckets as Endpoint L1 Acceptable Latency in the PCIe spec. On our platforms, DWC maps a delay in (2 us, 4 us] to encoding 010b (code 2, "maximum of 4 us" in the driver table). For that rule, order_base_2(us) after DIV_ROUND_UP(ns, 1000) selects the correct code (e.g. 4000 ns → 4 us → 010b). - Manikanta -- nvpublic