On Wed, Oct 06, 2021 at 12:37:47PM -0700, Stephen Boyd wrote: > The component driver only provides 'bind' and 'unbind' callbacks to tell > the host driver that it is time to assemble the aggregate driver now > that all the components have probed. The component driver model doesn't > attempt to resolve runtime PM or suspend/resume ordering, and explicitly > mentions this in the code. This lack of support leads to some pretty > gnarly usages of the 'prepare' and 'complete' power management hooks in > drivers that host the aggregate device, and it fully breaks down when > faced with ordering shutdown between the various components, the > aggregate driver, and the host driver that registers the whole thing. > > In a concrete example, the MSM display driver at drivers/gpu/drm/msm is > using 'prepare' and 'complete' to call the drm helpers > drm_mode_config_helper_suspend() and drm_mode_config_helper_resume() > respectively, so that it can move the aggregate driver suspend/resume > callbacks to be before and after the components that make up the drm > device call any suspend/resume hooks they have. This only works as long > as the component devices don't do anything in their own 'prepare' and > 'complete' callbacks. If they did, then the ordering would be incorrect > and we would be doing something in the component drivers before the > aggregate driver could do anything. Yuck! > > Similarly, when trying to add shutdown support to the MSM driver we run > across a problem where we're trying to shutdown the drm device via > drm_atomic_helper_shutdown(), but some of the devices in the encoder > chain have already been shutdown. This time, the component devices > aren't the problem (although they could be if they did anything in their > shutdown callbacks), but there's a DSI to eDP bridge in the encoder > chain that has already been shutdown before the driver hosting the > aggregate device runs shutdown. The ordering of driver probe is like > this: > > 1. msm_pdev_probe() (host driver) > 2. DSI bridge > 3. aggregate bind > > When it comes to shutdown we have this order: > > 1. DSI bridge > 2. msm_pdev_shutdown() (host driver) > > and so the bridge is already off, but we want to communicate to it to > turn things off on the display during msm_pdev_shutdown(). Double yuck! > Unfortunately, this time we can't split shutdown into multiple phases > and swap msm_pdev_shutdown() with the DSI bridge. > > Let's make the component driver into an actual device driver that has > probe/remove/shutdown functions. The driver will only be bound to the > aggregate device once all component drivers have called component_add() > to indicate they're ready to assemble the aggregate driver. This allows > us to attach shutdown logic (and in the future runtime PM logic) to the > aggregate driver so that it runs the hooks in the correct order. Why are you creating a new bus type and not using the auxiliary bus instead? You have seen Documentation/driver-api/auxiliary_bus.rst, right? thanks, greg k-h
