On Mon, 2022-09-26 at 11:47 +0200, Ulf Hansson wrote: > On Fri, 23 Sept 2022 at 17:23, Liu Ying <victor.liu@xxxxxxx> wrote: > > On Fri, 2022-09-23 at 15:48 +0200, Ulf Hansson wrote: > > > On Fri, 23 Sept 2022 at 14:47, Liu Ying <victor.liu@xxxxxxx> wrote: > > > > After a device transitions to sleep state through it's system > > > > suspend > > > > callback pm_runtime_force_suspend(), the device's driver may still > > > > try > > > > to do runtime PM for the device(runtime suspend first and then > > > > runtime > > > > resume) although runtime PM is disabled by that callback. The > > > > runtime > > > > PM operations would not touch the device effectively and the device > > > > is > > > > assumed to be resumed through it's system resume callback > > > > pm_runtime_force_resume(). > > > > > > This sounds like a fragile use case to me. In principle you want to > > > allow the device to be runtime resumed/suspended, after the device > > > has > > > already been put into a low power state through the regular system > > > suspend callback. Normally it seems better to prevent this from > > > happening, completely. > > > > Not sure if we really may prevent this from happening completely. > > > > > That said, in this case, I wonder if a better option would be to > > > point > > > ->suspend_late() to pm_runtime_force_suspend() and ->resume_early() > > > to > > > pm_runtime_force_resume(), rather than using the regular > > > ->suspend|resume() callbacks. This should avoid the problem, I think, > > > no? > > > > I thought about this and it actually works for my particular > > panel-simple case. What worries me is that the device(DRM device in my > > case) which triggers the runtime PM operations may also use > > ->suspend_late/resume_early() callbacks for whatever reasons, hence no > > fixed order to suspend/resume the two devices(like panel device and DRM > > device). > > > > Also, not sure if there is any sequence issue by using the > > ->suspend_late/resume_early() callbacks in the panel-simple driver, > > since it's written for quite a few display panels which may work with > > various DRM devices - don't want to break any of them. > > What you are describing here, is the classical problem we have with > suspend/resume ordering of devices. > > There are in principle two ways to solve this. > 1. If it makes sense, the devices might be assigned as parent/child. > 2. If it's more a consumer/supplier thing, we can add a device-link > between them. I thought about the two ways for my particular panel-simple case and the first impression is that it's not straightforward to use them. For DSI panels(with DRM_MODE_CONNECTOR_DSI connector type), it looks like panel device's parent is DSI host device(set in mipi_dsi_device_alloc() ). For other types of panels, like DPI panels, many show up in device tree as child-node of root node and connect a display controller or a display bridge through OF graph. Seems that DRM architecture level lacks some sort of glue code to use the two ways. Regards, Liu Ying > > In this way, the PM core can guarantee that the order becomes correct. > > Kind regards > Uffe
