Linux Adding A Quirk To Improve Power Management For Intel Arc "Alchemist" GPUs

In addition to Linux 5.19 being the kernel set to have DG2/Alchemist graphics support in better shape with the IDs now (finally) being added and compute support being ready, this next kernel should boast improved power management handling for these "Alchemist" Arc Graphics GPUs.

Intel's DG2/Alchemist discrete GPUs advertise an acceptable L1 exit latency to be less than one microsecond even though they can tolerate exit latencies beyond that threshold. So coming by way of the Linux PCI (PCIe) subsystem for Linux 5.19 is a quirk that treats the initial batch of DG2/Alchemist GPUs as "unlimited" so that PCI Express Active State Power Management (ASPM) L1 power-savings can be enabled in more configurations -- beyond where it can offer less than 1us exits when resuming PCIe activity.

Active State Power Management (PCIe ASPM) is very useful for power savings during idle as we have shown with various components over the years and presumably will yield big savings too for the Intel discrete graphics cards now expected for launch in Q3. ASPM though has been a recurring headache with various quirks and other issues with different motherboards/chipsets and PCIe hardware over time. ASPM L1 allows putting the PCIe link into a low-power mode during periods of inactivity and to then restart quickly when needed. ASPM L1 offers greater power-savings than the shallower ASPM L0s mode by allowing the PCIe link to be completely powered down when not in use.

At least ahead of Intel's desktop graphics cards appearing is now this quirk in PCI-next with the Linux 5.19 merge window opening later this month. So far all indications seem to be Linux 5.19 likely being the safe minimum bet for the kernel version needed for Intel Arc Graphics discrete graphics cards and Mesa 22.0+. But as with most new hardware and especially GPUs, the newest Linux/Mesa releases of the time will be recommended for whenever these graphics cards start shipping.