On Mon, Feb 03, 2020 at 04:16:36PM -0500, Alex Deucher wrote: > AMD has had a micro-controller on the GPU handling pcie link speeds > and widths dynamically (in addition to GPU clocks and voltages) for > about 12 years or so at this point to save power when the GPU is idle > and improve performance when it's required. The micro-controller > changes the link parameters dynamically based on load independent of > the driver. The driver can tweak the heuristics, or even disable the > dynamic changes, but by default it's enabled when the driver loads. > The ucode for this micro-controller is loaded by the driver so you'll > see fixed clocks and widths prior to the driver loading. We'd need > some sort of opt out I suppose for periods when the driver has enabled > dynamic pcie power management in the micro-controller. Note that there are *two* bits in the Link Status Register: * Link Autonomous Bandwidth Status "This bit is Set by hardware to indicate that hardware has autonomously changed Link speed or width, without the Port transitioning through DL_Down status, for reasons other than to attempt to correct unreliable Link operation. This bit must be set if the Physical Layer reports a speed or width change was initiated by the Downstream component that was indicated as an autonomous change." * Link Bandwidth Management Status "This bit is Set by hardware to indicate that either of the following has occurred without the Port transitioning through DL_Down status. [...] Hardware has changed Link speed or width to attempt to correct unreliable Link operation, either through an LTSSM timeout or a higher level process." See PCIe Base Spec 4.0 sec 7.8.8, 7.8.7, 4.2.6.3.3.1. The two bits generate *separate* interrupts. We only enable the interrupt for the latter. If AMD GPUs generate a Link Bandwidth Management Interrupt upon autonomously changing bandwidth for power management reasons (instead of to correct unreliability issues), that would be a spec violation. So the question is, do your GPUs violate the spec in this regard? Thanks, Lukas
