> 2020. október 5., hétfő 14:58 keltezéssel, Limonciello, Mario írta: > > > On modern systems CPU/GPU/... performance is often dynamically > configurable > > > in the form of e.g. variable clock-speeds and TPD. The performance is > often > > > automatically adjusted to the load by some automatic-mechanism (which may > > > very well live outside the kernel). > > > These auto performance-adjustment mechanisms often can be configured with > > > one of several performance-profiles, with either a bias towards low-power > > > consumption (and cool and quiet) or towards performance (and higher power > > > consumption and thermals). > > > Introduce a new performance_profile class/sysfs API which offers a generic > > > API for selecting the performance-profile of these automatic-mechanisms. > > > > If introducing an API for this - let me ask the question, why even let each > > driver offer a class interface and userspace need to change "each" driver's > > performance setting? > > > > I would think that you could just offer something kernel-wide like > > /sys/power/performance-profile > > > > Userspace can read and write to a single file. All drivers can get notified > > on this sysfs file changing. > > > > That makes sense, in my opinion, from the regular user's perspective: > one switch to rule them all, no fuss. However, I don't think that scales well. > What if the hypothetical users wants to run a CPU-heavy workload, and thus > wants > to put the GPU into "low-power" mode and the CPU into "performance" mode? What > if > the users wants to put one GPU into "low-power" mode, but the other one into > "performance"? With the current specification, the user's needs could be > easily > satisfied. I don't see how that's possible with a single switch. Nonetheless, > I think > that a single global switch *in addition* to the class devices could possibly > simplify the userspace-kernel interaction for most users.
I think that the moment you represent a platform/system device as a switch you lose the ability to set different policies for other types of devices separately. What if using the platform/system device actually /also/ orchestrates changes to those other devices you mention? Then it becomes an order of events problem, or worse a problem where one switch shows "wrong" value. > > > > The systems that react in firmware (such as the two that prompted > > this discussion) can change at that time. It leaves the possibility for a > > more open kernel implementation that can do the same thing though too by > > directly modifying device registers instead of ACPI devices. > > > > Excuse my ignorance, but I don't really see why this interface would be tied > to > ACPI devices? Why is it not possible to write a driver that implements this > interface > and directly modifies device registers? Am I missing something obvious here? > When implemented for the two vendors mentioned here, it would be using a proprietary "firmware API" implemented by those two vendors. For example write arguments (0x1, 0x2) to ACPI-WMI method WMFT and it will cause firmware to coordinate using undisclosed protocol to affect the platform changes desirable. This is different in my mind from "kernel writes to a specific register" to set power properties of a specific device.
